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authorbors[bot] <26634292+bors[bot]@users.noreply.github.com>2021-05-31 22:01:52 +0100
committerGitHub <[email protected]>2021-05-31 22:01:52 +0100
commit71117e6812f87e014bc8e984e195a75e222ac227 (patch)
tree8f5a74076cc2802c5de8fdb1b40e3eaf12252d51
parent42dfdb87cb748e65d2c87687bde4d4712f9a850b (diff)
parente7c49666be180eba2720cce09d4d2116b1ef4d20 (diff)
Merge #8717
8717: Update match checking algorithm r=iDawer a=iDawer I've recently got interest in the match checking to extend the current algo to support reporting witnesses of non-exhaustiveness. It appears the algo is outdated from rustc's implementation. I decided to rewrite it based on the latest rustc's version. It is a diff-based port to ra codebase. That means you can diff-compare these files to rustc. I'm striving to keep minimal ra-related changes in the algo to make it easier to backport future changes from the upstream. Based on upstream algorithm of version rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22) https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs The goal of this PR is to cover the current `missing-match-arm` diagnostic. What is remaining to do: - [x] Error handling. The errors that are unrelated to match checking will be handled before the check. Just like how it made in rustc. - [x] Lowering `hir_def::expr::Pat` to `hir_ty::diagnostics::match_check::Pat`. rustc's match checking works on top of `rustc_mir_build::thir::Pat`, which is lowered from `hir::Pat` and carries some extra semantics used by the check. All unrelated checks are done there. RA could use this to rule out running the check on unimplemented cases (`Pat::ConstBlock`, etc). - [x] ~~Proper~~Loose typecheck of match arm patterns (https://github.com/rust-analyzer/rust-analyzer/pull/8840, https://github.com/rust-analyzer/rust-analyzer/pull/8875). - [x] Tests from `hir_ty::diagnostics::match_check::tests`. - [x] Clean up `todo`s - [x] Test run on real repos https://github.com/rust-analyzer/rust-analyzer/pull/8717#issuecomment-847120265. Co-authored-by: Dawer <[email protected]>
-rw-r--r--crates/hir_def/src/path.rs2
-rw-r--r--crates/hir_ty/Cargo.toml1
-rw-r--r--crates/hir_ty/src/diagnostics/expr.rs117
-rw-r--r--crates/hir_ty/src/diagnostics/match_check.rs1304
-rw-r--r--crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs907
-rw-r--r--crates/hir_ty/src/diagnostics/match_check/pat_util.rs56
-rw-r--r--crates/hir_ty/src/diagnostics/match_check/usefulness.rs1188
7 files changed, 2769 insertions, 806 deletions
diff --git a/crates/hir_def/src/path.rs b/crates/hir_def/src/path.rs
index 16440041d..4cdb5913d 100644
--- a/crates/hir_def/src/path.rs
+++ b/crates/hir_def/src/path.rs
@@ -166,7 +166,7 @@ impl Path {
166 } 166 }
167 167
168 /// Converts a known mod path to `Path`. 168 /// Converts a known mod path to `Path`.
169 pub(crate) fn from_known_path( 169 pub fn from_known_path(
170 path: ModPath, 170 path: ModPath,
171 generic_args: Vec<Option<Interned<GenericArgs>>>, 171 generic_args: Vec<Option<Interned<GenericArgs>>>,
172 ) -> Path { 172 ) -> Path {
diff --git a/crates/hir_ty/Cargo.toml b/crates/hir_ty/Cargo.toml
index c3d02424d..4b714c6d8 100644
--- a/crates/hir_ty/Cargo.toml
+++ b/crates/hir_ty/Cargo.toml
@@ -22,6 +22,7 @@ chalk-solve = { version = "0.68", default-features = false }
22chalk-ir = "0.68" 22chalk-ir = "0.68"
23chalk-recursive = "0.68" 23chalk-recursive = "0.68"
24la-arena = { version = "0.2.0", path = "../../lib/arena" } 24la-arena = { version = "0.2.0", path = "../../lib/arena" }
25once_cell = { version = "1.5.0" }
25 26
26stdx = { path = "../stdx", version = "0.0.0" } 27stdx = { path = "../stdx", version = "0.0.0" }
27hir_def = { path = "../hir_def", version = "0.0.0" } 28hir_def = { path = "../hir_def", version = "0.0.0" }
diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 86f82e3fa..3efbce773 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -2,9 +2,11 @@
2//! through the body using inference results: mismatched arg counts, missing 2//! through the body using inference results: mismatched arg counts, missing
3//! fields, etc. 3//! fields, etc.
4 4
5use std::sync::Arc; 5use std::{cell::RefCell, sync::Arc};
6 6
7use hir_def::{expr::Statement, path::path, resolver::HasResolver, AssocItemId, DefWithBodyId}; 7use hir_def::{
8 expr::Statement, path::path, resolver::HasResolver, AssocItemId, DefWithBodyId, HasModule,
9};
8use hir_expand::name; 10use hir_expand::name;
9use rustc_hash::FxHashSet; 11use rustc_hash::FxHashSet;
10use syntax::{ast, AstPtr}; 12use syntax::{ast, AstPtr};
@@ -12,7 +14,10 @@ use syntax::{ast, AstPtr};
12use crate::{ 14use crate::{
13 db::HirDatabase, 15 db::HirDatabase,
14 diagnostics::{ 16 diagnostics::{
15 match_check::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness}, 17 match_check::{
18 self,
19 usefulness::{compute_match_usefulness, expand_pattern, MatchCheckCtx, PatternArena},
20 },
16 MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkOrSomeInTailExpr, 21 MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkOrSomeInTailExpr,
17 MissingPatFields, RemoveThisSemicolon, 22 MissingPatFields, RemoveThisSemicolon,
18 }, 23 },
@@ -294,12 +299,12 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
294 &infer.type_of_expr[match_expr] 299 &infer.type_of_expr[match_expr]
295 }; 300 };
296 301
297 let cx = MatchCheckCtx { match_expr, body, infer: infer.clone(), db }; 302 let pattern_arena = RefCell::new(PatternArena::new());
298 let pats = arms.iter().map(|arm| arm.pat);
299 303
300 let mut seen = Matrix::empty(); 304 let mut m_arms = Vec::new();
301 for pat in pats { 305 let mut has_lowering_errors = false;
302 if let Some(pat_ty) = infer.type_of_pat.get(pat) { 306 for arm in arms {
307 if let Some(pat_ty) = infer.type_of_pat.get(arm.pat) {
303 // We only include patterns whose type matches the type 308 // We only include patterns whose type matches the type
304 // of the match expression. If we had a InvalidMatchArmPattern 309 // of the match expression. If we had a InvalidMatchArmPattern
305 // diagnostic or similar we could raise that in an else 310 // diagnostic or similar we could raise that in an else
@@ -315,14 +320,25 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
315 .as_reference() 320 .as_reference()
316 .map(|(match_expr_ty, ..)| match_expr_ty == pat_ty) 321 .map(|(match_expr_ty, ..)| match_expr_ty == pat_ty)
317 .unwrap_or(false)) 322 .unwrap_or(false))
318 && types_of_subpatterns_do_match(pat, &cx.body, &infer) 323 && types_of_subpatterns_do_match(arm.pat, &body, &infer)
319 { 324 {
320 // If we had a NotUsefulMatchArm diagnostic, we could 325 // If we had a NotUsefulMatchArm diagnostic, we could
321 // check the usefulness of each pattern as we added it 326 // check the usefulness of each pattern as we added it
322 // to the matrix here. 327 // to the matrix here.
323 let v = PatStack::from_pattern(pat); 328 let m_arm = match_check::MatchArm {
324 seen.push(&cx, v); 329 pat: self.lower_pattern(
325 continue; 330 arm.pat,
331 &mut pattern_arena.borrow_mut(),
332 db,
333 &body,
334 &mut has_lowering_errors,
335 ),
336 has_guard: arm.guard.is_some(),
337 };
338 m_arms.push(m_arm);
339 if !has_lowering_errors {
340 continue;
341 }
326 } 342 }
327 } 343 }
328 344
@@ -330,34 +346,73 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
330 // fit the match expression, we skip this diagnostic. Skipping the entire 346 // fit the match expression, we skip this diagnostic. Skipping the entire
331 // diagnostic rather than just not including this match arm is preferred 347 // diagnostic rather than just not including this match arm is preferred
332 // to avoid the chance of false positives. 348 // to avoid the chance of false positives.
349 #[cfg(test)]
350 match_check::tests::report_bail_out(db, self.owner, arm.pat, self.sink);
333 return; 351 return;
334 } 352 }
335 353
336 match is_useful(&cx, &seen, &PatStack::from_wild()) { 354 let cx = MatchCheckCtx {
337 Ok(Usefulness::Useful) => (), 355 module: self.owner.module(db.upcast()),
338 // if a wildcard pattern is not useful, then all patterns are covered 356 match_expr,
339 Ok(Usefulness::NotUseful) => return, 357 infer: &infer,
340 // this path is for unimplemented checks, so we err on the side of not 358 db,
341 // reporting any errors 359 pattern_arena: &pattern_arena,
342 _ => return, 360 eprint_panic_context: &|| {
343 } 361 use syntax::AstNode;
362 if let Ok(scrutinee_sptr) = source_map.expr_syntax(match_expr) {
363 let root = scrutinee_sptr.file_syntax(db.upcast());
364 if let Some(match_ast) = scrutinee_sptr.value.to_node(&root).syntax().parent() {
365 eprintln!(
366 "Match checking is about to panic on this expression:\n{}",
367 match_ast.to_string(),
368 );
369 }
370 }
371 },
372 };
373 let report = compute_match_usefulness(&cx, &m_arms);
344 374
345 if let Ok(source_ptr) = source_map.expr_syntax(id) { 375 // FIXME Report unreacheble arms
346 let root = source_ptr.file_syntax(db.upcast()); 376 // https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/check_match.rs#L200-L201
347 if let ast::Expr::MatchExpr(match_expr) = &source_ptr.value.to_node(&root) { 377
348 if let (Some(match_expr), Some(arms)) = 378 let witnesses = report.non_exhaustiveness_witnesses;
349 (match_expr.expr(), match_expr.match_arm_list()) 379 // FIXME Report witnesses
350 { 380 // eprintln!("compute_match_usefulness(..) -> {:?}", &witnesses);
351 self.sink.push(MissingMatchArms { 381 if !witnesses.is_empty() {
352 file: source_ptr.file_id, 382 if let Ok(source_ptr) = source_map.expr_syntax(id) {
353 match_expr: AstPtr::new(&match_expr), 383 let root = source_ptr.file_syntax(db.upcast());
354 arms: AstPtr::new(&arms), 384 if let ast::Expr::MatchExpr(match_expr) = &source_ptr.value.to_node(&root) {
355 }) 385 if let (Some(match_expr), Some(arms)) =
386 (match_expr.expr(), match_expr.match_arm_list())
387 {
388 self.sink.push(MissingMatchArms {
389 file: source_ptr.file_id,
390 match_expr: AstPtr::new(&match_expr),
391 arms: AstPtr::new(&arms),
392 })
393 }
356 } 394 }
357 } 395 }
358 } 396 }
359 } 397 }
360 398
399 fn lower_pattern(
400 &self,
401 pat: PatId,
402 pattern_arena: &mut PatternArena,
403 db: &dyn HirDatabase,
404 body: &Body,
405 have_errors: &mut bool,
406 ) -> match_check::PatId {
407 let mut patcx = match_check::PatCtxt::new(db, &self.infer, body);
408 let pattern = patcx.lower_pattern(pat);
409 let pattern = pattern_arena.alloc(expand_pattern(pattern));
410 if !patcx.errors.is_empty() {
411 *have_errors = true;
412 }
413 pattern
414 }
415
361 fn validate_results_in_tail_expr(&mut self, body_id: ExprId, id: ExprId, db: &dyn HirDatabase) { 416 fn validate_results_in_tail_expr(&mut self, body_id: ExprId, id: ExprId, db: &dyn HirDatabase) {
362 // the mismatch will be on the whole block currently 417 // the mismatch will be on the whole block currently
363 let mismatch = match self.infer.type_mismatch_for_expr(body_id) { 418 let mismatch = match self.infer.type_mismatch_for_expr(body_id) {
diff --git a/crates/hir_ty/src/diagnostics/match_check.rs b/crates/hir_ty/src/diagnostics/match_check.rs
index e8dd669bf..a9a99f57a 100644
--- a/crates/hir_ty/src/diagnostics/match_check.rs
+++ b/crates/hir_ty/src/diagnostics/match_check.rs
@@ -1,871 +1,416 @@
1//! This module implements match statement exhaustiveness checking and usefulness checking 1//! Validation of matches.
2//! for match arms.
3//! 2//!
4//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which 3//! This module provides lowering from [hir_def::expr::Pat] to [self::Pat] and match
5//! contains very detailed documentation about the algorithms used here. I've duplicated 4//! checking algorithm.
6//! most of that documentation below.
7//! 5//!
8//! This file includes the logic for exhaustiveness and usefulness checking for 6//! It is modeled on the rustc module `rustc_mir_build::thir::pattern`.
9//! pattern-matching. Specifically, given a list of patterns for a type, we can 7
10//! tell whether: 8mod deconstruct_pat;
11//! - (a) the patterns cover every possible constructor for the type (exhaustiveness). 9mod pat_util;
12//! - (b) each pattern is necessary (usefulness). 10pub(crate) mod usefulness;
13//! 11
14//! The algorithm implemented here is a modified version of the one described in 12use hir_def::{body::Body, EnumVariantId, LocalFieldId, VariantId};
15//! <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
16//! However, to save future implementors from reading the original paper, we
17//! summarize the algorithm here to hopefully save time and be a little clearer
18//! (without being so rigorous).
19//!
20//! The core of the algorithm revolves about a "usefulness" check. In particular, we
21//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
22//! a matrix). `U(P, p)` represents whether, given an existing list of patterns
23//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
24//! uncovered values of the type).
25//!
26//! If we have this predicate, then we can easily compute both exhaustiveness of an
27//! entire set of patterns and the individual usefulness of each one.
28//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
29//! match doesn't increase the number of values we're matching)
30//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
31//! pattern to those that have come before it doesn't increase the number of values
32//! we're matching).
33//!
34//! During the course of the algorithm, the rows of the matrix won't just be individual patterns,
35//! but rather partially-deconstructed patterns in the form of a list of patterns. The paper
36//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
37//! new pattern `p`.
38//!
39//! For example, say we have the following:
40//!
41//! ```ignore
42//! // x: (Option<bool>, Result<()>)
43//! match x {
44//! (Some(true), _) => (),
45//! (None, Err(())) => (),
46//! (None, Err(_)) => (),
47//! }
48//! ```
49//!
50//! Here, the matrix `P` starts as:
51//!
52//! ```text
53//! [
54//! [(Some(true), _)],
55//! [(None, Err(()))],
56//! [(None, Err(_))],
57//! ]
58//! ```
59//!
60//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
61//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
62//! all the values it covers are already covered by row 2.
63//!
64//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of
65//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
66//! To match the paper, the top of the stack is at the beginning / on the left.
67//!
68//! There are two important operations on pattern-stacks necessary to understand the algorithm:
69//!
70//! 1. We can pop a given constructor off the top of a stack. This operation is called
71//! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
72//! `None`) and `p` a pattern-stack.
73//! If the pattern on top of the stack can cover `c`, this removes the constructor and
74//! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
75//! Otherwise the pattern-stack is discarded.
76//! This essentially filters those pattern-stacks whose top covers the constructor `c` and
77//! discards the others.
78//!
79//! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
80//! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
81//! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
82//! nothing back.
83//!
84//! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
85//! on top of the stack, and we have four cases:
86//!
87//! * 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We push onto
88//! the stack the arguments of this constructor, and return the result:
89//!
90//! r_1, .., r_a, p_2, .., p_n
91//!
92//! * 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and return
93//! nothing.
94//! * 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
95//! arguments (its arity), and return the resulting stack:
96//!
97//! _, .., _, p_2, .., p_n
98//!
99//! * 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
100//!
101//! S(c, (r_1, p_2, .., p_n))
102//! S(c, (r_2, p_2, .., p_n))
103//!
104//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
105//! a pattern-stack.
106//! This is used when we know there are missing constructor cases, but there might be
107//! existing wildcard patterns, so to check the usefulness of the matrix, we have to check
108//! all its *other* components.
109//!
110//! It is computed as follows. We look at the pattern `p_1` on top of the stack,
111//! and we have three cases:
112//! * 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
113//! * 1.2. `p_1 = _`. We return the rest of the stack:
114//!
115//! p_2, .., p_n
116//!
117//! * 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
118//!
119//! D((r_1, p_2, .., p_n))
120//! D((r_2, p_2, .., p_n))
121//!
122//! Note that the OR-patterns are not always used directly in Rust, but are used to derive the
123//! exhaustive integer matching rules, so they're written here for posterity.
124//!
125//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
126//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
127//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
128//!
129//!
130//! The algorithm for computing `U`
131//! -------------------------------
132//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
133//! That means we're going to check the components from left-to-right, so the algorithm
134//! operates principally on the first component of the matrix and new pattern-stack `p`.
135//! This algorithm is realized in the `is_useful` function.
136//!
137//! Base case (`n = 0`, i.e., an empty tuple pattern):
138//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), then
139//! `U(P, p)` is false.
140//! - Otherwise, `P` must be empty, so `U(P, p)` is true.
141//!
142//! Inductive step (`n > 0`, i.e., whether there's at least one column [which may then be expanded
143//! into further columns later]). We're going to match on the top of the new pattern-stack, `p_1`:
144//!
145//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
146//! Then, the usefulness of `p_1` can be reduced to whether it is useful when
147//! we ignore all the patterns in the first column of `P` that involve other constructors.
148//! This is where `S(c, P)` comes in:
149//!
150//! ```text
151//! U(P, p) := U(S(c, P), S(c, p))
152//! ```
153//!
154//! This special case is handled in `is_useful_specialized`.
155//!
156//! For example, if `P` is:
157//!
158//! ```text
159//! [
160//! [Some(true), _],
161//! [None, 0],
162//! ]
163//! ```
164//!
165//! and `p` is `[Some(false), 0]`, then we don't care about row 2 since we know `p` only
166//! matches values that row 2 doesn't. For row 1 however, we need to dig into the
167//! arguments of `Some` to know whether some new value is covered. So we compute
168//! `U([[true, _]], [false, 0])`.
169//!
170//! - If `p_1 == _`, then we look at the list of constructors that appear in the first component of
171//! the rows of `P`:
172//! - If there are some constructors that aren't present, then we might think that the
173//! wildcard `_` is useful, since it covers those constructors that weren't covered
174//! before.
175//! That's almost correct, but only works if there were no wildcards in those first
176//! components. So we need to check that `p` is useful with respect to the rows that
177//! start with a wildcard, if there are any. This is where `D` comes in:
178//! `U(P, p) := U(D(P), D(p))`
179//!
180//! For example, if `P` is:
181//! ```text
182//! [
183//! [_, true, _],
184//! [None, false, 1],
185//! ]
186//! ```
187//! and `p` is `[_, false, _]`, the `Some` constructor doesn't appear in `P`. So if we
188//! only had row 2, we'd know that `p` is useful. However row 1 starts with a
189//! wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
190//!
191//! - Otherwise, all possible constructors (for the relevant type) are present. In this
192//! case we must check whether the wildcard pattern covers any unmatched value. For
193//! that, we can think of the `_` pattern as a big OR-pattern that covers all
194//! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
195//! example. The wildcard pattern is useful in this case if it is useful when
196//! specialized to one of the possible constructors. So we compute:
197//! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
198//!
199//! For example, if `P` is:
200//! ```text
201//! [
202//! [Some(true), _],
203//! [None, false],
204//! ]
205//! ```
206//! and `p` is `[_, false]`, both `None` and `Some` constructors appear in the first
207//! components of `P`. We will therefore try popping both constructors in turn: we
208//! compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]],
209//! [false])` for the `None` constructor. The first case returns true, so we know that
210//! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
211//! before.
212//!
213//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
214//!
215//! ```text
216//! U(P, p) := U(P, (r_1, p_2, .., p_n))
217//! || U(P, (r_2, p_2, .., p_n))
218//! ```
219use std::{iter, sync::Arc};
220
221use hir_def::{
222 adt::VariantData,
223 body::Body,
224 expr::{Expr, Literal, Pat, PatId},
225 EnumVariantId, StructId, VariantId,
226};
227use la_arena::Idx; 13use la_arena::Idx;
228use smallvec::{smallvec, SmallVec};
229
230use crate::{db::HirDatabase, AdtId, InferenceResult, Interner, TyExt, TyKind};
231
232#[derive(Debug, Clone, Copy)]
233/// Either a pattern from the source code being analyzed, represented as
234/// as `PatId`, or a `Wild` pattern which is created as an intermediate
235/// step in the match checking algorithm and thus is not backed by a
236/// real `PatId`.
237///
238/// Note that it is totally valid for the `PatId` variant to contain
239/// a `PatId` which resolves to a `Wild` pattern, if that wild pattern
240/// exists in the source code being analyzed.
241enum PatIdOrWild {
242 PatId(PatId),
243 Wild,
244}
245 14
246impl PatIdOrWild { 15use crate::{db::HirDatabase, InferenceResult, Interner, Substitution, Ty, TyKind};
247 fn as_pat(self, cx: &MatchCheckCtx) -> Pat {
248 match self {
249 PatIdOrWild::PatId(id) => cx.body.pats[id].clone(),
250 PatIdOrWild::Wild => Pat::Wild,
251 }
252 }
253 16
254 fn as_id(self) -> Option<PatId> { 17use self::pat_util::EnumerateAndAdjustIterator;
255 match self {
256 PatIdOrWild::PatId(id) => Some(id),
257 PatIdOrWild::Wild => None,
258 }
259 }
260}
261 18
262impl From<PatId> for PatIdOrWild { 19pub(crate) use self::usefulness::MatchArm;
263 fn from(pat_id: PatId) -> Self {
264 Self::PatId(pat_id)
265 }
266}
267 20
268impl From<&PatId> for PatIdOrWild { 21pub(crate) type PatId = Idx<Pat>;
269 fn from(pat_id: &PatId) -> Self {
270 Self::PatId(*pat_id)
271 }
272}
273 22
274#[derive(Debug, Clone, Copy, PartialEq)] 23#[derive(Clone, Debug)]
275pub(super) enum MatchCheckErr { 24pub(crate) enum PatternError {
276 NotImplemented, 25 Unimplemented,
277 MalformedMatchArm, 26 UnresolvedVariant,
278 /// Used when type inference cannot resolve the type of 27 MissingField,
279 /// a pattern or expression. 28 ExtraFields,
280 Unknown,
281} 29}
282 30
283/// The return type of `is_useful` is either an indication of usefulness 31#[derive(Clone, Debug, PartialEq)]
284/// of the match arm, or an error in the case the match statement 32pub(crate) struct FieldPat {
285/// is made up of types for which exhaustiveness checking is currently 33 pub(crate) field: LocalFieldId,
286/// not completely implemented. 34 pub(crate) pattern: Pat,
287/// 35}
288/// The `std::result::Result` type is used here rather than a custom enum
289/// to allow the use of `?`.
290pub(super) type MatchCheckResult<T> = Result<T, MatchCheckErr>;
291 36
292#[derive(Debug)] 37#[derive(Clone, Debug, PartialEq)]
293/// A row in a Matrix. 38pub(crate) struct Pat {
294/// 39 pub(crate) ty: Ty,
295/// This type is modeled from the struct of the same name in `rustc`. 40 pub(crate) kind: Box<PatKind>,
296pub(super) struct PatStack(PatStackInner); 41}
297type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
298 42
299impl PatStack { 43impl Pat {
300 pub(super) fn from_pattern(pat_id: PatId) -> PatStack { 44 pub(crate) fn wildcard_from_ty(ty: Ty) -> Self {
301 Self(smallvec!(pat_id.into())) 45 Pat { ty, kind: Box::new(PatKind::Wild) }
302 } 46 }
47}
303 48
304 pub(super) fn from_wild() -> PatStack { 49/// Close relative to `rustc_mir_build::thir::pattern::PatKind`
305 Self(smallvec!(PatIdOrWild::Wild)) 50#[derive(Clone, Debug, PartialEq)]
306 } 51pub(crate) enum PatKind {
52 Wild,
307 53
308 fn from_slice(slice: &[PatIdOrWild]) -> PatStack { 54 /// `x`, `ref x`, `x @ P`, etc.
309 Self(SmallVec::from_slice(slice)) 55 Binding {
310 } 56 subpattern: Option<Pat>,
57 },
58
59 /// `Foo(...)` or `Foo{...}` or `Foo`, where `Foo` is a variant name from an ADT with
60 /// multiple variants.
61 Variant {
62 substs: Substitution,
63 enum_variant: EnumVariantId,
64 subpatterns: Vec<FieldPat>,
65 },
66
67 /// `(...)`, `Foo(...)`, `Foo{...}`, or `Foo`, where `Foo` is a variant name from an ADT with
68 /// a single variant.
69 Leaf {
70 subpatterns: Vec<FieldPat>,
71 },
72
73 /// `box P`, `&P`, `&mut P`, etc.
74 Deref {
75 subpattern: Pat,
76 },
77
78 // FIXME: for now, only bool literals are implemented
79 LiteralBool {
80 value: bool,
81 },
82
83 /// An or-pattern, e.g. `p | q`.
84 /// Invariant: `pats.len() >= 2`.
85 Or {
86 pats: Vec<Pat>,
87 },
88}
311 89
312 fn from_vec(v: PatStackInner) -> PatStack { 90pub(crate) struct PatCtxt<'a> {
313 Self(v) 91 db: &'a dyn HirDatabase,
314 } 92 infer: &'a InferenceResult,
93 body: &'a Body,
94 pub(crate) errors: Vec<PatternError>,
95}
315 96
316 fn get_head(&self) -> Option<PatIdOrWild> { 97impl<'a> PatCtxt<'a> {
317 self.0.first().copied() 98 pub(crate) fn new(db: &'a dyn HirDatabase, infer: &'a InferenceResult, body: &'a Body) -> Self {
99 Self { db, infer, body, errors: Vec::new() }
318 } 100 }
319 101
320 fn tail(&self) -> &[PatIdOrWild] { 102 pub(crate) fn lower_pattern(&mut self, pat: hir_def::expr::PatId) -> Pat {
321 self.0.get(1..).unwrap_or(&[]) 103 // FIXME: implement pattern adjustments (implicit pattern dereference; "RFC 2005-match-ergonomics")
104 // More info https://github.com/rust-lang/rust/issues/42640#issuecomment-313535089
105 let unadjusted_pat = self.lower_pattern_unadjusted(pat);
106 unadjusted_pat
322 } 107 }
323 108
324 fn to_tail(&self) -> PatStack { 109 fn lower_pattern_unadjusted(&mut self, pat: hir_def::expr::PatId) -> Pat {
325 Self::from_slice(self.tail()) 110 let mut ty = &self.infer[pat];
326 } 111 let variant = self.infer.variant_resolution_for_pat(pat);
327 112
328 fn replace_head_with<I, T>(&self, pats: I) -> PatStack 113 let kind = match self.body[pat] {
329 where 114 hir_def::expr::Pat::Wild => PatKind::Wild,
330 I: Iterator<Item = T>,
331 T: Into<PatIdOrWild>,
332 {
333 let mut patterns: PatStackInner = smallvec![];
334 for pat in pats {
335 patterns.push(pat.into());
336 }
337 for pat in &self.0[1..] {
338 patterns.push(*pat);
339 }
340 PatStack::from_vec(patterns)
341 }
342 115
343 /// Computes `D(self)`. 116 hir_def::expr::Pat::Lit(expr) => self.lower_lit(expr),
344 ///
345 /// See the module docs and the associated documentation in rustc for details.
346 fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
347 if matches!(self.get_head()?.as_pat(cx), Pat::Wild) {
348 Some(self.to_tail())
349 } else {
350 None
351 }
352 }
353 117
354 /// Computes `S(constructor, self)`. 118 hir_def::expr::Pat::Path(ref path) => {
355 /// 119 return self.lower_path(pat, path);
356 /// See the module docs and the associated documentation in rustc for details. 120 }
357 fn specialize_constructor(
358 &self,
359 cx: &MatchCheckCtx,
360 constructor: &Constructor,
361 ) -> MatchCheckResult<Option<PatStack>> {
362 let head = match self.get_head() {
363 Some(head) => head,
364 None => return Ok(None),
365 };
366 121
367 let head_pat = head.as_pat(cx); 122 hir_def::expr::Pat::Tuple { ref args, ellipsis } => {
368 let result = match (head_pat, constructor) { 123 let arity = match *ty.kind(&Interner) {
369 (Pat::Tuple { args: pat_ids, ellipsis }, &Constructor::Tuple { arity }) => { 124 TyKind::Tuple(arity, _) => arity,
370 if let Some(ellipsis) = ellipsis { 125 _ => panic!("unexpected type for tuple pattern: {:?}", ty),
371 let (pre, post) = pat_ids.split_at(ellipsis); 126 };
372 let n_wild_pats = arity.saturating_sub(pat_ids.len()); 127 let subpatterns = self.lower_tuple_subpats(args, arity, ellipsis);
373 let pre_iter = pre.iter().map(Into::into); 128 PatKind::Leaf { subpatterns }
374 let wildcards = iter::repeat(PatIdOrWild::Wild).take(n_wild_pats);
375 let post_iter = post.iter().map(Into::into);
376 Some(self.replace_head_with(pre_iter.chain(wildcards).chain(post_iter)))
377 } else {
378 Some(self.replace_head_with(pat_ids.iter()))
379 }
380 } 129 }
381 (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => { 130
382 match cx.body.exprs[lit_expr] { 131 hir_def::expr::Pat::Bind { subpat, .. } => {
383 Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => { 132 if let TyKind::Ref(.., rty) = ty.kind(&Interner) {
384 Some(self.to_tail()) 133 ty = rty;
385 }
386 // it was a bool but the value doesn't match
387 Expr::Literal(Literal::Bool(_)) => None,
388 // perhaps this is actually unreachable given we have
389 // already checked that these match arms have the appropriate type?
390 _ => return Err(MatchCheckErr::NotImplemented),
391 } 134 }
135 PatKind::Binding { subpattern: self.lower_opt_pattern(subpat) }
392 } 136 }
393 (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?), 137
394 (Pat::Path(_), constructor) => { 138 hir_def::expr::Pat::TupleStruct { ref args, ellipsis, .. } if variant.is_some() => {
395 // unit enum variants become `Pat::Path` 139 let expected_len = variant.unwrap().variant_data(self.db.upcast()).fields().len();
396 let pat_id = head.as_id().expect("we know this isn't a wild"); 140 let subpatterns = self.lower_tuple_subpats(args, expected_len, ellipsis);
397 let variant_id: VariantId = match constructor { 141 self.lower_variant_or_leaf(pat, ty, subpatterns)
398 &Constructor::Enum(e) => e.into(),
399 &Constructor::Struct(s) => s.into(),
400 _ => return Err(MatchCheckErr::NotImplemented),
401 };
402 if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) {
403 None
404 } else {
405 Some(self.to_tail())
406 }
407 } 142 }
408 (Pat::TupleStruct { args: ref pat_ids, ellipsis, .. }, constructor) => { 143
409 let pat_id = head.as_id().expect("we know this isn't a wild"); 144 hir_def::expr::Pat::Record { ref args, .. } if variant.is_some() => {
410 let variant_id: VariantId = match constructor { 145 let variant_data = variant.unwrap().variant_data(self.db.upcast());
411 &Constructor::Enum(e) => e.into(), 146 let subpatterns = args
412 &Constructor::Struct(s) => s.into(), 147 .iter()
413 _ => return Err(MatchCheckErr::MalformedMatchArm), 148 .map(|field| {
414 }; 149 // XXX(iDawer): field lookup is inefficient
415 if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) { 150 variant_data.field(&field.name).map(|lfield_id| FieldPat {
416 None 151 field: lfield_id,
417 } else { 152 pattern: self.lower_pattern(field.pat),
418 let constructor_arity = constructor.arity(cx)?; 153 })
419 if let Some(ellipsis_position) = ellipsis { 154 })
420 // If there are ellipsis in the pattern, the ellipsis must take the place 155 .collect();
421 // of at least one sub-pattern, so `pat_ids` should be smaller than the 156 match subpatterns {
422 // constructor arity. 157 Some(subpatterns) => self.lower_variant_or_leaf(pat, ty, subpatterns),
423 if pat_ids.len() < constructor_arity { 158 None => {
424 let mut new_patterns: Vec<PatIdOrWild> = vec![]; 159 self.errors.push(PatternError::MissingField);
425 160 PatKind::Wild
426 for pat_id in &pat_ids[0..ellipsis_position] {
427 new_patterns.push((*pat_id).into());
428 }
429
430 for _ in 0..(constructor_arity - pat_ids.len()) {
431 new_patterns.push(PatIdOrWild::Wild);
432 }
433
434 for pat_id in &pat_ids[ellipsis_position..pat_ids.len()] {
435 new_patterns.push((*pat_id).into());
436 }
437
438 Some(self.replace_head_with(new_patterns.into_iter()))
439 } else {
440 return Err(MatchCheckErr::MalformedMatchArm);
441 }
442 } else {
443 // If there is no ellipsis in the tuple pattern, the number
444 // of patterns must equal the constructor arity.
445 if pat_ids.len() == constructor_arity {
446 Some(self.replace_head_with(pat_ids.into_iter()))
447 } else {
448 return Err(MatchCheckErr::MalformedMatchArm);
449 }
450 } 161 }
451 } 162 }
452 } 163 }
453 (Pat::Record { args: ref arg_patterns, .. }, constructor) => { 164 hir_def::expr::Pat::TupleStruct { .. } | hir_def::expr::Pat::Record { .. } => {
454 let pat_id = head.as_id().expect("we know this isn't a wild"); 165 self.errors.push(PatternError::UnresolvedVariant);
455 let (variant_id, variant_data) = match constructor { 166 PatKind::Wild
456 &Constructor::Enum(e) => (
457 e.into(),
458 cx.db.enum_data(e.parent).variants[e.local_id].variant_data.clone(),
459 ),
460 &Constructor::Struct(s) => {
461 (s.into(), cx.db.struct_data(s).variant_data.clone())
462 }
463 _ => return Err(MatchCheckErr::MalformedMatchArm),
464 };
465 if Some(variant_id) != cx.infer.variant_resolution_for_pat(pat_id) {
466 None
467 } else {
468 match variant_data.as_ref() {
469 VariantData::Record(struct_field_arena) => {
470 // Here we treat any missing fields in the record as the wild pattern, as
471 // if the record has ellipsis. We want to do this here even if the
472 // record does not contain ellipsis, because it allows us to continue
473 // enforcing exhaustiveness for the rest of the match statement.
474 //
475 // Creating the diagnostic for the missing field in the pattern
476 // should be done in a different diagnostic.
477 let patterns = struct_field_arena.iter().map(|(_, struct_field)| {
478 arg_patterns
479 .iter()
480 .find(|pat| pat.name == struct_field.name)
481 .map(|pat| PatIdOrWild::from(pat.pat))
482 .unwrap_or(PatIdOrWild::Wild)
483 });
484
485 Some(self.replace_head_with(patterns))
486 }
487 _ => return Err(MatchCheckErr::Unknown),
488 }
489 }
490 } 167 }
491 (Pat::Or(_), _) => return Err(MatchCheckErr::NotImplemented),
492 (_, _) => return Err(MatchCheckErr::NotImplemented),
493 };
494 168
495 Ok(result) 169 hir_def::expr::Pat::Or(ref pats) => PatKind::Or { pats: self.lower_patterns(pats) },
496 }
497
498 /// A special case of `specialize_constructor` where the head of the pattern stack
499 /// is a Wild pattern.
500 ///
501 /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns
502 /// (N >= 0), where N is the arity of the given constructor.
503 fn expand_wildcard(
504 &self,
505 cx: &MatchCheckCtx,
506 constructor: &Constructor,
507 ) -> MatchCheckResult<PatStack> {
508 assert_eq!(
509 Pat::Wild,
510 self.get_head().expect("expand_wildcard called on empty PatStack").as_pat(cx),
511 "expand_wildcard must only be called on PatStack with wild at head",
512 );
513 170
514 let mut patterns: PatStackInner = smallvec![]; 171 _ => {
172 self.errors.push(PatternError::Unimplemented);
173 PatKind::Wild
174 }
175 };
515 176
516 for _ in 0..constructor.arity(cx)? { 177 Pat { ty: ty.clone(), kind: Box::new(kind) }
517 patterns.push(PatIdOrWild::Wild); 178 }
518 }
519 179
520 for pat in &self.0[1..] { 180 fn lower_tuple_subpats(
521 patterns.push(*pat); 181 &mut self,
182 pats: &[hir_def::expr::PatId],
183 expected_len: usize,
184 ellipsis: Option<usize>,
185 ) -> Vec<FieldPat> {
186 if pats.len() > expected_len {
187 self.errors.push(PatternError::ExtraFields);
188 return Vec::new();
522 } 189 }
523 190
524 Ok(PatStack::from_vec(patterns)) 191 pats.iter()
192 .enumerate_and_adjust(expected_len, ellipsis)
193 .map(|(i, &subpattern)| FieldPat {
194 field: LocalFieldId::from_raw((i as u32).into()),
195 pattern: self.lower_pattern(subpattern),
196 })
197 .collect()
525 } 198 }
526}
527 199
528/// A collection of PatStack. 200 fn lower_patterns(&mut self, pats: &[hir_def::expr::PatId]) -> Vec<Pat> {
529/// 201 pats.iter().map(|&p| self.lower_pattern(p)).collect()
530/// This type is modeled from the struct of the same name in `rustc`. 202 }
531pub(super) struct Matrix(Vec<PatStack>);
532 203
533impl Matrix { 204 fn lower_opt_pattern(&mut self, pat: Option<hir_def::expr::PatId>) -> Option<Pat> {
534 pub(super) fn empty() -> Self { 205 pat.map(|p| self.lower_pattern(p))
535 Self(vec![])
536 } 206 }
537 207
538 pub(super) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) { 208 fn lower_variant_or_leaf(
539 if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) { 209 &mut self,
540 // Or patterns are expanded here 210 pat: hir_def::expr::PatId,
541 for pat_id in pat_ids { 211 ty: &Ty,
542 self.0.push(row.replace_head_with([pat_id].iter())); 212 subpatterns: Vec<FieldPat>,
213 ) -> PatKind {
214 let kind = match self.infer.variant_resolution_for_pat(pat) {
215 Some(variant_id) => {
216 if let VariantId::EnumVariantId(enum_variant) = variant_id {
217 let substs = match ty.kind(&Interner) {
218 TyKind::Adt(_, substs) | TyKind::FnDef(_, substs) => substs.clone(),
219 TyKind::Error => {
220 return PatKind::Wild;
221 }
222 _ => panic!("inappropriate type for def: {:?}", ty),
223 };
224 PatKind::Variant { substs, enum_variant, subpatterns }
225 } else {
226 PatKind::Leaf { subpatterns }
227 }
543 } 228 }
544 } else { 229 None => {
545 self.0.push(row); 230 self.errors.push(PatternError::UnresolvedVariant);
546 } 231 PatKind::Wild
232 }
233 };
234 kind
547 } 235 }
548 236
549 fn is_empty(&self) -> bool { 237 fn lower_path(&mut self, pat: hir_def::expr::PatId, _path: &hir_def::path::Path) -> Pat {
550 self.0.is_empty() 238 let ty = &self.infer[pat];
551 }
552 239
553 fn heads(&self) -> Vec<PatIdOrWild> { 240 let pat_from_kind = |kind| Pat { ty: ty.clone(), kind: Box::new(kind) };
554 self.0.iter().flat_map(|p| p.get_head()).collect()
555 }
556 241
557 /// Computes `D(self)` for each contained PatStack. 242 match self.infer.variant_resolution_for_pat(pat) {
558 /// 243 Some(_) => pat_from_kind(self.lower_variant_or_leaf(pat, ty, Vec::new())),
559 /// See the module docs and the associated documentation in rustc for details. 244 None => {
560 fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self { 245 self.errors.push(PatternError::UnresolvedVariant);
561 Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx))) 246 pat_from_kind(PatKind::Wild)
247 }
248 }
562 } 249 }
563 250
564 /// Computes `S(constructor, self)` for each contained PatStack. 251 fn lower_lit(&mut self, expr: hir_def::expr::ExprId) -> PatKind {
565 /// 252 use hir_def::expr::{Expr, Literal::Bool};
566 /// See the module docs and the associated documentation in rustc for details. 253
567 fn specialize_constructor( 254 match self.body[expr] {
568 &self, 255 Expr::Literal(Bool(value)) => PatKind::LiteralBool { value },
569 cx: &MatchCheckCtx, 256 _ => {
570 constructor: &Constructor, 257 self.errors.push(PatternError::Unimplemented);
571 ) -> MatchCheckResult<Self> { 258 PatKind::Wild
572 let mut new_matrix = Matrix::empty();
573 for pat in &self.0 {
574 if let Some(pat) = pat.specialize_constructor(cx, constructor)? {
575 new_matrix.push(cx, pat);
576 } 259 }
577 } 260 }
261 }
262}
578 263
579 Ok(new_matrix) 264pub(crate) trait PatternFoldable: Sized {
265 fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
266 self.super_fold_with(folder)
580 } 267 }
581 268
582 fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self { 269 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self;
583 let mut matrix = Matrix::empty(); 270}
584 271
585 for pat in iter { 272pub(crate) trait PatternFolder: Sized {
586 // using push ensures we expand or-patterns 273 fn fold_pattern(&mut self, pattern: &Pat) -> Pat {
587 matrix.push(cx, pat); 274 pattern.super_fold_with(self)
588 } 275 }
589 276
590 matrix 277 fn fold_pattern_kind(&mut self, kind: &PatKind) -> PatKind {
278 kind.super_fold_with(self)
591 } 279 }
592} 280}
593 281
594#[derive(Clone, Debug, PartialEq)] 282impl<T: PatternFoldable> PatternFoldable for Box<T> {
595/// An indication of the usefulness of a given match arm, where 283 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
596/// usefulness is defined as matching some patterns which were 284 let content: T = (**self).fold_with(folder);
597/// not matched by an prior match arms. 285 Box::new(content)
598/// 286 }
599/// We may eventually need an `Unknown` variant here.
600pub(super) enum Usefulness {
601 Useful,
602 NotUseful,
603} 287}
604 288
605pub(super) struct MatchCheckCtx<'a> { 289impl<T: PatternFoldable> PatternFoldable for Vec<T> {
606 pub(super) match_expr: Idx<Expr>, 290 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
607 pub(super) body: Arc<Body>, 291 self.iter().map(|t| t.fold_with(folder)).collect()
608 pub(super) infer: Arc<InferenceResult>, 292 }
609 pub(super) db: &'a dyn HirDatabase,
610} 293}
611 294
612/// Given a set of patterns `matrix`, and pattern to consider `v`, determines 295impl<T: PatternFoldable> PatternFoldable for Option<T> {
613/// whether `v` is useful. A pattern is useful if it covers cases which were 296 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
614/// not previously covered. 297 self.as_ref().map(|t| t.fold_with(folder))
615///
616/// When calling this function externally (that is, not the recursive calls) it
617/// expected that you have already type checked the match arms. All patterns in
618/// matrix should be the same type as v, as well as they should all be the same
619/// type as the match expression.
620pub(super) fn is_useful(
621 cx: &MatchCheckCtx,
622 matrix: &Matrix,
623 v: &PatStack,
624) -> MatchCheckResult<Usefulness> {
625 // Handle two special cases:
626 // - enum with no variants
627 // - `!` type
628 // In those cases, no match arm is useful.
629 match cx.infer[cx.match_expr].strip_references().kind(&Interner) {
630 TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ..) => {
631 if cx.db.enum_data(*enum_id).variants.is_empty() {
632 return Ok(Usefulness::NotUseful);
633 }
634 }
635 TyKind::Never => return Ok(Usefulness::NotUseful),
636 _ => (),
637 } 298 }
299}
638 300
639 let head = match v.get_head() { 301macro_rules! clone_impls {
640 Some(head) => head, 302 ($($ty:ty),+) => {
641 None => { 303 $(
642 let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful }; 304 impl PatternFoldable for $ty {
643 305 fn super_fold_with<F: PatternFolder>(&self, _: &mut F) -> Self {
644 return Ok(result); 306 Clone::clone(self)
645 }
646 };
647
648 if let Pat::Or(pat_ids) = head.as_pat(cx) {
649 let mut found_unimplemented = false;
650 let any_useful = pat_ids.iter().any(|&pat_id| {
651 let v = PatStack::from_pattern(pat_id);
652
653 match is_useful(cx, matrix, &v) {
654 Ok(Usefulness::Useful) => true,
655 Ok(Usefulness::NotUseful) => false,
656 _ => {
657 found_unimplemented = true;
658 false
659 } 307 }
660 } 308 }
661 }); 309 )+
662
663 return if any_useful {
664 Ok(Usefulness::Useful)
665 } else if found_unimplemented {
666 Err(MatchCheckErr::NotImplemented)
667 } else {
668 Ok(Usefulness::NotUseful)
669 };
670 } 310 }
311}
671 312
672 if let Some(constructor) = pat_constructor(cx, head)? { 313clone_impls! { LocalFieldId, Ty, Substitution, EnumVariantId }
673 let matrix = matrix.specialize_constructor(&cx, &constructor)?;
674 let v = v
675 .specialize_constructor(&cx, &constructor)?
676 .expect("we know this can't fail because we get the constructor from `v.head()` above");
677
678 is_useful(&cx, &matrix, &v)
679 } else {
680 // expanding wildcard
681 let mut used_constructors: Vec<Constructor> = vec![];
682 for pat in matrix.heads() {
683 if let Some(constructor) = pat_constructor(cx, pat)? {
684 used_constructors.push(constructor);
685 }
686 }
687
688 // We assume here that the first constructor is the "correct" type. Since we
689 // only care about the "type" of the constructor (i.e. if it is a bool we
690 // don't care about the value), this assumption should be valid as long as
691 // the match statement is well formed. We currently uphold this invariant by
692 // filtering match arms before calling `is_useful`, only passing in match arms
693 // whose type matches the type of the match expression.
694 match &used_constructors.first() {
695 Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
696 // If all constructors are covered, then we need to consider whether
697 // any values are covered by this wildcard.
698 //
699 // For example, with matrix '[[Some(true)], [None]]', all
700 // constructors are covered (`Some`/`None`), so we need
701 // to perform specialization to see that our wildcard will cover
702 // the `Some(false)` case.
703 //
704 // Here we create a constructor for each variant and then check
705 // usefulness after specializing for that constructor.
706 let mut found_unimplemented = false;
707 for constructor in constructor.all_constructors(cx) {
708 let matrix = matrix.specialize_constructor(&cx, &constructor)?;
709 let v = v.expand_wildcard(&cx, &constructor)?;
710
711 match is_useful(&cx, &matrix, &v) {
712 Ok(Usefulness::Useful) => return Ok(Usefulness::Useful),
713 Ok(Usefulness::NotUseful) => continue,
714 _ => found_unimplemented = true,
715 };
716 }
717 314
718 if found_unimplemented { 315impl PatternFoldable for FieldPat {
719 Err(MatchCheckErr::NotImplemented) 316 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
720 } else { 317 FieldPat { field: self.field.fold_with(folder), pattern: self.pattern.fold_with(folder) }
721 Ok(Usefulness::NotUseful)
722 }
723 }
724 _ => {
725 // Either not all constructors are covered, or the only other arms
726 // are wildcards. Either way, this pattern is useful if it is useful
727 // when compared to those arms with wildcards.
728 let matrix = matrix.specialize_wildcard(&cx);
729 let v = v.to_tail();
730
731 is_useful(&cx, &matrix, &v)
732 }
733 }
734 } 318 }
735} 319}
736 320
737#[derive(Debug, Clone, Copy)] 321impl PatternFoldable for Pat {
738/// Similar to TypeCtor, but includes additional information about the specific 322 fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
739/// value being instantiated. For example, TypeCtor::Bool doesn't contain the 323 folder.fold_pattern(self)
740/// boolean value. 324 }
741enum Constructor {
742 Bool(bool),
743 Tuple { arity: usize },
744 Enum(EnumVariantId),
745 Struct(StructId),
746}
747 325
748impl Constructor { 326 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
749 fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> { 327 Pat { ty: self.ty.fold_with(folder), kind: self.kind.fold_with(folder) }
750 let arity = match self { 328 }
751 Constructor::Bool(_) => 0, 329}
752 Constructor::Tuple { arity } => *arity,
753 Constructor::Enum(e) => {
754 match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
755 VariantData::Tuple(struct_field_data) => struct_field_data.len(),
756 VariantData::Record(struct_field_data) => struct_field_data.len(),
757 VariantData::Unit => 0,
758 }
759 }
760 &Constructor::Struct(s) => match cx.db.struct_data(s).variant_data.as_ref() {
761 VariantData::Tuple(struct_field_data) => struct_field_data.len(),
762 VariantData::Record(struct_field_data) => struct_field_data.len(),
763 VariantData::Unit => 0,
764 },
765 };
766 330
767 Ok(arity) 331impl PatternFoldable for PatKind {
332 fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
333 folder.fold_pattern_kind(self)
768 } 334 }
769 335
770 fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> { 336 fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
771 match self { 337 match self {
772 Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)], 338 PatKind::Wild => PatKind::Wild,
773 Constructor::Tuple { .. } | Constructor::Struct(_) => vec![*self], 339 PatKind::Binding { subpattern } => {
774 Constructor::Enum(e) => cx 340 PatKind::Binding { subpattern: subpattern.fold_with(folder) }
775 .db 341 }
776 .enum_data(e.parent) 342 PatKind::Variant { substs, enum_variant, subpatterns } => PatKind::Variant {
777 .variants 343 substs: substs.fold_with(folder),
778 .iter() 344 enum_variant: enum_variant.fold_with(folder),
779 .map(|(local_id, _)| { 345 subpatterns: subpatterns.fold_with(folder),
780 Constructor::Enum(EnumVariantId { parent: e.parent, local_id }) 346 },
781 }) 347 PatKind::Leaf { subpatterns } => {
782 .collect(), 348 PatKind::Leaf { subpatterns: subpatterns.fold_with(folder) }
349 }
350 PatKind::Deref { subpattern } => {
351 PatKind::Deref { subpattern: subpattern.fold_with(folder) }
352 }
353 &PatKind::LiteralBool { value } => PatKind::LiteralBool { value },
354 PatKind::Or { pats } => PatKind::Or { pats: pats.fold_with(folder) },
783 } 355 }
784 } 356 }
785} 357}
786 358
787/// Returns the constructor for the given pattern. Should only return None 359#[cfg(test)]
788/// in the case of a Wild pattern. 360pub(super) mod tests {
789fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> { 361 mod report {
790 let res = match pat.as_pat(cx) { 362 use std::any::Any;
791 Pat::Wild => None,
792 Pat::Tuple { .. } => {
793 let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
794 Some(Constructor::Tuple {
795 arity: cx.infer.type_of_pat[pat_id]
796 .as_tuple()
797 .ok_or(MatchCheckErr::Unknown)?
798 .len(&Interner),
799 })
800 }
801 Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
802 Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
803 _ => return Err(MatchCheckErr::NotImplemented),
804 },
805 Pat::TupleStruct { .. } | Pat::Path(_) | Pat::Record { .. } => {
806 let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
807 let variant_id =
808 cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckErr::Unknown)?;
809 match variant_id {
810 VariantId::EnumVariantId(enum_variant_id) => {
811 Some(Constructor::Enum(enum_variant_id))
812 }
813 VariantId::StructId(struct_id) => Some(Constructor::Struct(struct_id)),
814 _ => return Err(MatchCheckErr::NotImplemented),
815 }
816 }
817 _ => return Err(MatchCheckErr::NotImplemented),
818 };
819 363
820 Ok(res) 364 use hir_def::{expr::PatId, DefWithBodyId};
821} 365 use hir_expand::{HirFileId, InFile};
366 use syntax::SyntaxNodePtr;
822 367
823fn all_constructors_covered( 368 use crate::{
824 cx: &MatchCheckCtx, 369 db::HirDatabase,
825 constructor: &Constructor, 370 diagnostics_sink::{Diagnostic, DiagnosticCode, DiagnosticSink},
826 used_constructors: &[Constructor], 371 };
827) -> bool {
828 match constructor {
829 Constructor::Tuple { arity } => {
830 used_constructors.iter().any(|constructor| match constructor {
831 Constructor::Tuple { arity: used_arity } => arity == used_arity,
832 _ => false,
833 })
834 }
835 Constructor::Bool(_) => {
836 if used_constructors.is_empty() {
837 return false;
838 }
839 372
840 let covers_true = 373 /// In tests, match check bails out loudly.
841 used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true))); 374 /// This helps to catch incorrect tests that pass due to false negatives.
842 let covers_false = 375 pub(crate) fn report_bail_out(
843 used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false))); 376 db: &dyn HirDatabase,
377 def: DefWithBodyId,
378 pat: PatId,
379 sink: &mut DiagnosticSink,
380 ) {
381 let (_, source_map) = db.body_with_source_map(def);
382 if let Ok(source_ptr) = source_map.pat_syntax(pat) {
383 let pat_syntax_ptr = source_ptr.value.either(Into::into, Into::into);
384 sink.push(BailedOut { file: source_ptr.file_id, pat_syntax_ptr });
385 }
386 }
844 387
845 covers_true && covers_false 388 #[derive(Debug)]
389 struct BailedOut {
390 file: HirFileId,
391 pat_syntax_ptr: SyntaxNodePtr,
846 } 392 }
847 Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
848 for constructor in used_constructors {
849 if let Constructor::Enum(e) = constructor {
850 if id == e.local_id {
851 return true;
852 }
853 }
854 }
855 393
856 false 394 impl Diagnostic for BailedOut {
857 }), 395 fn code(&self) -> DiagnosticCode {
858 &Constructor::Struct(s) => used_constructors.iter().any(|constructor| match constructor { 396 DiagnosticCode("internal:match-check-bailed-out")
859 &Constructor::Struct(sid) => sid == s, 397 }
860 _ => false, 398 fn message(&self) -> String {
861 }), 399 format!("Internal: match check bailed out")
400 }
401 fn display_source(&self) -> InFile<SyntaxNodePtr> {
402 InFile { file_id: self.file, value: self.pat_syntax_ptr.clone() }
403 }
404 fn as_any(&self) -> &(dyn Any + Send + 'static) {
405 self
406 }
407 }
862 } 408 }
863}
864 409
865#[cfg(test)]
866mod tests {
867 use crate::diagnostics::tests::check_diagnostics; 410 use crate::diagnostics::tests::check_diagnostics;
868 411
412 pub(crate) use self::report::report_bail_out;
413
869 #[test] 414 #[test]
870 fn empty_tuple() { 415 fn empty_tuple() {
871 check_diagnostics( 416 check_diagnostics(
@@ -1113,14 +658,18 @@ enum Either2 { C, D }
1113fn main() { 658fn main() {
1114 match Either::A { 659 match Either::A {
1115 Either2::C => (), 660 Either2::C => (),
661 // ^^^^^^^^^^ Internal: match check bailed out
1116 Either2::D => (), 662 Either2::D => (),
1117 } 663 }
1118 match (true, false) { 664 match (true, false) {
1119 (true, false, true) => (), 665 (true, false, true) => (),
666 // ^^^^^^^^^^^^^^^^^^^ Internal: match check bailed out
1120 (true) => (), 667 (true) => (),
1121 } 668 }
1122 match (true, false) { (true,) => {} } 669 match (true, false) { (true,) => {} }
670 // ^^^^^^^ Internal: match check bailed out
1123 match (0) { () => () } 671 match (0) { () => () }
672 // ^^ Internal: match check bailed out
1124 match Unresolved::Bar { Unresolved::Baz => () } 673 match Unresolved::Bar { Unresolved::Baz => () }
1125} 674}
1126 "#, 675 "#,
@@ -1133,7 +682,9 @@ fn main() {
1133 r#" 682 r#"
1134fn main() { 683fn main() {
1135 match false { true | () => {} } 684 match false { true | () => {} }
685 // ^^^^^^^^^ Internal: match check bailed out
1136 match (false,) { (true | (),) => {} } 686 match (false,) { (true | (),) => {} }
687 // ^^^^^^^^^^^^ Internal: match check bailed out
1137} 688}
1138"#, 689"#,
1139 ); 690 );
@@ -1158,6 +709,25 @@ fn main() {
1158 } 709 }
1159 710
1160 #[test] 711 #[test]
712 fn malformed_match_arm_extra_fields() {
713 check_diagnostics(
714 r#"
715enum A { B(isize, isize), C }
716fn main() {
717 match A::B(1, 2) {
718 A::B(_, _, _) => (),
719 // ^^^^^^^^^^^^^ Internal: match check bailed out
720 }
721 match A::B(1, 2) {
722 A::C(_) => (),
723 // ^^^^^^^ Internal: match check bailed out
724 }
725}
726"#,
727 );
728 }
729
730 #[test]
1161 fn expr_diverges() { 731 fn expr_diverges() {
1162 check_diagnostics( 732 check_diagnostics(
1163 r#" 733 r#"
@@ -1166,10 +736,12 @@ enum Either { A, B }
1166fn main() { 736fn main() {
1167 match loop {} { 737 match loop {} {
1168 Either::A => (), 738 Either::A => (),
739 // ^^^^^^^^^ Internal: match check bailed out
1169 Either::B => (), 740 Either::B => (),
1170 } 741 }
1171 match loop {} { 742 match loop {} {
1172 Either::A => (), 743 Either::A => (),
744 // ^^^^^^^^^ Internal: match check bailed out
1173 } 745 }
1174 match loop { break Foo::A } { 746 match loop { break Foo::A } {
1175 //^^^^^^^^^^^^^^^^^^^^^ Missing match arm 747 //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
@@ -1357,6 +929,7 @@ fn enum_(never: Never) {
1357} 929}
1358fn enum_ref(never: &Never) { 930fn enum_ref(never: &Never) {
1359 match never {} 931 match never {}
932 //^^^^^ Missing match arm
1360} 933}
1361fn bang(never: !) { 934fn bang(never: !) {
1362 match never {} 935 match never {}
@@ -1376,6 +949,11 @@ fn main() {
1376 match Option::<Never>::None { 949 match Option::<Never>::None {
1377 None => (), 950 None => (),
1378 Some(never) => match never {}, 951 Some(never) => match never {},
952 // ^^^^^^^^^^^ Internal: match check bailed out
953 }
954 match Option::<Never>::None {
955 //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
956 Option::Some(_never) => {},
1379 } 957 }
1380} 958}
1381"#, 959"#,
@@ -1513,6 +1091,151 @@ fn main() {
1513"#, 1091"#,
1514 ); 1092 );
1515 } 1093 }
1094
1095 #[test]
1096 fn no_panic_at_unimplemented_subpattern_type() {
1097 check_diagnostics(
1098 r#"
1099struct S { a: char}
1100fn main(v: S) {
1101 match v { S{ a } => {} }
1102 match v { S{ a: _x } => {} }
1103 match v { S{ a: 'a' } => {} }
1104 //^^^^^^^^^^^ Internal: match check bailed out
1105 match v { S{..} => {} }
1106 match v { _ => {} }
1107 match v { }
1108 //^ Missing match arm
1109}
1110"#,
1111 );
1112 }
1113
1114 #[test]
1115 fn binding() {
1116 check_diagnostics(
1117 r#"
1118fn main() {
1119 match true {
1120 _x @ true => {}
1121 false => {}
1122 }
1123 match true { _x @ true => {} }
1124 //^^^^ Missing match arm
1125}
1126"#,
1127 );
1128 }
1129
1130 #[test]
1131 fn binding_ref_has_correct_type() {
1132 // Asserts `PatKind::Binding(ref _x): bool`, not &bool.
1133 // If that's not true match checking will panic with "incompatible constructors"
1134 // FIXME: make facilities to test this directly like `tests::check_infer(..)`
1135 check_diagnostics(
1136 r#"
1137enum Foo { A }
1138fn main() {
1139 // FIXME: this should not bail out but current behavior is such as the old algorithm.
1140 // ExprValidator::validate_match(..) checks types of top level patterns incorrecly.
1141 match Foo::A {
1142 ref _x => {}
1143 // ^^^^^^ Internal: match check bailed out
1144 Foo::A => {}
1145 }
1146 match (true,) {
1147 (ref _x,) => {}
1148 (true,) => {}
1149 }
1150}
1151"#,
1152 );
1153 }
1154
1155 #[test]
1156 fn enum_non_exhaustive() {
1157 check_diagnostics(
1158 r#"
1159//- /lib.rs crate:lib
1160#[non_exhaustive]
1161pub enum E { A, B }
1162fn _local() {
1163 match E::A { _ => {} }
1164 match E::A {
1165 E::A => {}
1166 E::B => {}
1167 }
1168 match E::A {
1169 E::A | E::B => {}
1170 }
1171}
1172
1173//- /main.rs crate:main deps:lib
1174use lib::E;
1175fn main() {
1176 match E::A { _ => {} }
1177 match E::A {
1178 //^^^^ Missing match arm
1179 E::A => {}
1180 E::B => {}
1181 }
1182 match E::A {
1183 //^^^^ Missing match arm
1184 E::A | E::B => {}
1185 }
1186}
1187"#,
1188 );
1189 }
1190
1191 #[test]
1192 fn match_guard() {
1193 check_diagnostics(
1194 r#"
1195fn main() {
1196 match true {
1197 true if false => {}
1198 true => {}
1199 false => {}
1200 }
1201 match true {
1202 //^^^^ Missing match arm
1203 true if false => {}
1204 false => {}
1205}
1206"#,
1207 );
1208 }
1209
1210 #[test]
1211 fn pattern_type_is_of_substitution() {
1212 cov_mark::check!(match_check_wildcard_expanded_to_substitutions);
1213 check_diagnostics(
1214 r#"
1215struct Foo<T>(T);
1216struct Bar;
1217fn main() {
1218 match Foo(Bar) {
1219 _ | Foo(Bar) => {}
1220 }
1221}
1222"#,
1223 );
1224 }
1225
1226 #[test]
1227 fn record_struct_no_such_field() {
1228 check_diagnostics(
1229 r#"
1230struct Foo { }
1231fn main(f: Foo) {
1232 match f { Foo { bar } => () }
1233 // ^^^^^^^^^^^ Internal: match check bailed out
1234}
1235"#,
1236 );
1237 }
1238
1516 mod false_negatives { 1239 mod false_negatives {
1517 //! The implementation of match checking here is a work in progress. As we roll this out, we 1240 //! The implementation of match checking here is a work in progress. As we roll this out, we
1518 //! prefer false negatives to false positives (ideally there would be no false positives). This 1241 //! prefer false negatives to false positives (ideally there would be no false positives). This
@@ -1533,11 +1256,44 @@ fn main() {
1533fn main() { 1256fn main() {
1534 match 5 { 1257 match 5 {
1535 10 => (), 1258 10 => (),
1259 // ^^ Internal: match check bailed out
1536 11..20 => (), 1260 11..20 => (),
1537 } 1261 }
1538} 1262}
1539"#, 1263"#,
1540 ); 1264 );
1541 } 1265 }
1266
1267 #[test]
1268 fn reference_patterns_at_top_level() {
1269 check_diagnostics(
1270 r#"
1271fn main() {
1272 match &false {
1273 &true => {}
1274 // ^^^^^ Internal: match check bailed out
1275 }
1276}
1277 "#,
1278 );
1279 }
1280
1281 #[test]
1282 fn reference_patterns_in_fields() {
1283 check_diagnostics(
1284 r#"
1285fn main() {
1286 match (&false,) {
1287 (true,) => {}
1288 // ^^^^^^^ Internal: match check bailed out
1289 }
1290 match (&false,) {
1291 (&true,) => {}
1292 // ^^^^^^^^ Internal: match check bailed out
1293 }
1294}
1295 "#,
1296 );
1297 }
1542 } 1298 }
1543} 1299}
diff --git a/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
new file mode 100644
index 000000000..1f4219b42
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/deconstruct_pat.rs
@@ -0,0 +1,907 @@
1//! [`super::usefulness`] explains most of what is happening in this file. As explained there,
2//! values and patterns are made from constructors applied to fields. This file defines a
3//! `Constructor` enum, a `Fields` struct, and various operations to manipulate them and convert
4//! them from/to patterns.
5//!
6//! There's one idea that is not detailed in [`super::usefulness`] because the details are not
7//! needed there: _constructor splitting_.
8//!
9//! # Constructor splitting
10//!
11//! The idea is as follows: given a constructor `c` and a matrix, we want to specialize in turn
12//! with all the value constructors that are covered by `c`, and compute usefulness for each.
13//! Instead of listing all those constructors (which is intractable), we group those value
14//! constructors together as much as possible. Example:
15//!
16//! ```
17//! match (0, false) {
18//! (0 ..=100, true) => {} // `p_1`
19//! (50..=150, false) => {} // `p_2`
20//! (0 ..=200, _) => {} // `q`
21//! }
22//! ```
23//!
24//! The naive approach would try all numbers in the range `0..=200`. But we can be a lot more
25//! clever: `0` and `1` for example will match the exact same rows, and return equivalent
26//! witnesses. In fact all of `0..50` would. We can thus restrict our exploration to 4
27//! constructors: `0..50`, `50..=100`, `101..=150` and `151..=200`. That is enough and infinitely
28//! more tractable.
29//!
30//! We capture this idea in a function `split(p_1 ... p_n, c)` which returns a list of constructors
31//! `c'` covered by `c`. Given such a `c'`, we require that all value ctors `c''` covered by `c'`
32//! return an equivalent set of witnesses after specializing and computing usefulness.
33//! In the example above, witnesses for specializing by `c''` covered by `0..50` will only differ
34//! in their first element.
35//!
36//! We usually also ask that the `c'` together cover all of the original `c`. However we allow
37//! skipping some constructors as long as it doesn't change whether the resulting list of witnesses
38//! is empty of not. We use this in the wildcard `_` case.
39//!
40//! Splitting is implemented in the [`Constructor::split`] function. We don't do splitting for
41//! or-patterns; instead we just try the alternatives one-by-one. For details on splitting
42//! wildcards, see [`SplitWildcard`]; for integer ranges, see [`SplitIntRange`]; for slices, see
43//! [`SplitVarLenSlice`].
44
45use std::{
46 cmp::{max, min},
47 iter::once,
48 ops::RangeInclusive,
49};
50
51use hir_def::{EnumVariantId, HasModule, LocalFieldId, VariantId};
52use smallvec::{smallvec, SmallVec};
53
54use crate::{AdtId, Interner, Scalar, Ty, TyExt, TyKind};
55
56use super::{
57 usefulness::{MatchCheckCtx, PatCtxt},
58 FieldPat, Pat, PatId, PatKind,
59};
60
61use self::Constructor::*;
62
63/// [Constructor] uses this in umimplemented variants.
64/// It allows porting match expressions from upstream algorithm without losing semantics.
65#[derive(Copy, Clone, Debug, PartialEq, Eq)]
66pub(super) enum Void {}
67
68/// An inclusive interval, used for precise integer exhaustiveness checking.
69/// `IntRange`s always store a contiguous range. This means that values are
70/// encoded such that `0` encodes the minimum value for the integer,
71/// regardless of the signedness.
72/// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
73/// This makes comparisons and arithmetic on interval endpoints much more
74/// straightforward. See `signed_bias` for details.
75///
76/// `IntRange` is never used to encode an empty range or a "range" that wraps
77/// around the (offset) space: i.e., `range.lo <= range.hi`.
78#[derive(Clone, Debug, PartialEq, Eq)]
79pub(super) struct IntRange {
80 range: RangeInclusive<u128>,
81}
82
83impl IntRange {
84 #[inline]
85 fn is_integral(ty: &Ty) -> bool {
86 match ty.kind(&Interner) {
87 TyKind::Scalar(Scalar::Char)
88 | TyKind::Scalar(Scalar::Int(_))
89 | TyKind::Scalar(Scalar::Uint(_))
90 | TyKind::Scalar(Scalar::Bool) => true,
91 _ => false,
92 }
93 }
94
95 fn is_singleton(&self) -> bool {
96 self.range.start() == self.range.end()
97 }
98
99 fn boundaries(&self) -> (u128, u128) {
100 (*self.range.start(), *self.range.end())
101 }
102
103 #[inline]
104 fn from_bool(value: bool) -> IntRange {
105 let val = value as u128;
106 IntRange { range: val..=val }
107 }
108
109 #[inline]
110 fn from_range(lo: u128, hi: u128, scalar_ty: Scalar) -> IntRange {
111 if let Scalar::Bool = scalar_ty {
112 IntRange { range: lo..=hi }
113 } else {
114 unimplemented!()
115 }
116 }
117
118 fn is_subrange(&self, other: &Self) -> bool {
119 other.range.start() <= self.range.start() && self.range.end() <= other.range.end()
120 }
121
122 fn intersection(&self, other: &Self) -> Option<Self> {
123 let (lo, hi) = self.boundaries();
124 let (other_lo, other_hi) = other.boundaries();
125 if lo <= other_hi && other_lo <= hi {
126 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi) })
127 } else {
128 None
129 }
130 }
131
132 /// See `Constructor::is_covered_by`
133 fn is_covered_by(&self, other: &Self) -> bool {
134 if self.intersection(other).is_some() {
135 // Constructor splitting should ensure that all intersections we encounter are actually
136 // inclusions.
137 assert!(self.is_subrange(other));
138 true
139 } else {
140 false
141 }
142 }
143}
144
145/// Represents a border between 2 integers. Because the intervals spanning borders must be able to
146/// cover every integer, we need to be able to represent 2^128 + 1 such borders.
147#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
148enum IntBorder {
149 JustBefore(u128),
150 AfterMax,
151}
152
153/// A range of integers that is partitioned into disjoint subranges. This does constructor
154/// splitting for integer ranges as explained at the top of the file.
155///
156/// This is fed multiple ranges, and returns an output that covers the input, but is split so that
157/// the only intersections between an output range and a seen range are inclusions. No output range
158/// straddles the boundary of one of the inputs.
159///
160/// The following input:
161/// ```
162/// |-------------------------| // `self`
163/// |------| |----------| |----|
164/// |-------| |-------|
165/// ```
166/// would be iterated over as follows:
167/// ```
168/// ||---|--||-|---|---|---|--|
169/// ```
170#[derive(Debug, Clone)]
171struct SplitIntRange {
172 /// The range we are splitting
173 range: IntRange,
174 /// The borders of ranges we have seen. They are all contained within `range`. This is kept
175 /// sorted.
176 borders: Vec<IntBorder>,
177}
178
179impl SplitIntRange {
180 fn new(range: IntRange) -> Self {
181 SplitIntRange { range, borders: Vec::new() }
182 }
183
184 /// Internal use
185 fn to_borders(r: IntRange) -> [IntBorder; 2] {
186 use IntBorder::*;
187 let (lo, hi) = r.boundaries();
188 let lo = JustBefore(lo);
189 let hi = match hi.checked_add(1) {
190 Some(m) => JustBefore(m),
191 None => AfterMax,
192 };
193 [lo, hi]
194 }
195
196 /// Add ranges relative to which we split.
197 fn split(&mut self, ranges: impl Iterator<Item = IntRange>) {
198 let this_range = &self.range;
199 let included_ranges = ranges.filter_map(|r| this_range.intersection(&r));
200 let included_borders = included_ranges.flat_map(|r| {
201 let borders = Self::to_borders(r);
202 once(borders[0]).chain(once(borders[1]))
203 });
204 self.borders.extend(included_borders);
205 self.borders.sort_unstable();
206 }
207
208 /// Iterate over the contained ranges.
209 fn iter(&self) -> impl Iterator<Item = IntRange> + '_ {
210 use IntBorder::*;
211
212 let self_range = Self::to_borders(self.range.clone());
213 // Start with the start of the range.
214 let mut prev_border = self_range[0];
215 self.borders
216 .iter()
217 .copied()
218 // End with the end of the range.
219 .chain(once(self_range[1]))
220 // List pairs of adjacent borders.
221 .map(move |border| {
222 let ret = (prev_border, border);
223 prev_border = border;
224 ret
225 })
226 // Skip duplicates.
227 .filter(|(prev_border, border)| prev_border != border)
228 // Finally, convert to ranges.
229 .map(|(prev_border, border)| {
230 let range = match (prev_border, border) {
231 (JustBefore(n), JustBefore(m)) if n < m => n..=(m - 1),
232 (JustBefore(n), AfterMax) => n..=u128::MAX,
233 _ => unreachable!(), // Ruled out by the sorting and filtering we did
234 };
235 IntRange { range }
236 })
237 }
238}
239
240/// A constructor for array and slice patterns.
241#[derive(Copy, Clone, Debug, PartialEq, Eq)]
242pub(super) struct Slice {
243 _unimplemented: Void,
244}
245
246impl Slice {
247 /// See `Constructor::is_covered_by`
248 fn is_covered_by(self, _other: Self) -> bool {
249 unimplemented!() // never called as Slice contains Void
250 }
251}
252
253/// A value can be decomposed into a constructor applied to some fields. This struct represents
254/// the constructor. See also `Fields`.
255///
256/// `pat_constructor` retrieves the constructor corresponding to a pattern.
257/// `specialize_constructor` returns the list of fields corresponding to a pattern, given a
258/// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
259/// `Fields`.
260#[allow(dead_code)]
261#[derive(Clone, Debug, PartialEq)]
262pub(super) enum Constructor {
263 /// The constructor for patterns that have a single constructor, like tuples, struct patterns
264 /// and fixed-length arrays.
265 Single,
266 /// Enum variants.
267 Variant(EnumVariantId),
268 /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
269 IntRange(IntRange),
270 /// Ranges of floating-point literal values (`2.0..=5.2`).
271 FloatRange(Void),
272 /// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
273 Str(Void),
274 /// Array and slice patterns.
275 Slice(Slice),
276 /// Constants that must not be matched structurally. They are treated as black
277 /// boxes for the purposes of exhaustiveness: we must not inspect them, and they
278 /// don't count towards making a match exhaustive.
279 Opaque,
280 /// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
281 /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
282 NonExhaustive,
283 /// Stands for constructors that are not seen in the matrix, as explained in the documentation
284 /// for [`SplitWildcard`].
285 Missing,
286 /// Wildcard pattern.
287 Wildcard,
288}
289
290impl Constructor {
291 pub(super) fn is_wildcard(&self) -> bool {
292 matches!(self, Wildcard)
293 }
294
295 fn as_int_range(&self) -> Option<&IntRange> {
296 match self {
297 IntRange(range) => Some(range),
298 _ => None,
299 }
300 }
301
302 fn as_slice(&self) -> Option<Slice> {
303 match self {
304 Slice(slice) => Some(*slice),
305 _ => None,
306 }
307 }
308
309 fn variant_id_for_adt(&self, adt: hir_def::AdtId) -> VariantId {
310 match *self {
311 Variant(id) => id.into(),
312 Single => {
313 assert!(!matches!(adt, hir_def::AdtId::EnumId(_)));
314 match adt {
315 hir_def::AdtId::EnumId(_) => unreachable!(),
316 hir_def::AdtId::StructId(id) => id.into(),
317 hir_def::AdtId::UnionId(id) => id.into(),
318 }
319 }
320 _ => panic!("bad constructor {:?} for adt {:?}", self, adt),
321 }
322 }
323
324 /// Determines the constructor that the given pattern can be specialized to.
325 pub(super) fn from_pat(cx: &MatchCheckCtx<'_>, pat: PatId) -> Self {
326 match cx.pattern_arena.borrow()[pat].kind.as_ref() {
327 PatKind::Binding { .. } | PatKind::Wild => Wildcard,
328 PatKind::Leaf { .. } | PatKind::Deref { .. } => Single,
329 &PatKind::Variant { enum_variant, .. } => Variant(enum_variant),
330 &PatKind::LiteralBool { value } => IntRange(IntRange::from_bool(value)),
331 PatKind::Or { .. } => cx.bug("Or-pattern should have been expanded earlier on."),
332 }
333 }
334
335 /// Some constructors (namely `Wildcard`, `IntRange` and `Slice`) actually stand for a set of actual
336 /// constructors (like variants, integers or fixed-sized slices). When specializing for these
337 /// constructors, we want to be specialising for the actual underlying constructors.
338 /// Naively, we would simply return the list of constructors they correspond to. We instead are
339 /// more clever: if there are constructors that we know will behave the same wrt the current
340 /// matrix, we keep them grouped. For example, all slices of a sufficiently large length
341 /// will either be all useful or all non-useful with a given matrix.
342 ///
343 /// See the branches for details on how the splitting is done.
344 ///
345 /// This function may discard some irrelevant constructors if this preserves behavior and
346 /// diagnostics. Eg. for the `_` case, we ignore the constructors already present in the
347 /// matrix, unless all of them are.
348 pub(super) fn split<'a>(
349 &self,
350 pcx: PatCtxt<'_>,
351 ctors: impl Iterator<Item = &'a Constructor> + Clone,
352 ) -> SmallVec<[Self; 1]> {
353 match self {
354 Wildcard => {
355 let mut split_wildcard = SplitWildcard::new(pcx);
356 split_wildcard.split(pcx, ctors);
357 split_wildcard.into_ctors(pcx)
358 }
359 // Fast-track if the range is trivial. In particular, we don't do the overlapping
360 // ranges check.
361 IntRange(ctor_range) if !ctor_range.is_singleton() => {
362 let mut split_range = SplitIntRange::new(ctor_range.clone());
363 let int_ranges = ctors.filter_map(|ctor| ctor.as_int_range());
364 split_range.split(int_ranges.cloned());
365 split_range.iter().map(IntRange).collect()
366 }
367 Slice(_) => unimplemented!(),
368 // Any other constructor can be used unchanged.
369 _ => smallvec![self.clone()],
370 }
371 }
372
373 /// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
374 /// For the simple cases, this is simply checking for equality. For the "grouped" constructors,
375 /// this checks for inclusion.
376 // We inline because this has a single call site in `Matrix::specialize_constructor`.
377 #[inline]
378 pub(super) fn is_covered_by(&self, pcx: PatCtxt<'_>, other: &Self) -> bool {
379 // This must be kept in sync with `is_covered_by_any`.
380 match (self, other) {
381 // Wildcards cover anything
382 (_, Wildcard) => true,
383 // The missing ctors are not covered by anything in the matrix except wildcards.
384 (Missing, _) | (Wildcard, _) => false,
385
386 (Single, Single) => true,
387 (Variant(self_id), Variant(other_id)) => self_id == other_id,
388
389 (IntRange(self_range), IntRange(other_range)) => self_range.is_covered_by(other_range),
390 (FloatRange(..), FloatRange(..)) => {
391 unimplemented!()
392 }
393 (Str(..), Str(..)) => {
394 unimplemented!()
395 }
396 (Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),
397
398 // We are trying to inspect an opaque constant. Thus we skip the row.
399 (Opaque, _) | (_, Opaque) => false,
400 // Only a wildcard pattern can match the special extra constructor.
401 (NonExhaustive, _) => false,
402
403 _ => pcx.cx.bug(&format!(
404 "trying to compare incompatible constructors {:?} and {:?}",
405 self, other
406 )),
407 }
408 }
409
410 /// Faster version of `is_covered_by` when applied to many constructors. `used_ctors` is
411 /// assumed to be built from `matrix.head_ctors()` with wildcards filtered out, and `self` is
412 /// assumed to have been split from a wildcard.
413 fn is_covered_by_any(&self, pcx: PatCtxt<'_>, used_ctors: &[Constructor]) -> bool {
414 if used_ctors.is_empty() {
415 return false;
416 }
417
418 // This must be kept in sync with `is_covered_by`.
419 match self {
420 // If `self` is `Single`, `used_ctors` cannot contain anything else than `Single`s.
421 Single => !used_ctors.is_empty(),
422 Variant(_) => used_ctors.iter().any(|c| c == self),
423 IntRange(range) => used_ctors
424 .iter()
425 .filter_map(|c| c.as_int_range())
426 .any(|other| range.is_covered_by(other)),
427 Slice(slice) => used_ctors
428 .iter()
429 .filter_map(|c| c.as_slice())
430 .any(|other| slice.is_covered_by(other)),
431 // This constructor is never covered by anything else
432 NonExhaustive => false,
433 Str(..) | FloatRange(..) | Opaque | Missing | Wildcard => {
434 pcx.cx.bug(&format!("found unexpected ctor in all_ctors: {:?}", self))
435 }
436 }
437 }
438}
439
440/// A wildcard constructor that we split relative to the constructors in the matrix, as explained
441/// at the top of the file.
442///
443/// A constructor that is not present in the matrix rows will only be covered by the rows that have
444/// wildcards. Thus we can group all of those constructors together; we call them "missing
445/// constructors". Splitting a wildcard would therefore list all present constructors individually
446/// (or grouped if they are integers or slices), and then all missing constructors together as a
447/// group.
448///
449/// However we can go further: since any constructor will match the wildcard rows, and having more
450/// rows can only reduce the amount of usefulness witnesses, we can skip the present constructors
451/// and only try the missing ones.
452/// This will not preserve the whole list of witnesses, but will preserve whether the list is empty
453/// or not. In fact this is quite natural from the point of view of diagnostics too. This is done
454/// in `to_ctors`: in some cases we only return `Missing`.
455#[derive(Debug)]
456pub(super) struct SplitWildcard {
457 /// Constructors seen in the matrix.
458 matrix_ctors: Vec<Constructor>,
459 /// All the constructors for this type
460 all_ctors: SmallVec<[Constructor; 1]>,
461}
462
463impl SplitWildcard {
464 pub(super) fn new(pcx: PatCtxt<'_>) -> Self {
465 let cx = pcx.cx;
466 let make_range = |start, end, scalar| IntRange(IntRange::from_range(start, end, scalar));
467
468 // Unhandled types are treated as non-exhaustive. Being explicit here instead of falling
469 // to catchall arm to ease further implementation.
470 let unhandled = || smallvec![NonExhaustive];
471
472 // This determines the set of all possible constructors for the type `pcx.ty`. For numbers,
473 // arrays and slices we use ranges and variable-length slices when appropriate.
474 //
475 // If the `exhaustive_patterns` feature is enabled, we make sure to omit constructors that
476 // are statically impossible. E.g., for `Option<!>`, we do not include `Some(_)` in the
477 // returned list of constructors.
478 // Invariant: this is empty if and only if the type is uninhabited (as determined by
479 // `cx.is_uninhabited()`).
480 let all_ctors = match pcx.ty.kind(&Interner) {
481 TyKind::Scalar(Scalar::Bool) => smallvec![make_range(0, 1, Scalar::Bool)],
482 // TyKind::Array(..) if ... => unhandled(),
483 TyKind::Array(..) | TyKind::Slice(..) => unhandled(),
484 &TyKind::Adt(AdtId(hir_def::AdtId::EnumId(enum_id)), ref _substs) => {
485 let enum_data = cx.db.enum_data(enum_id);
486
487 // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
488 // additional "unknown" constructor.
489 // There is no point in enumerating all possible variants, because the user can't
490 // actually match against them all themselves. So we always return only the fictitious
491 // constructor.
492 // E.g., in an example like:
493 //
494 // ```
495 // let err: io::ErrorKind = ...;
496 // match err {
497 // io::ErrorKind::NotFound => {},
498 // }
499 // ```
500 //
501 // we don't want to show every possible IO error, but instead have only `_` as the
502 // witness.
503 let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(enum_id);
504
505 // If `exhaustive_patterns` is disabled and our scrutinee is an empty enum, we treat it
506 // as though it had an "unknown" constructor to avoid exposing its emptiness. The
507 // exception is if the pattern is at the top level, because we want empty matches to be
508 // considered exhaustive.
509 let is_secretly_empty = enum_data.variants.is_empty()
510 && !cx.feature_exhaustive_patterns()
511 && !pcx.is_top_level;
512
513 if is_secretly_empty || is_declared_nonexhaustive {
514 smallvec![NonExhaustive]
515 } else if cx.feature_exhaustive_patterns() {
516 unimplemented!() // see MatchCheckCtx.feature_exhaustive_patterns()
517 } else {
518 enum_data
519 .variants
520 .iter()
521 .map(|(local_id, ..)| Variant(EnumVariantId { parent: enum_id, local_id }))
522 .collect()
523 }
524 }
525 TyKind::Scalar(Scalar::Char) => unhandled(),
526 TyKind::Scalar(Scalar::Int(..)) | TyKind::Scalar(Scalar::Uint(..)) => unhandled(),
527 TyKind::Never if !cx.feature_exhaustive_patterns() && !pcx.is_top_level => {
528 smallvec![NonExhaustive]
529 }
530 TyKind::Never => SmallVec::new(),
531 _ if cx.is_uninhabited(&pcx.ty) => SmallVec::new(),
532 TyKind::Adt(..) | TyKind::Tuple(..) | TyKind::Ref(..) => smallvec![Single],
533 // This type is one for which we cannot list constructors, like `str` or `f64`.
534 _ => smallvec![NonExhaustive],
535 };
536 SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
537 }
538
539 /// Pass a set of constructors relative to which to split this one. Don't call twice, it won't
540 /// do what you want.
541 pub(super) fn split<'a>(
542 &mut self,
543 pcx: PatCtxt<'_>,
544 ctors: impl Iterator<Item = &'a Constructor> + Clone,
545 ) {
546 // Since `all_ctors` never contains wildcards, this won't recurse further.
547 self.all_ctors =
548 self.all_ctors.iter().flat_map(|ctor| ctor.split(pcx, ctors.clone())).collect();
549 self.matrix_ctors = ctors.filter(|c| !c.is_wildcard()).cloned().collect();
550 }
551
552 /// Whether there are any value constructors for this type that are not present in the matrix.
553 fn any_missing(&self, pcx: PatCtxt<'_>) -> bool {
554 self.iter_missing(pcx).next().is_some()
555 }
556
557 /// Iterate over the constructors for this type that are not present in the matrix.
558 pub(super) fn iter_missing<'a>(
559 &'a self,
560 pcx: PatCtxt<'a>,
561 ) -> impl Iterator<Item = &'a Constructor> {
562 self.all_ctors.iter().filter(move |ctor| !ctor.is_covered_by_any(pcx, &self.matrix_ctors))
563 }
564
565 /// Return the set of constructors resulting from splitting the wildcard. As explained at the
566 /// top of the file, if any constructors are missing we can ignore the present ones.
567 fn into_ctors(self, pcx: PatCtxt<'_>) -> SmallVec<[Constructor; 1]> {
568 if self.any_missing(pcx) {
569 // Some constructors are missing, thus we can specialize with the special `Missing`
570 // constructor, which stands for those constructors that are not seen in the matrix,
571 // and matches the same rows as any of them (namely the wildcard rows). See the top of
572 // the file for details.
573 // However, when all constructors are missing we can also specialize with the full
574 // `Wildcard` constructor. The difference will depend on what we want in diagnostics.
575
576 // If some constructors are missing, we typically want to report those constructors,
577 // e.g.:
578 // ```
579 // enum Direction { N, S, E, W }
580 // let Direction::N = ...;
581 // ```
582 // we can report 3 witnesses: `S`, `E`, and `W`.
583 //
584 // However, if the user didn't actually specify a constructor
585 // in this arm, e.g., in
586 // ```
587 // let x: (Direction, Direction, bool) = ...;
588 // let (_, _, false) = x;
589 // ```
590 // we don't want to show all 16 possible witnesses `(<direction-1>, <direction-2>,
591 // true)` - we are satisfied with `(_, _, true)`. So if all constructors are missing we
592 // prefer to report just a wildcard `_`.
593 //
594 // The exception is: if we are at the top-level, for example in an empty match, we
595 // sometimes prefer reporting the list of constructors instead of just `_`.
596 let report_when_all_missing = pcx.is_top_level && !IntRange::is_integral(pcx.ty);
597 let ctor = if !self.matrix_ctors.is_empty() || report_when_all_missing {
598 Missing
599 } else {
600 Wildcard
601 };
602 return smallvec![ctor];
603 }
604
605 // All the constructors are present in the matrix, so we just go through them all.
606 self.all_ctors
607 }
608}
609
610/// A value can be decomposed into a constructor applied to some fields. This struct represents
611/// those fields, generalized to allow patterns in each field. See also `Constructor`.
612/// This is constructed from a constructor using [`Fields::wildcards()`].
613///
614/// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
615/// uninhabited. For that, we filter these fields out of the matrix. This is handled automatically
616/// in `Fields`. This filtering is uncommon in practice, because uninhabited fields are rarely used,
617/// so we avoid it when possible to preserve performance.
618#[derive(Debug, Clone)]
619pub(super) enum Fields {
620 /// Lists of patterns that don't contain any filtered fields.
621 /// `Slice` and `Vec` behave the same; the difference is only to avoid allocating and
622 /// triple-dereferences when possible. Frankly this is premature optimization, I (Nadrieril)
623 /// have not measured if it really made a difference.
624 Vec(SmallVec<[PatId; 2]>),
625}
626
627impl Fields {
628 /// Internal use. Use `Fields::wildcards()` instead.
629 /// Must not be used if the pattern is a field of a struct/tuple/variant.
630 fn from_single_pattern(pat: PatId) -> Self {
631 Fields::Vec(smallvec![pat])
632 }
633
634 /// Convenience; internal use.
635 fn wildcards_from_tys(cx: &MatchCheckCtx<'_>, tys: impl IntoIterator<Item = Ty>) -> Self {
636 let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
637 let pats = wilds.map(|pat| cx.alloc_pat(pat)).collect();
638 Fields::Vec(pats)
639 }
640
641 /// Creates a new list of wildcard fields for a given constructor.
642 pub(crate) fn wildcards(pcx: PatCtxt<'_>, constructor: &Constructor) -> Self {
643 let ty = pcx.ty;
644 let cx = pcx.cx;
645 let wildcard_from_ty = |ty: &Ty| cx.alloc_pat(Pat::wildcard_from_ty(ty.clone()));
646
647 let ret = match constructor {
648 Single | Variant(_) => match ty.kind(&Interner) {
649 TyKind::Tuple(_, substs) => {
650 let tys = substs.iter(&Interner).map(|ty| ty.assert_ty_ref(&Interner));
651 Fields::wildcards_from_tys(cx, tys.cloned())
652 }
653 TyKind::Ref(.., rty) => Fields::from_single_pattern(wildcard_from_ty(rty)),
654 &TyKind::Adt(AdtId(adt), ref substs) => {
655 if adt_is_box(adt, cx) {
656 // Use T as the sub pattern type of Box<T>.
657 let subst_ty = substs.at(&Interner, 0).assert_ty_ref(&Interner);
658 Fields::from_single_pattern(wildcard_from_ty(subst_ty))
659 } else {
660 let variant_id = constructor.variant_id_for_adt(adt);
661 let adt_is_local =
662 variant_id.module(cx.db.upcast()).krate() == cx.module.krate();
663 // Whether we must not match the fields of this variant exhaustively.
664 let is_non_exhaustive =
665 is_field_list_non_exhaustive(variant_id, cx) && !adt_is_local;
666
667 cov_mark::hit!(match_check_wildcard_expanded_to_substitutions);
668 let field_ty_data = cx.db.field_types(variant_id);
669 let field_tys = || {
670 field_ty_data
671 .iter()
672 .map(|(_, binders)| binders.clone().substitute(&Interner, substs))
673 };
674
675 // In the following cases, we don't need to filter out any fields. This is
676 // the vast majority of real cases, since uninhabited fields are uncommon.
677 let has_no_hidden_fields = (matches!(adt, hir_def::AdtId::EnumId(_))
678 && !is_non_exhaustive)
679 || !field_tys().any(|ty| cx.is_uninhabited(&ty));
680
681 if has_no_hidden_fields {
682 Fields::wildcards_from_tys(cx, field_tys())
683 } else {
684 //FIXME(iDawer): see MatchCheckCtx::is_uninhabited, has_no_hidden_fields is always true
685 unimplemented!("exhaustive_patterns feature")
686 }
687 }
688 }
689 ty_kind => {
690 cx.bug(&format!("Unexpected type for `Single` constructor: {:?}", ty_kind))
691 }
692 },
693 Slice(..) => {
694 unimplemented!()
695 }
696 Str(..) | FloatRange(..) | IntRange(..) | NonExhaustive | Opaque | Missing
697 | Wildcard => Fields::Vec(Default::default()),
698 };
699 ret
700 }
701
702 /// Apply a constructor to a list of patterns, yielding a new pattern. `self`
703 /// must have as many elements as this constructor's arity.
704 ///
705 /// This is roughly the inverse of `specialize_constructor`.
706 ///
707 /// Examples:
708 /// `ctor`: `Constructor::Single`
709 /// `ty`: `Foo(u32, u32, u32)`
710 /// `self`: `[10, 20, _]`
711 /// returns `Foo(10, 20, _)`
712 ///
713 /// `ctor`: `Constructor::Variant(Option::Some)`
714 /// `ty`: `Option<bool>`
715 /// `self`: `[false]`
716 /// returns `Some(false)`
717 pub(super) fn apply(self, pcx: PatCtxt<'_>, ctor: &Constructor) -> Pat {
718 let subpatterns_and_indices = self.patterns_and_indices();
719 let mut subpatterns =
720 subpatterns_and_indices.iter().map(|&(_, p)| pcx.cx.pattern_arena.borrow()[p].clone());
721 // FIXME(iDawer) witnesses are not yet used
722 const UNHANDLED: PatKind = PatKind::Wild;
723
724 let pat = match ctor {
725 Single | Variant(_) => match pcx.ty.kind(&Interner) {
726 TyKind::Adt(..) | TyKind::Tuple(..) => {
727 // We want the real indices here.
728 let subpatterns = subpatterns_and_indices
729 .iter()
730 .map(|&(field, pat)| FieldPat {
731 field,
732 pattern: pcx.cx.pattern_arena.borrow()[pat].clone(),
733 })
734 .collect();
735
736 if let Some((adt, substs)) = pcx.ty.as_adt() {
737 if let hir_def::AdtId::EnumId(_) = adt {
738 let enum_variant = match ctor {
739 &Variant(id) => id,
740 _ => unreachable!(),
741 };
742 PatKind::Variant { substs: substs.clone(), enum_variant, subpatterns }
743 } else {
744 PatKind::Leaf { subpatterns }
745 }
746 } else {
747 PatKind::Leaf { subpatterns }
748 }
749 }
750 // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
751 // be careful to reconstruct the correct constant pattern here. However a string
752 // literal pattern will never be reported as a non-exhaustiveness witness, so we
753 // can ignore this issue.
754 TyKind::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
755 TyKind::Slice(..) | TyKind::Array(..) => {
756 pcx.cx.bug(&format!("bad slice pattern {:?} {:?}", ctor, pcx.ty))
757 }
758 _ => PatKind::Wild,
759 },
760 Constructor::Slice(_) => UNHANDLED,
761 Str(_) => UNHANDLED,
762 FloatRange(..) => UNHANDLED,
763 Constructor::IntRange(_) => UNHANDLED,
764 NonExhaustive => PatKind::Wild,
765 Wildcard => return Pat::wildcard_from_ty(pcx.ty.clone()),
766 Opaque => pcx.cx.bug("we should not try to apply an opaque constructor"),
767 Missing => pcx.cx.bug(
768 "trying to apply the `Missing` constructor;\
769 this should have been done in `apply_constructors`",
770 ),
771 };
772
773 Pat { ty: pcx.ty.clone(), kind: Box::new(pat) }
774 }
775
776 /// Returns the number of patterns. This is the same as the arity of the constructor used to
777 /// construct `self`.
778 pub(super) fn len(&self) -> usize {
779 match self {
780 Fields::Vec(pats) => pats.len(),
781 }
782 }
783
784 /// Returns the list of patterns along with the corresponding field indices.
785 fn patterns_and_indices(&self) -> SmallVec<[(LocalFieldId, PatId); 2]> {
786 match self {
787 Fields::Vec(pats) => pats
788 .iter()
789 .copied()
790 .enumerate()
791 .map(|(i, p)| (LocalFieldId::from_raw((i as u32).into()), p))
792 .collect(),
793 }
794 }
795
796 pub(super) fn into_patterns(self) -> SmallVec<[PatId; 2]> {
797 match self {
798 Fields::Vec(pats) => pats,
799 }
800 }
801
802 /// Overrides some of the fields with the provided patterns. Exactly like
803 /// `replace_fields_indexed`, except that it takes `FieldPat`s as input.
804 fn replace_with_fieldpats(
805 &self,
806 new_pats: impl IntoIterator<Item = (LocalFieldId, PatId)>,
807 ) -> Self {
808 self.replace_fields_indexed(
809 new_pats.into_iter().map(|(field, pat)| (u32::from(field.into_raw()) as usize, pat)),
810 )
811 }
812
813 /// Overrides some of the fields with the provided patterns. This is used when a pattern
814 /// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start
815 /// with a `Fields` that is just one wildcard per field of the `Foo` struct, and override the
816 /// entry corresponding to `field1` with the pattern `Some(_)`. This is also used for slice
817 /// patterns for the same reason.
818 fn replace_fields_indexed(&self, new_pats: impl IntoIterator<Item = (usize, PatId)>) -> Self {
819 let mut fields = self.clone();
820
821 match &mut fields {
822 Fields::Vec(pats) => {
823 for (i, pat) in new_pats {
824 if let Some(p) = pats.get_mut(i) {
825 *p = pat;
826 }
827 }
828 }
829 }
830 fields
831 }
832
833 /// Replaces contained fields with the given list of patterns. There must be `len()` patterns
834 /// in `pats`.
835 pub(super) fn replace_fields(
836 &self,
837 cx: &MatchCheckCtx<'_>,
838 pats: impl IntoIterator<Item = Pat>,
839 ) -> Self {
840 let pats = pats.into_iter().map(|pat| cx.alloc_pat(pat)).collect();
841
842 match self {
843 Fields::Vec(_) => Fields::Vec(pats),
844 }
845 }
846
847 /// Replaces contained fields with the arguments of the given pattern. Only use on a pattern
848 /// that is compatible with the constructor used to build `self`.
849 /// This is meant to be used on the result of `Fields::wildcards()`. The idea is that
850 /// `wildcards` constructs a list of fields where all entries are wildcards, and the pattern
851 /// provided to this function fills some of the fields with non-wildcards.
852 /// In the following example `Fields::wildcards` would return `[_, _, _, _]`. If we call
853 /// `replace_with_pattern_arguments` on it with the pattern, the result will be `[Some(0), _,
854 /// _, _]`.
855 /// ```rust
856 /// let x: [Option<u8>; 4] = foo();
857 /// match x {
858 /// [Some(0), ..] => {}
859 /// }
860 /// ```
861 /// This is guaranteed to preserve the number of patterns in `self`.
862 pub(super) fn replace_with_pattern_arguments(
863 &self,
864 pat: PatId,
865 cx: &MatchCheckCtx<'_>,
866 ) -> Self {
867 // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
868 // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
869 let mut arena = cx.pattern_arena.borrow_mut();
870 match arena[pat].kind.as_ref() {
871 PatKind::Deref { subpattern } => {
872 assert_eq!(self.len(), 1);
873 let subpattern = subpattern.clone();
874 Fields::from_single_pattern(arena.alloc(subpattern))
875 }
876 PatKind::Leaf { subpatterns } | PatKind::Variant { subpatterns, .. } => {
877 let subpatterns = subpatterns.clone();
878 let subpatterns = subpatterns
879 .iter()
880 .map(|field_pat| (field_pat.field, arena.alloc(field_pat.pattern.clone())));
881 self.replace_with_fieldpats(subpatterns)
882 }
883
884 PatKind::Wild
885 | PatKind::Binding { .. }
886 | PatKind::LiteralBool { .. }
887 | PatKind::Or { .. } => self.clone(),
888 }
889 }
890}
891
892fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_>) -> bool {
893 let attr_def_id = match variant_id {
894 VariantId::EnumVariantId(id) => id.into(),
895 VariantId::StructId(id) => id.into(),
896 VariantId::UnionId(id) => id.into(),
897 };
898 cx.db.attrs(attr_def_id).by_key("non_exhaustive").exists()
899}
900
901fn adt_is_box(adt: hir_def::AdtId, cx: &MatchCheckCtx<'_>) -> bool {
902 use hir_def::lang_item::LangItemTarget;
903 match cx.db.lang_item(cx.module.krate(), "owned_box".into()) {
904 Some(LangItemTarget::StructId(box_id)) => adt == box_id.into(),
905 _ => false,
906 }
907}
diff --git a/crates/hir_ty/src/diagnostics/match_check/pat_util.rs b/crates/hir_ty/src/diagnostics/match_check/pat_util.rs
new file mode 100644
index 000000000..b89b4f2bf
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/pat_util.rs
@@ -0,0 +1,56 @@
1//! Pattern untilities.
2//!
3//! Originates from `rustc_hir::pat_util`
4
5use std::iter::{Enumerate, ExactSizeIterator};
6
7pub(crate) struct EnumerateAndAdjust<I> {
8 enumerate: Enumerate<I>,
9 gap_pos: usize,
10 gap_len: usize,
11}
12
13impl<I> Iterator for EnumerateAndAdjust<I>
14where
15 I: Iterator,
16{
17 type Item = (usize, <I as Iterator>::Item);
18
19 fn next(&mut self) -> Option<(usize, <I as Iterator>::Item)> {
20 self.enumerate
21 .next()
22 .map(|(i, elem)| (if i < self.gap_pos { i } else { i + self.gap_len }, elem))
23 }
24
25 fn size_hint(&self) -> (usize, Option<usize>) {
26 self.enumerate.size_hint()
27 }
28}
29
30pub(crate) trait EnumerateAndAdjustIterator {
31 fn enumerate_and_adjust(
32 self,
33 expected_len: usize,
34 gap_pos: Option<usize>,
35 ) -> EnumerateAndAdjust<Self>
36 where
37 Self: Sized;
38}
39
40impl<T: ExactSizeIterator> EnumerateAndAdjustIterator for T {
41 fn enumerate_and_adjust(
42 self,
43 expected_len: usize,
44 gap_pos: Option<usize>,
45 ) -> EnumerateAndAdjust<Self>
46 where
47 Self: Sized,
48 {
49 let actual_len = self.len();
50 EnumerateAndAdjust {
51 enumerate: self.enumerate(),
52 gap_pos: gap_pos.unwrap_or(expected_len),
53 gap_len: expected_len - actual_len,
54 }
55 }
56}
diff --git a/crates/hir_ty/src/diagnostics/match_check/usefulness.rs b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
new file mode 100644
index 000000000..83b094a89
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs
@@ -0,0 +1,1188 @@
1//! Based on rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22)
2//! https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
3//!
4//! -----
5//!
6//! This file includes the logic for exhaustiveness and reachability checking for pattern-matching.
7//! Specifically, given a list of patterns for a type, we can tell whether:
8//! (a) each pattern is reachable (reachability)
9//! (b) the patterns cover every possible value for the type (exhaustiveness)
10//!
11//! The algorithm implemented here is a modified version of the one described in [this
12//! paper](http://moscova.inria.fr/~maranget/papers/warn/index.html). We have however generalized
13//! it to accommodate the variety of patterns that Rust supports. We thus explain our version here,
14//! without being as rigorous.
15//!
16//!
17//! # Summary
18//!
19//! The core of the algorithm is the notion of "usefulness". A pattern `q` is said to be *useful*
20//! relative to another pattern `p` of the same type if there is a value that is matched by `q` and
21//! not matched by `p`. This generalizes to many `p`s: `q` is useful w.r.t. a list of patterns
22//! `p_1 .. p_n` if there is a value that is matched by `q` and by none of the `p_i`. We write
23//! `usefulness(p_1 .. p_n, q)` for a function that returns a list of such values. The aim of this
24//! file is to compute it efficiently.
25//!
26//! This is enough to compute reachability: a pattern in a `match` expression is reachable iff it
27//! is useful w.r.t. the patterns above it:
28//! ```rust
29//! match x {
30//! Some(_) => ...,
31//! None => ..., // reachable: `None` is matched by this but not the branch above
32//! Some(0) => ..., // unreachable: all the values this matches are already matched by
33//! // `Some(_)` above
34//! }
35//! ```
36//!
37//! This is also enough to compute exhaustiveness: a match is exhaustive iff the wildcard `_`
38//! pattern is _not_ useful w.r.t. the patterns in the match. The values returned by `usefulness`
39//! are used to tell the user which values are missing.
40//! ```rust
41//! match x {
42//! Some(0) => ...,
43//! None => ...,
44//! // not exhaustive: `_` is useful because it matches `Some(1)`
45//! }
46//! ```
47//!
48//! The entrypoint of this file is the [`compute_match_usefulness`] function, which computes
49//! reachability for each match branch and exhaustiveness for the whole match.
50//!
51//!
52//! # Constructors and fields
53//!
54//! Note: we will often abbreviate "constructor" as "ctor".
55//!
56//! The idea that powers everything that is done in this file is the following: a (matcheable)
57//! value is made from a constructor applied to a number of subvalues. Examples of constructors are
58//! `Some`, `None`, `(,)` (the 2-tuple constructor), `Foo {..}` (the constructor for a struct
59//! `Foo`), and `2` (the constructor for the number `2`). This is natural when we think of
60//! pattern-matching, and this is the basis for what follows.
61//!
62//! Some of the ctors listed above might feel weird: `None` and `2` don't take any arguments.
63//! That's ok: those are ctors that take a list of 0 arguments; they are the simplest case of
64//! ctors. We treat `2` as a ctor because `u64` and other number types behave exactly like a huge
65//! `enum`, with one variant for each number. This allows us to see any matcheable value as made up
66//! from a tree of ctors, each having a set number of children. For example: `Foo { bar: None,
67//! baz: Ok(0) }` is made from 4 different ctors, namely `Foo{..}`, `None`, `Ok` and `0`.
68//!
69//! This idea can be extended to patterns: they are also made from constructors applied to fields.
70//! A pattern for a given type is allowed to use all the ctors for values of that type (which we
71//! call "value constructors"), but there are also pattern-only ctors. The most important one is
72//! the wildcard (`_`), and the others are integer ranges (`0..=10`), variable-length slices (`[x,
73//! ..]`), and or-patterns (`Ok(0) | Err(_)`). Examples of valid patterns are `42`, `Some(_)`, `Foo
74//! { bar: Some(0) | None, baz: _ }`. Note that a binder in a pattern (e.g. `Some(x)`) matches the
75//! same values as a wildcard (e.g. `Some(_)`), so we treat both as wildcards.
76//!
77//! From this deconstruction we can compute whether a given value matches a given pattern; we
78//! simply look at ctors one at a time. Given a pattern `p` and a value `v`, we want to compute
79//! `matches!(v, p)`. It's mostly straightforward: we compare the head ctors and when they match
80//! we compare their fields recursively. A few representative examples:
81//!
82//! - `matches!(v, _) := true`
83//! - `matches!((v0, v1), (p0, p1)) := matches!(v0, p0) && matches!(v1, p1)`
84//! - `matches!(Foo { bar: v0, baz: v1 }, Foo { bar: p0, baz: p1 }) := matches!(v0, p0) && matches!(v1, p1)`
85//! - `matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)`
86//! - `matches!(Ok(v0), Err(p0)) := false` (incompatible variants)
87//! - `matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)`
88//! - `matches!([v0], [p0, .., p1]) := false` (incompatible lengths)
89//! - `matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)`
90//! - `matches!(v, p0 | p1) := matches!(v, p0) || matches!(v, p1)`
91//!
92//! Constructors, fields and relevant operations are defined in the [`super::deconstruct_pat`] module.
93//!
94//! Note: this constructors/fields distinction may not straightforwardly apply to every Rust type.
95//! For example a value of type `Rc<u64>` can't be deconstructed that way, and `&str` has an
96//! infinitude of constructors. There are also subtleties with visibility of fields and
97//! uninhabitedness and various other things. The constructors idea can be extended to handle most
98//! of these subtleties though; caveats are documented where relevant throughout the code.
99//!
100//! Whether constructors cover each other is computed by [`Constructor::is_covered_by`].
101//!
102//!
103//! # Specialization
104//!
105//! Recall that we wish to compute `usefulness(p_1 .. p_n, q)`: given a list of patterns `p_1 ..
106//! p_n` and a pattern `q`, all of the same type, we want to find a list of values (called
107//! "witnesses") that are matched by `q` and by none of the `p_i`. We obviously don't just
108//! enumerate all possible values. From the discussion above we see that we can proceed
109//! ctor-by-ctor: for each value ctor of the given type, we ask "is there a value that starts with
110//! this constructor and matches `q` and none of the `p_i`?". As we saw above, there's a lot we can
111//! say from knowing only the first constructor of our candidate value.
112//!
113//! Let's take the following example:
114//! ```
115//! match x {
116//! Enum::Variant1(_) => {} // `p1`
117//! Enum::Variant2(None, 0) => {} // `p2`
118//! Enum::Variant2(Some(_), 0) => {} // `q`
119//! }
120//! ```
121//!
122//! We can easily see that if our candidate value `v` starts with `Variant1` it will not match `q`.
123//! If `v = Variant2(v0, v1)` however, whether or not it matches `p2` and `q` will depend on `v0`
124//! and `v1`. In fact, such a `v` will be a witness of usefulness of `q` exactly when the tuple
125//! `(v0, v1)` is a witness of usefulness of `q'` in the following reduced match:
126//!
127//! ```
128//! match x {
129//! (None, 0) => {} // `p2'`
130//! (Some(_), 0) => {} // `q'`
131//! }
132//! ```
133//!
134//! This motivates a new step in computing usefulness, that we call _specialization_.
135//! Specialization consist of filtering a list of patterns for those that match a constructor, and
136//! then looking into the constructor's fields. This enables usefulness to be computed recursively.
137//!
138//! Instead of acting on a single pattern in each row, we will consider a list of patterns for each
139//! row, and we call such a list a _pattern-stack_. The idea is that we will specialize the
140//! leftmost pattern, which amounts to popping the constructor and pushing its fields, which feels
141//! like a stack. We note a pattern-stack simply with `[p_1 ... p_n]`.
142//! Here's a sequence of specializations of a list of pattern-stacks, to illustrate what's
143//! happening:
144//! ```
145//! [Enum::Variant1(_)]
146//! [Enum::Variant2(None, 0)]
147//! [Enum::Variant2(Some(_), 0)]
148//! //==>> specialize with `Variant2`
149//! [None, 0]
150//! [Some(_), 0]
151//! //==>> specialize with `Some`
152//! [_, 0]
153//! //==>> specialize with `true` (say the type was `bool`)
154//! [0]
155//! //==>> specialize with `0`
156//! []
157//! ```
158//!
159//! The function `specialize(c, p)` takes a value constructor `c` and a pattern `p`, and returns 0
160//! or more pattern-stacks. If `c` does not match the head constructor of `p`, it returns nothing;
161//! otherwise if returns the fields of the constructor. This only returns more than one
162//! pattern-stack if `p` has a pattern-only constructor.
163//!
164//! - Specializing for the wrong constructor returns nothing
165//!
166//! `specialize(None, Some(p0)) := []`
167//!
168//! - Specializing for the correct constructor returns a single row with the fields
169//!
170//! `specialize(Variant1, Variant1(p0, p1, p2)) := [[p0, p1, p2]]`
171//!
172//! `specialize(Foo{..}, Foo { bar: p0, baz: p1 }) := [[p0, p1]]`
173//!
174//! - For or-patterns, we specialize each branch and concatenate the results
175//!
176//! `specialize(c, p0 | p1) := specialize(c, p0) ++ specialize(c, p1)`
177//!
178//! - We treat the other pattern constructors as if they were a large or-pattern of all the
179//! possibilities:
180//!
181//! `specialize(c, _) := specialize(c, Variant1(_) | Variant2(_, _) | ...)`
182//!
183//! `specialize(c, 1..=100) := specialize(c, 1 | ... | 100)`
184//!
185//! `specialize(c, [p0, .., p1]) := specialize(c, [p0, p1] | [p0, _, p1] | [p0, _, _, p1] | ...)`
186//!
187//! - If `c` is a pattern-only constructor, `specialize` is defined on a case-by-case basis. See
188//! the discussion about constructor splitting in [`super::deconstruct_pat`].
189//!
190//!
191//! We then extend this function to work with pattern-stacks as input, by acting on the first
192//! column and keeping the other columns untouched.
193//!
194//! Specialization for the whole matrix is done in [`Matrix::specialize_constructor`]. Note that
195//! or-patterns in the first column are expanded before being stored in the matrix. Specialization
196//! for a single patstack is done from a combination of [`Constructor::is_covered_by`] and
197//! [`PatStack::pop_head_constructor`]. The internals of how it's done mostly live in the
198//! [`Fields`] struct.
199//!
200//!
201//! # Computing usefulness
202//!
203//! We now have all we need to compute usefulness. The inputs to usefulness are a list of
204//! pattern-stacks `p_1 ... p_n` (one per row), and a new pattern_stack `q`. The paper and this
205//! file calls the list of patstacks a _matrix_. They must all have the same number of columns and
206//! the patterns in a given column must all have the same type. `usefulness` returns a (possibly
207//! empty) list of witnesses of usefulness. These witnesses will also be pattern-stacks.
208//!
209//! - base case: `n_columns == 0`.
210//! Since a pattern-stack functions like a tuple of patterns, an empty one functions like the
211//! unit type. Thus `q` is useful iff there are no rows above it, i.e. if `n == 0`.
212//!
213//! - inductive case: `n_columns > 0`.
214//! We need a way to list the constructors we want to try. We will be more clever in the next
215//! section but for now assume we list all value constructors for the type of the first column.
216//!
217//! - for each such ctor `c`:
218//!
219//! - for each `q'` returned by `specialize(c, q)`:
220//!
221//! - we compute `usefulness(specialize(c, p_1) ... specialize(c, p_n), q')`
222//!
223//! - for each witness found, we revert specialization by pushing the constructor `c` on top.
224//!
225//! - We return the concatenation of all the witnesses found, if any.
226//!
227//! Example:
228//! ```
229//! [Some(true)] // p_1
230//! [None] // p_2
231//! [Some(_)] // q
232//! //==>> try `None`: `specialize(None, q)` returns nothing
233//! //==>> try `Some`: `specialize(Some, q)` returns a single row
234//! [true] // p_1'
235//! [_] // q'
236//! //==>> try `true`: `specialize(true, q')` returns a single row
237//! [] // p_1''
238//! [] // q''
239//! //==>> base case; `n != 0` so `q''` is not useful.
240//! //==>> go back up a step
241//! [true] // p_1'
242//! [_] // q'
243//! //==>> try `false`: `specialize(false, q')` returns a single row
244//! [] // q''
245//! //==>> base case; `n == 0` so `q''` is useful. We return the single witness `[]`
246//! witnesses:
247//! []
248//! //==>> undo the specialization with `false`
249//! witnesses:
250//! [false]
251//! //==>> undo the specialization with `Some`
252//! witnesses:
253//! [Some(false)]
254//! //==>> we have tried all the constructors. The output is the single witness `[Some(false)]`.
255//! ```
256//!
257//! This computation is done in [`is_useful`]. In practice we don't care about the list of
258//! witnesses when computing reachability; we only need to know whether any exist. We do keep the
259//! witnesses when computing exhaustiveness to report them to the user.
260//!
261//!
262//! # Making usefulness tractable: constructor splitting
263//!
264//! We're missing one last detail: which constructors do we list? Naively listing all value
265//! constructors cannot work for types like `u64` or `&str`, so we need to be more clever. The
266//! first obvious insight is that we only want to list constructors that are covered by the head
267//! constructor of `q`. If it's a value constructor, we only try that one. If it's a pattern-only
268//! constructor, we use the final clever idea for this algorithm: _constructor splitting_, where we
269//! group together constructors that behave the same.
270//!
271//! The details are not necessary to understand this file, so we explain them in
272//! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function.
273
274use std::{cell::RefCell, iter::FromIterator};
275
276use hir_def::{expr::ExprId, HasModule, ModuleId};
277use la_arena::Arena;
278use once_cell::unsync::OnceCell;
279use rustc_hash::FxHashMap;
280use smallvec::{smallvec, SmallVec};
281
282use crate::{db::HirDatabase, InferenceResult, Interner, Ty};
283
284use super::{
285 deconstruct_pat::{Constructor, Fields, SplitWildcard},
286 Pat, PatId, PatKind, PatternFoldable, PatternFolder,
287};
288
289use self::{helper::PatIdExt, Usefulness::*, WitnessPreference::*};
290
291pub(crate) struct MatchCheckCtx<'a> {
292 pub(crate) module: ModuleId,
293 pub(crate) match_expr: ExprId,
294 pub(crate) infer: &'a InferenceResult,
295 pub(crate) db: &'a dyn HirDatabase,
296 /// Lowered patterns from arms plus generated by the check.
297 pub(crate) pattern_arena: &'a RefCell<PatternArena>,
298 pub(crate) eprint_panic_context: &'a dyn Fn(),
299}
300
301impl<'a> MatchCheckCtx<'a> {
302 pub(super) fn is_uninhabited(&self, _ty: &Ty) -> bool {
303 // FIXME(iDawer) implement exhaustive_patterns feature. More info in:
304 // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085
305 false
306 }
307
308 /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
309 pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool {
310 let has_non_exhaustive_attr =
311 self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists();
312 let is_local =
313 hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
314 has_non_exhaustive_attr && !is_local
315 }
316
317 // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
318 pub(super) fn feature_exhaustive_patterns(&self) -> bool {
319 // FIXME see MatchCheckCtx::is_uninhabited
320 false
321 }
322
323 pub(super) fn alloc_pat(&self, pat: Pat) -> PatId {
324 self.pattern_arena.borrow_mut().alloc(pat)
325 }
326
327 /// Get type of a pattern. Handles expanded patterns.
328 pub(super) fn type_of(&self, pat: PatId) -> Ty {
329 self.pattern_arena.borrow()[pat].ty.clone()
330 }
331
332 #[track_caller]
333 pub(super) fn bug(&self, info: &str) -> ! {
334 (self.eprint_panic_context)();
335 panic!("bug: {}", info);
336 }
337}
338
339#[derive(Copy, Clone)]
340pub(super) struct PatCtxt<'a> {
341 pub(super) cx: &'a MatchCheckCtx<'a>,
342 /// Type of the current column under investigation.
343 pub(super) ty: &'a Ty,
344 /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
345 /// subpattern.
346 pub(super) is_top_level: bool,
347}
348
349pub(crate) fn expand_pattern(pat: Pat) -> Pat {
350 LiteralExpander.fold_pattern(&pat)
351}
352
353struct LiteralExpander;
354
355impl PatternFolder for LiteralExpander {
356 fn fold_pattern(&mut self, pat: &Pat) -> Pat {
357 match (pat.ty.kind(&Interner), pat.kind.as_ref()) {
358 (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self),
359 _ => pat.super_fold_with(self),
360 }
361 }
362}
363
364impl Pat {
365 fn _is_wildcard(&self) -> bool {
366 matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild)
367 }
368}
369
370impl PatIdExt for PatId {
371 fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool {
372 matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. })
373 }
374
375 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
376 fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self> {
377 fn expand(pat: PatId, vec: &mut Vec<PatId>, pat_arena: &mut PatternArena) {
378 if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() {
379 // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
380 // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
381 let pats = pats.clone();
382 for pat in pats {
383 let pat = pat_arena.alloc(pat.clone());
384 expand(pat, vec, pat_arena);
385 }
386 } else {
387 vec.push(pat)
388 }
389 }
390
391 let mut pat_arena = cx.pattern_arena.borrow_mut();
392 let mut pats = Vec::new();
393 expand(self, &mut pats, &mut pat_arena);
394 pats
395 }
396}
397
398/// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
399/// works well.
400#[derive(Clone)]
401pub(super) struct PatStack {
402 pats: SmallVec<[PatId; 2]>,
403 /// Cache for the constructor of the head
404 head_ctor: OnceCell<Constructor>,
405}
406
407impl PatStack {
408 fn from_pattern(pat: PatId) -> Self {
409 Self::from_vec(smallvec![pat])
410 }
411
412 fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self {
413 PatStack { pats: vec, head_ctor: OnceCell::new() }
414 }
415
416 fn is_empty(&self) -> bool {
417 self.pats.is_empty()
418 }
419
420 fn len(&self) -> usize {
421 self.pats.len()
422 }
423
424 fn head(&self) -> PatId {
425 self.pats[0]
426 }
427
428 #[inline]
429 fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor {
430 self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head()))
431 }
432
433 // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an
434 // or-pattern. Panics if `self` is empty.
435 fn expand_or_pat(&self, cx: &MatchCheckCtx<'_>) -> impl Iterator<Item = PatStack> + '_ {
436 self.head().expand_or_pat(cx).into_iter().map(move |pat| {
437 let mut new_patstack = PatStack::from_pattern(pat);
438 new_patstack.pats.extend_from_slice(&self.pats[1..]);
439 new_patstack
440 })
441 }
442
443 /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations.
444 ///
445 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
446 /// fields filled with wild patterns.
447 ///
448 /// This is roughly the inverse of `Constructor::apply`.
449 fn pop_head_constructor(
450 &self,
451 ctor_wild_subpatterns: &Fields,
452 cx: &MatchCheckCtx<'_>,
453 ) -> PatStack {
454 // We pop the head pattern and push the new fields extracted from the arguments of
455 // `self.head()`.
456 let mut new_fields =
457 ctor_wild_subpatterns.replace_with_pattern_arguments(self.head(), cx).into_patterns();
458 new_fields.extend_from_slice(&self.pats[1..]);
459 PatStack::from_vec(new_fields)
460 }
461}
462
463impl Default for PatStack {
464 fn default() -> Self {
465 Self::from_vec(smallvec![])
466 }
467}
468
469impl PartialEq for PatStack {
470 fn eq(&self, other: &Self) -> bool {
471 self.pats == other.pats
472 }
473}
474
475impl FromIterator<PatId> for PatStack {
476 fn from_iter<T>(iter: T) -> Self
477 where
478 T: IntoIterator<Item = PatId>,
479 {
480 Self::from_vec(iter.into_iter().collect())
481 }
482}
483
484/// A 2D matrix.
485#[derive(Clone)]
486pub(super) struct Matrix {
487 patterns: Vec<PatStack>,
488}
489
490impl Matrix {
491 fn empty() -> Self {
492 Matrix { patterns: vec![] }
493 }
494
495 /// Number of columns of this matrix. `None` is the matrix is empty.
496 pub(super) fn _column_count(&self) -> Option<usize> {
497 self.patterns.get(0).map(|r| r.len())
498 }
499
500 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
501 /// expands it.
502 fn push(&mut self, row: PatStack, cx: &MatchCheckCtx<'_>) {
503 if !row.is_empty() && row.head().is_or_pat(cx) {
504 for row in row.expand_or_pat(cx) {
505 self.patterns.push(row);
506 }
507 } else {
508 self.patterns.push(row);
509 }
510 }
511
512 /// Iterate over the first component of each row
513 fn heads(&self) -> impl Iterator<Item = PatId> + '_ {
514 self.patterns.iter().map(|r| r.head())
515 }
516
517 /// Iterate over the first constructor of each row.
518 fn head_ctors<'a>(
519 &'a self,
520 cx: &'a MatchCheckCtx<'_>,
521 ) -> impl Iterator<Item = &'a Constructor> + Clone {
522 self.patterns.iter().map(move |r| r.head_ctor(cx))
523 }
524
525 /// This computes `S(constructor, self)`. See top of the file for explanations.
526 fn specialize_constructor(
527 &self,
528 pcx: PatCtxt<'_>,
529 ctor: &Constructor,
530 ctor_wild_subpatterns: &Fields,
531 ) -> Matrix {
532 let rows = self
533 .patterns
534 .iter()
535 .filter(|r| ctor.is_covered_by(pcx, r.head_ctor(pcx.cx)))
536 .map(|r| r.pop_head_constructor(ctor_wild_subpatterns, pcx.cx));
537 Matrix::from_iter(rows, pcx.cx)
538 }
539
540 fn from_iter(rows: impl IntoIterator<Item = PatStack>, cx: &MatchCheckCtx<'_>) -> Matrix {
541 let mut matrix = Matrix::empty();
542 for x in rows {
543 // Using `push` ensures we correctly expand or-patterns.
544 matrix.push(x, cx);
545 }
546 matrix
547 }
548}
549
550/// Given a pattern or a pattern-stack, this struct captures a set of its subpatterns. We use that
551/// to track reachable sub-patterns arising from or-patterns. In the absence of or-patterns this
552/// will always be either `Empty` (the whole pattern is unreachable) or `Full` (the whole pattern
553/// is reachable). When there are or-patterns, some subpatterns may be reachable while others
554/// aren't. In this case the whole pattern still counts as reachable, but we will lint the
555/// unreachable subpatterns.
556///
557/// This supports a limited set of operations, so not all possible sets of subpatterns can be
558/// represented. That's ok, we only want the ones that make sense for our usage.
559///
560/// What we're doing is illustrated by this:
561/// ```
562/// match (true, 0) {
563/// (true, 0) => {}
564/// (_, 1) => {}
565/// (true | false, 0 | 1) => {}
566/// }
567/// ```
568/// When we try the alternatives of the `true | false` or-pattern, the last `0` is reachable in the
569/// `false` alternative but not the `true`. So overall it is reachable. By contrast, the last `1`
570/// is not reachable in either alternative, so we want to signal this to the user.
571/// Therefore we take the union of sets of reachable patterns coming from different alternatives in
572/// order to figure out which subpatterns are overall reachable.
573///
574/// Invariant: we try to construct the smallest representation we can. In particular if
575/// `self.is_empty()` we ensure that `self` is `Empty`, and same with `Full`. This is not important
576/// for correctness currently.
577#[derive(Debug, Clone)]
578enum SubPatSet {
579 /// The empty set. This means the pattern is unreachable.
580 Empty,
581 /// The set containing the full pattern.
582 Full,
583 /// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each
584 /// of its subpatterns. Missing entries in the map are implicitly full, because that's the
585 /// common case.
586 Seq { subpats: FxHashMap<usize, SubPatSet> },
587 /// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing
588 /// entries in the map are implicitly empty. Note: we always flatten nested or-patterns.
589 Alt {
590 subpats: FxHashMap<usize, SubPatSet>,
591 /// Counts the total number of alternatives in the pattern
592 alt_count: usize,
593 /// We keep the pattern around to retrieve spans.
594 pat: PatId,
595 },
596}
597
598impl SubPatSet {
599 fn full() -> Self {
600 SubPatSet::Full
601 }
602
603 fn empty() -> Self {
604 SubPatSet::Empty
605 }
606
607 fn is_empty(&self) -> bool {
608 match self {
609 SubPatSet::Empty => true,
610 SubPatSet::Full => false,
611 // If any subpattern in a sequence is unreachable, the whole pattern is unreachable.
612 SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_empty()),
613 // An or-pattern is reachable if any of its alternatives is.
614 SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_empty()),
615 }
616 }
617
618 fn is_full(&self) -> bool {
619 match self {
620 SubPatSet::Empty => false,
621 SubPatSet::Full => true,
622 // The whole pattern is reachable only when all its alternatives are.
623 SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_full()),
624 // The whole or-pattern is reachable only when all its alternatives are.
625 SubPatSet::Alt { subpats, alt_count, .. } => {
626 subpats.len() == *alt_count && subpats.values().all(|set| set.is_full())
627 }
628 }
629 }
630
631 /// Union `self` with `other`, mutating `self`.
632 fn union(&mut self, other: Self) {
633 use SubPatSet::*;
634 // Union with full stays full; union with empty changes nothing.
635 if self.is_full() || other.is_empty() {
636 return;
637 } else if self.is_empty() {
638 *self = other;
639 return;
640 } else if other.is_full() {
641 *self = Full;
642 return;
643 }
644
645 match (&mut *self, other) {
646 (Seq { subpats: s_set }, Seq { subpats: mut o_set }) => {
647 s_set.retain(|i, s_sub_set| {
648 // Missing entries count as full.
649 let o_sub_set = o_set.remove(&i).unwrap_or(Full);
650 s_sub_set.union(o_sub_set);
651 // We drop full entries.
652 !s_sub_set.is_full()
653 });
654 // Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since
655 // unioning with full returns full, we can drop those entries.
656 }
657 (Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => {
658 s_set.retain(|i, s_sub_set| {
659 // Missing entries count as empty.
660 let o_sub_set = o_set.remove(&i).unwrap_or(Empty);
661 s_sub_set.union(o_sub_set);
662 // We drop empty entries.
663 !s_sub_set.is_empty()
664 });
665 // Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since
666 // unioning with empty changes nothing, we can take those entries as is.
667 s_set.extend(o_set);
668 }
669 _ => panic!("bug"),
670 }
671
672 if self.is_full() {
673 *self = Full;
674 }
675 }
676
677 /// Returns a list of the unreachable subpatterns. If `self` is empty (i.e. the
678 /// whole pattern is unreachable) we return `None`.
679 fn list_unreachable_subpatterns(&self, cx: &MatchCheckCtx<'_>) -> Option<Vec<PatId>> {
680 /// Panics if `set.is_empty()`.
681 fn fill_subpats(
682 set: &SubPatSet,
683 unreachable_pats: &mut Vec<PatId>,
684 cx: &MatchCheckCtx<'_>,
685 ) {
686 match set {
687 SubPatSet::Empty => panic!("bug"),
688 SubPatSet::Full => {}
689 SubPatSet::Seq { subpats } => {
690 for (_, sub_set) in subpats {
691 fill_subpats(sub_set, unreachable_pats, cx);
692 }
693 }
694 SubPatSet::Alt { subpats, pat, alt_count, .. } => {
695 let expanded = pat.expand_or_pat(cx);
696 for i in 0..*alt_count {
697 let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Empty);
698 if sub_set.is_empty() {
699 // Found a unreachable subpattern.
700 unreachable_pats.push(expanded[i]);
701 } else {
702 fill_subpats(sub_set, unreachable_pats, cx);
703 }
704 }
705 }
706 }
707 }
708
709 if self.is_empty() {
710 return None;
711 }
712 if self.is_full() {
713 // No subpatterns are unreachable.
714 return Some(Vec::new());
715 }
716 let mut unreachable_pats = Vec::new();
717 fill_subpats(self, &mut unreachable_pats, cx);
718 Some(unreachable_pats)
719 }
720
721 /// When `self` refers to a patstack that was obtained from specialization, after running
722 /// `unspecialize` it will refer to the original patstack before specialization.
723 fn unspecialize(self, arity: usize) -> Self {
724 use SubPatSet::*;
725 match self {
726 Full => Full,
727 Empty => Empty,
728 Seq { subpats } => {
729 // We gather the first `arity` subpatterns together and shift the remaining ones.
730 let mut new_subpats = FxHashMap::default();
731 let mut new_subpats_first_col = FxHashMap::default();
732 for (i, sub_set) in subpats {
733 if i < arity {
734 // The first `arity` indices are now part of the pattern in the first
735 // column.
736 new_subpats_first_col.insert(i, sub_set);
737 } else {
738 // Indices after `arity` are simply shifted
739 new_subpats.insert(i - arity + 1, sub_set);
740 }
741 }
742 // If `new_subpats_first_col` has no entries it counts as full, so we can omit it.
743 if !new_subpats_first_col.is_empty() {
744 new_subpats.insert(0, Seq { subpats: new_subpats_first_col });
745 }
746 Seq { subpats: new_subpats }
747 }
748 Alt { .. } => panic!("bug"), // `self` is a patstack
749 }
750 }
751
752 /// When `self` refers to a patstack that was obtained from splitting an or-pattern, after
753 /// running `unspecialize` it will refer to the original patstack before splitting.
754 ///
755 /// For example:
756 /// ```
757 /// match Some(true) {
758 /// Some(true) => {}
759 /// None | Some(true | false) => {}
760 /// }
761 /// ```
762 /// Here `None` would return the full set and `Some(true | false)` would return the set
763 /// containing `false`. After `unsplit_or_pat`, we want the set to contain `None` and `false`.
764 /// This is what this function does.
765 fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
766 use SubPatSet::*;
767 if self.is_empty() {
768 return Empty;
769 }
770
771 // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0
772 // | 1)`.
773 let set_first_col = match &mut self {
774 Full => Full,
775 Seq { subpats } => subpats.remove(&0).unwrap_or(Full),
776 Empty => unreachable!(),
777 Alt { .. } => panic!("bug"), // `self` is a patstack
778 };
779 let mut subpats_first_col = FxHashMap::default();
780 subpats_first_col.insert(alt_id, set_first_col);
781 let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count };
782
783 let mut subpats = match self {
784 Full => FxHashMap::default(),
785 Seq { subpats } => subpats,
786 Empty => unreachable!(),
787 Alt { .. } => panic!("bug"), // `self` is a patstack
788 };
789 subpats.insert(0, set_first_col);
790 Seq { subpats }
791 }
792}
793
794/// This carries the results of computing usefulness, as described at the top of the file. When
795/// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track
796/// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
797/// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
798/// witnesses of non-exhaustiveness when there are any.
799/// Which variant to use is dictated by `WitnessPreference`.
800#[derive(Clone, Debug)]
801enum Usefulness {
802 /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates
803 /// the whole pattern is unreachable. If not, this indicates that the pattern is reachable but
804 /// that some sub-patterns may be unreachable (due to or-patterns). In the absence of
805 /// or-patterns this will always be either `Empty` (the whole pattern is unreachable) or `Full`
806 /// (the whole pattern is reachable).
807 NoWitnesses(SubPatSet),
808 /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole
809 /// pattern is unreachable.
810 WithWitnesses(Vec<Witness>),
811}
812
813impl Usefulness {
814 fn new_useful(preference: WitnessPreference) -> Self {
815 match preference {
816 ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
817 LeaveOutWitness => NoWitnesses(SubPatSet::full()),
818 }
819 }
820 fn new_not_useful(preference: WitnessPreference) -> Self {
821 match preference {
822 ConstructWitness => WithWitnesses(vec![]),
823 LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
824 }
825 }
826
827 /// Combine usefulnesses from two branches. This is an associative operation.
828 fn extend(&mut self, other: Self) {
829 match (&mut *self, other) {
830 (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {}
831 (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o),
832 (WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
833 (NoWitnesses(s), NoWitnesses(o)) => s.union(o),
834 _ => unreachable!(),
835 }
836 }
837
838 /// When trying several branches and each returns a `Usefulness`, we need to combine the
839 /// results together.
840 fn merge(pref: WitnessPreference, usefulnesses: impl Iterator<Item = Self>) -> Self {
841 let mut ret = Self::new_not_useful(pref);
842 for u in usefulnesses {
843 ret.extend(u);
844 if let NoWitnesses(subpats) = &ret {
845 if subpats.is_full() {
846 // Once we reach the full set, more unions won't change the result.
847 return ret;
848 }
849 }
850 }
851 ret
852 }
853
854 /// After calculating the usefulness for a branch of an or-pattern, call this to make this
855 /// usefulness mergeable with those from the other branches.
856 fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
857 match self {
858 NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)),
859 WithWitnesses(_) => panic!("bug"),
860 }
861 }
862
863 /// After calculating usefulness after a specialization, call this to recontruct a usefulness
864 /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
865 /// with the results of specializing with the other constructors.
866 fn apply_constructor(
867 self,
868 pcx: PatCtxt<'_>,
869 matrix: &Matrix,
870 ctor: &Constructor,
871 ctor_wild_subpatterns: &Fields,
872 ) -> Self {
873 match self {
874 WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses),
875 WithWitnesses(witnesses) => {
876 let new_witnesses = if matches!(ctor, Constructor::Missing) {
877 let mut split_wildcard = SplitWildcard::new(pcx);
878 split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
879 // Construct for each missing constructor a "wild" version of this
880 // constructor, that matches everything that can be built with
881 // it. For example, if `ctor` is a `Constructor::Variant` for
882 // `Option::Some`, we get the pattern `Some(_)`.
883 let new_patterns: Vec<_> = split_wildcard
884 .iter_missing(pcx)
885 .map(|missing_ctor| {
886 Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
887 })
888 .collect();
889 witnesses
890 .into_iter()
891 .flat_map(|witness| {
892 new_patterns.iter().map(move |pat| {
893 let mut witness = witness.clone();
894 witness.0.push(pat.clone());
895 witness
896 })
897 })
898 .collect()
899 } else {
900 witnesses
901 .into_iter()
902 .map(|witness| witness.apply_constructor(pcx, &ctor, ctor_wild_subpatterns))
903 .collect()
904 };
905 WithWitnesses(new_witnesses)
906 }
907 NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
908 }
909 }
910}
911
912#[derive(Copy, Clone, Debug)]
913enum WitnessPreference {
914 ConstructWitness,
915 LeaveOutWitness,
916}
917
918/// A witness of non-exhaustiveness for error reporting, represented
919/// as a list of patterns (in reverse order of construction) with
920/// wildcards inside to represent elements that can take any inhabitant
921/// of the type as a value.
922///
923/// A witness against a list of patterns should have the same types
924/// and length as the pattern matched against. Because Rust `match`
925/// is always against a single pattern, at the end the witness will
926/// have length 1, but in the middle of the algorithm, it can contain
927/// multiple patterns.
928///
929/// For example, if we are constructing a witness for the match against
930///
931/// ```
932/// struct Pair(Option<(u32, u32)>, bool);
933///
934/// match (p: Pair) {
935/// Pair(None, _) => {}
936/// Pair(_, false) => {}
937/// }
938/// ```
939///
940/// We'll perform the following steps:
941/// 1. Start with an empty witness
942/// `Witness(vec![])`
943/// 2. Push a witness `true` against the `false`
944/// `Witness(vec![true])`
945/// 3. Push a witness `Some(_)` against the `None`
946/// `Witness(vec![true, Some(_)])`
947/// 4. Apply the `Pair` constructor to the witnesses
948/// `Witness(vec![Pair(Some(_), true)])`
949///
950/// The final `Pair(Some(_), true)` is then the resulting witness.
951#[derive(Clone, Debug)]
952pub(crate) struct Witness(Vec<Pat>);
953
954impl Witness {
955 /// Asserts that the witness contains a single pattern, and returns it.
956 fn single_pattern(self) -> Pat {
957 assert_eq!(self.0.len(), 1);
958 self.0.into_iter().next().unwrap()
959 }
960
961 /// Constructs a partial witness for a pattern given a list of
962 /// patterns expanded by the specialization step.
963 ///
964 /// When a pattern P is discovered to be useful, this function is used bottom-up
965 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
966 /// of values, V, where each value in that set is not covered by any previously
967 /// used patterns and is covered by the pattern P'. Examples:
968 ///
969 /// left_ty: tuple of 3 elements
970 /// pats: [10, 20, _] => (10, 20, _)
971 ///
972 /// left_ty: struct X { a: (bool, &'static str), b: usize}
973 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
974 fn apply_constructor(
975 mut self,
976 pcx: PatCtxt<'_>,
977 ctor: &Constructor,
978 ctor_wild_subpatterns: &Fields,
979 ) -> Self {
980 let pat = {
981 let len = self.0.len();
982 let arity = ctor_wild_subpatterns.len();
983 let pats = self.0.drain((len - arity)..).rev();
984 ctor_wild_subpatterns.replace_fields(pcx.cx, pats).apply(pcx, ctor)
985 };
986
987 self.0.push(pat);
988
989 self
990 }
991}
992
993/// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
994/// The algorithm from the paper has been modified to correctly handle empty
995/// types. The changes are:
996/// (0) We don't exit early if the pattern matrix has zero rows. We just
997/// continue to recurse over columns.
998/// (1) all_constructors will only return constructors that are statically
999/// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1000///
1001/// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1002/// to a set of such vectors `m` - this is defined as there being a set of
1003/// inputs that will match `v` but not any of the sets in `m`.
1004///
1005/// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
1006///
1007/// This is used both for reachability checking (if a pattern isn't useful in
1008/// relation to preceding patterns, it is not reachable) and exhaustiveness
1009/// checking (if a wildcard pattern is useful in relation to a matrix, the
1010/// matrix isn't exhaustive).
1011///
1012/// `is_under_guard` is used to inform if the pattern has a guard. If it
1013/// has one it must not be inserted into the matrix. This shouldn't be
1014/// relied on for soundness.
1015fn is_useful(
1016 cx: &MatchCheckCtx<'_>,
1017 matrix: &Matrix,
1018 v: &PatStack,
1019 witness_preference: WitnessPreference,
1020 is_under_guard: bool,
1021 is_top_level: bool,
1022) -> Usefulness {
1023 let Matrix { patterns: rows, .. } = matrix;
1024
1025 // The base case. We are pattern-matching on () and the return value is
1026 // based on whether our matrix has a row or not.
1027 // NOTE: This could potentially be optimized by checking rows.is_empty()
1028 // first and then, if v is non-empty, the return value is based on whether
1029 // the type of the tuple we're checking is inhabited or not.
1030 if v.is_empty() {
1031 let ret = if rows.is_empty() {
1032 Usefulness::new_useful(witness_preference)
1033 } else {
1034 Usefulness::new_not_useful(witness_preference)
1035 };
1036 return ret;
1037 }
1038
1039 assert!(rows.iter().all(|r| r.len() == v.len()));
1040
1041 // FIXME(Nadrieril): Hack to work around type normalization issues (see rust-lang/rust#72476).
1042 let ty = matrix.heads().next().map_or(cx.type_of(v.head()), |r| cx.type_of(r));
1043 let pcx = PatCtxt { cx, ty: &ty, is_top_level };
1044
1045 // If the first pattern is an or-pattern, expand it.
1046 let ret = if v.head().is_or_pat(cx) {
1047 //expanding or-pattern
1048 let v_head = v.head();
1049 let vs: Vec<_> = v.expand_or_pat(cx).collect();
1050 let alt_count = vs.len();
1051 // We try each or-pattern branch in turn.
1052 let mut matrix = matrix.clone();
1053 let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| {
1054 let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false);
1055 // If pattern has a guard don't add it to the matrix.
1056 if !is_under_guard {
1057 // We push the already-seen patterns into the matrix in order to detect redundant
1058 // branches like `Some(_) | Some(0)`.
1059 matrix.push(v, cx);
1060 }
1061 usefulness.unsplit_or_pat(i, alt_count, v_head)
1062 });
1063 Usefulness::merge(witness_preference, usefulnesses)
1064 } else {
1065 let v_ctor = v.head_ctor(cx);
1066 // if let Constructor::IntRange(ctor_range) = v_ctor {
1067 // // Lint on likely incorrect range patterns (#63987)
1068 // ctor_range.lint_overlapping_range_endpoints(
1069 // pcx,
1070 // matrix.head_ctors_and_spans(cx),
1071 // matrix.column_count().unwrap_or(0),
1072 // hir_id,
1073 // )
1074 // }
1075
1076 // We split the head constructor of `v`.
1077 let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx));
1078 // For each constructor, we compute whether there's a value that starts with it that would
1079 // witness the usefulness of `v`.
1080 let start_matrix = matrix;
1081 let usefulnesses = split_ctors.into_iter().map(|ctor| {
1082 // debug!("specialize({:?})", ctor);
1083 // We cache the result of `Fields::wildcards` because it is used a lot.
1084 let ctor_wild_subpatterns = Fields::wildcards(pcx, &ctor);
1085 let spec_matrix =
1086 start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns);
1087 let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx);
1088 let usefulness =
1089 is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
1090 usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
1091 });
1092 Usefulness::merge(witness_preference, usefulnesses)
1093 };
1094
1095 ret
1096}
1097
1098/// The arm of a match expression.
1099#[derive(Clone, Copy)]
1100pub(crate) struct MatchArm {
1101 pub(crate) pat: PatId,
1102 pub(crate) has_guard: bool,
1103}
1104
1105/// Indicates whether or not a given arm is reachable.
1106#[derive(Clone, Debug)]
1107pub(crate) enum Reachability {
1108 /// The arm is reachable. This additionally carries a set of or-pattern branches that have been
1109 /// found to be unreachable despite the overall arm being reachable. Used only in the presence
1110 /// of or-patterns, otherwise it stays empty.
1111 Reachable(Vec<PatId>),
1112 /// The arm is unreachable.
1113 Unreachable,
1114}
1115
1116/// The output of checking a match for exhaustiveness and arm reachability.
1117pub(crate) struct UsefulnessReport {
1118 /// For each arm of the input, whether that arm is reachable after the arms above it.
1119 pub(crate) _arm_usefulness: Vec<(MatchArm, Reachability)>,
1120 /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
1121 /// exhaustiveness.
1122 pub(crate) non_exhaustiveness_witnesses: Vec<Pat>,
1123}
1124
1125/// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
1126/// of its arms are reachable.
1127///
1128/// Note: the input patterns must have been lowered through
1129/// `check_match::MatchVisitor::lower_pattern`.
1130pub(crate) fn compute_match_usefulness(
1131 cx: &MatchCheckCtx<'_>,
1132 arms: &[MatchArm],
1133) -> UsefulnessReport {
1134 let mut matrix = Matrix::empty();
1135 let arm_usefulness: Vec<_> = arms
1136 .iter()
1137 .copied()
1138 .map(|arm| {
1139 let v = PatStack::from_pattern(arm.pat);
1140 let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true);
1141 if !arm.has_guard {
1142 matrix.push(v, cx);
1143 }
1144 let reachability = match usefulness {
1145 NoWitnesses(subpats) if subpats.is_empty() => Reachability::Unreachable,
1146 NoWitnesses(subpats) => {
1147 Reachability::Reachable(subpats.list_unreachable_subpatterns(cx).unwrap())
1148 }
1149 WithWitnesses(..) => panic!("bug"),
1150 };
1151 (arm, reachability)
1152 })
1153 .collect();
1154
1155 let wild_pattern =
1156 cx.pattern_arena.borrow_mut().alloc(Pat::wildcard_from_ty(cx.infer[cx.match_expr].clone()));
1157 let v = PatStack::from_pattern(wild_pattern);
1158 let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, false, true);
1159 let non_exhaustiveness_witnesses = match usefulness {
1160 WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(),
1161 NoWitnesses(_) => panic!("bug"),
1162 };
1163 UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
1164}
1165
1166pub(crate) type PatternArena = Arena<Pat>;
1167
1168mod helper {
1169 use super::MatchCheckCtx;
1170
1171 pub(super) trait PatIdExt: Sized {
1172 // fn is_wildcard(self, cx: &MatchCheckCtx<'_>) -> bool;
1173 fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool;
1174 fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self>;
1175 }
1176
1177 // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs
1178 /// "Signaling" trait used in impl trait to tag lifetimes that you may
1179 /// need to capture but don't really need for other reasons.
1180 /// Basically a workaround; see [this comment] for details.
1181 ///
1182 /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
1183 // FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed.
1184 #[allow(unused_lifetimes)]
1185 pub(crate) trait Captures<'a> {}
1186
1187 impl<'a, T: ?Sized> Captures<'a> for T {}
1188}