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1//! This module implements match statement exhaustiveness checking and usefulness checking
2//! for match arms.
3//!
4//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which
5//! contains very detailed documentation about the algorithms used here. I've duplicated
6//! most of that documentation below.
7//!
8//! This file includes the logic for exhaustiveness and usefulness checking for
9//! pattern-matching. Specifically, given a list of patterns for a type, we can
10//! tell whether:
11//! - (a) the patterns cover every possible constructor for the type (exhaustiveness).
12//! - (b) each pattern is necessary (usefulness).
13//!
14//! The algorithm implemented here is a modified version of the one described in
15//! <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
16//! However, to save future implementors from reading the original paper, we
17//! summarise 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 realised 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::sync::Arc;
220
221use hir_def::{
222 adt::VariantData,
223 body::Body,
224 expr::{Expr, Literal, Pat, PatId},
225 AdtId, EnumVariantId, VariantId,
226};
227use ra_arena::Idx;
228use smallvec::{smallvec, SmallVec};
229
230use crate::{db::HirDatabase, ApplicationTy, InferenceResult, Ty, TypeCtor};
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
246impl PatIdOrWild {
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
254 fn as_id(self) -> Option<PatId> {
255 match self {
256 PatIdOrWild::PatId(id) => Some(id),
257 PatIdOrWild::Wild => None,
258 }
259 }
260}
261
262impl From<PatId> for PatIdOrWild {
263 fn from(pat_id: PatId) -> Self {
264 Self::PatId(pat_id)
265 }
266}
267
268impl From<&PatId> for PatIdOrWild {
269 fn from(pat_id: &PatId) -> Self {
270 Self::PatId(*pat_id)
271 }
272}
273
274#[derive(Debug, Clone, Copy, PartialEq)]
275pub(super) enum MatchCheckErr {
276 NotImplemented,
277 MalformedMatchArm,
278 /// Used when type inference cannot resolve the type of
279 /// a pattern or expression.
280 Unknown,
281}
282
283/// The return type of `is_useful` is either an indication of usefulness
284/// of the match arm, or an error in the case the match statement
285/// is made up of types for which exhaustiveness checking is currently
286/// not completely implemented.
287///
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
292#[derive(Debug)]
293/// A row in a Matrix.
294///
295/// This type is modeled from the struct of the same name in `rustc`.
296pub(super) struct PatStack(PatStackInner);
297type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
298
299impl PatStack {
300 pub(super) fn from_pattern(pat_id: PatId) -> PatStack {
301 Self(smallvec!(pat_id.into()))
302 }
303
304 pub(super) fn from_wild() -> PatStack {
305 Self(smallvec!(PatIdOrWild::Wild))
306 }
307
308 fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
309 Self(SmallVec::from_slice(slice))
310 }
311
312 fn from_vec(v: PatStackInner) -> PatStack {
313 Self(v)
314 }
315
316 fn get_head(&self) -> Option<PatIdOrWild> {
317 self.0.first().copied()
318 }
319
320 fn tail(&self) -> &[PatIdOrWild] {
321 self.0.get(1..).unwrap_or(&[])
322 }
323
324 fn to_tail(&self) -> PatStack {
325 Self::from_slice(self.tail())
326 }
327
328 fn replace_head_with<I, T>(&self, pats: I) -> PatStack
329 where
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
343 /// Computes `D(self)`.
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
354 /// Computes `S(constructor, self)`.
355 ///
356 /// See the module docs and the associated documentation in rustc for details.
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
367 let head_pat = head.as_pat(cx);
368 let result = match (head_pat, constructor) {
369 (Pat::Tuple { args: ref pat_ids, ellipsis }, Constructor::Tuple { arity: _ }) => {
370 if ellipsis.is_some() {
371 // If there are ellipsis here, we should add the correct number of
372 // Pat::Wild patterns to `pat_ids`. We should be able to use the
373 // constructors arity for this, but at the time of writing we aren't
374 // correctly calculating this arity when ellipsis are present.
375 return Err(MatchCheckErr::NotImplemented);
376 }
377
378 Some(self.replace_head_with(pat_ids.iter()))
379 }
380 (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => {
381 match cx.body.exprs[lit_expr] {
382 Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => {
383 Some(self.to_tail())
384 }
385 // it was a bool but the value doesn't match
386 Expr::Literal(Literal::Bool(_)) => None,
387 // perhaps this is actually unreachable given we have
388 // already checked that these match arms have the appropriate type?
389 _ => return Err(MatchCheckErr::NotImplemented),
390 }
391 }
392 (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?),
393 (Pat::Path(_), Constructor::Enum(constructor)) => {
394 // unit enum variants become `Pat::Path`
395 let pat_id = head.as_id().expect("we know this isn't a wild");
396 if !enum_variant_matches(cx, pat_id, *constructor) {
397 None
398 } else {
399 Some(self.to_tail())
400 }
401 }
402 (
403 Pat::TupleStruct { args: ref pat_ids, ellipsis, .. },
404 Constructor::Enum(enum_constructor),
405 ) => {
406 let pat_id = head.as_id().expect("we know this isn't a wild");
407 if !enum_variant_matches(cx, pat_id, *enum_constructor) {
408 None
409 } else {
410 let constructor_arity = constructor.arity(cx)?;
411 if let Some(ellipsis_position) = ellipsis {
412 // If there are ellipsis in the pattern, the ellipsis must take the place
413 // of at least one sub-pattern, so `pat_ids` should be smaller than the
414 // constructor arity.
415 if pat_ids.len() < constructor_arity {
416 let mut new_patterns: Vec<PatIdOrWild> = vec![];
417
418 for pat_id in &pat_ids[0..ellipsis_position] {
419 new_patterns.push((*pat_id).into());
420 }
421
422 for _ in 0..(constructor_arity - pat_ids.len()) {
423 new_patterns.push(PatIdOrWild::Wild);
424 }
425
426 for pat_id in &pat_ids[ellipsis_position..pat_ids.len()] {
427 new_patterns.push((*pat_id).into());
428 }
429
430 Some(self.replace_head_with(new_patterns.into_iter()))
431 } else {
432 return Err(MatchCheckErr::MalformedMatchArm);
433 }
434 } else {
435 // If there is no ellipsis in the tuple pattern, the number
436 // of patterns must equal the constructor arity.
437 if pat_ids.len() == constructor_arity {
438 Some(self.replace_head_with(pat_ids.into_iter()))
439 } else {
440 return Err(MatchCheckErr::MalformedMatchArm);
441 }
442 }
443 }
444 }
445 (Pat::Record { args: ref arg_patterns, .. }, Constructor::Enum(e)) => {
446 let pat_id = head.as_id().expect("we know this isn't a wild");
447 if !enum_variant_matches(cx, pat_id, *e) {
448 None
449 } else {
450 match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
451 VariantData::Record(struct_field_arena) => {
452 // Here we treat any missing fields in the record as the wild pattern, as
453 // if the record has ellipsis. We want to do this here even if the
454 // record does not contain ellipsis, because it allows us to continue
455 // enforcing exhaustiveness for the rest of the match statement.
456 //
457 // Creating the diagnostic for the missing field in the pattern
458 // should be done in a different diagnostic.
459 let patterns = struct_field_arena.iter().map(|(_, struct_field)| {
460 arg_patterns
461 .iter()
462 .find(|pat| pat.name == struct_field.name)
463 .map(|pat| PatIdOrWild::from(pat.pat))
464 .unwrap_or(PatIdOrWild::Wild)
465 });
466
467 Some(self.replace_head_with(patterns))
468 }
469 _ => return Err(MatchCheckErr::Unknown),
470 }
471 }
472 }
473 (Pat::Or(_), _) => return Err(MatchCheckErr::NotImplemented),
474 (_, _) => return Err(MatchCheckErr::NotImplemented),
475 };
476
477 Ok(result)
478 }
479
480 /// A special case of `specialize_constructor` where the head of the pattern stack
481 /// is a Wild pattern.
482 ///
483 /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns
484 /// (N >= 0), where N is the arity of the given constructor.
485 fn expand_wildcard(
486 &self,
487 cx: &MatchCheckCtx,
488 constructor: &Constructor,
489 ) -> MatchCheckResult<PatStack> {
490 assert_eq!(
491 Pat::Wild,
492 self.get_head().expect("expand_wildcard called on empty PatStack").as_pat(cx),
493 "expand_wildcard must only be called on PatStack with wild at head",
494 );
495
496 let mut patterns: PatStackInner = smallvec![];
497
498 for _ in 0..constructor.arity(cx)? {
499 patterns.push(PatIdOrWild::Wild);
500 }
501
502 for pat in &self.0[1..] {
503 patterns.push(*pat);
504 }
505
506 Ok(PatStack::from_vec(patterns))
507 }
508}
509
510/// A collection of PatStack.
511///
512/// This type is modeled from the struct of the same name in `rustc`.
513pub(super) struct Matrix(Vec<PatStack>);
514
515impl Matrix {
516 pub(super) fn empty() -> Self {
517 Self(vec![])
518 }
519
520 pub(super) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
521 if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
522 // Or patterns are expanded here
523 for pat_id in pat_ids {
524 self.0.push(PatStack::from_pattern(pat_id));
525 }
526 } else {
527 self.0.push(row);
528 }
529 }
530
531 fn is_empty(&self) -> bool {
532 self.0.is_empty()
533 }
534
535 fn heads(&self) -> Vec<PatIdOrWild> {
536 self.0.iter().flat_map(|p| p.get_head()).collect()
537 }
538
539 /// Computes `D(self)` for each contained PatStack.
540 ///
541 /// See the module docs and the associated documentation in rustc for details.
542 fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
543 Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
544 }
545
546 /// Computes `S(constructor, self)` for each contained PatStack.
547 ///
548 /// See the module docs and the associated documentation in rustc for details.
549 fn specialize_constructor(
550 &self,
551 cx: &MatchCheckCtx,
552 constructor: &Constructor,
553 ) -> MatchCheckResult<Self> {
554 let mut new_matrix = Matrix::empty();
555 for pat in &self.0 {
556 if let Some(pat) = pat.specialize_constructor(cx, constructor)? {
557 new_matrix.push(cx, pat);
558 }
559 }
560
561 Ok(new_matrix)
562 }
563
564 fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
565 let mut matrix = Matrix::empty();
566
567 for pat in iter {
568 // using push ensures we expand or-patterns
569 matrix.push(cx, pat);
570 }
571
572 matrix
573 }
574}
575
576#[derive(Clone, Debug, PartialEq)]
577/// An indication of the usefulness of a given match arm, where
578/// usefulness is defined as matching some patterns which were
579/// not matched by an prior match arms.
580///
581/// We may eventually need an `Unknown` variant here.
582pub(super) enum Usefulness {
583 Useful,
584 NotUseful,
585}
586
587pub(super) struct MatchCheckCtx<'a> {
588 pub(super) match_expr: Idx<Expr>,
589 pub(super) body: Arc<Body>,
590 pub(super) infer: Arc<InferenceResult>,
591 pub(super) db: &'a dyn HirDatabase,
592}
593
594/// Given a set of patterns `matrix`, and pattern to consider `v`, determines
595/// whether `v` is useful. A pattern is useful if it covers cases which were
596/// not previously covered.
597///
598/// When calling this function externally (that is, not the recursive calls) it
599/// expected that you have already type checked the match arms. All patterns in
600/// matrix should be the same type as v, as well as they should all be the same
601/// type as the match expression.
602pub(super) fn is_useful(
603 cx: &MatchCheckCtx,
604 matrix: &Matrix,
605 v: &PatStack,
606) -> MatchCheckResult<Usefulness> {
607 // Handle two special cases:
608 // - enum with no variants
609 // - `!` type
610 // In those cases, no match arm is useful.
611 match cx.infer[cx.match_expr].strip_references() {
612 Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) => {
613 if cx.db.enum_data(*enum_id).variants.is_empty() {
614 return Ok(Usefulness::NotUseful);
615 }
616 }
617 Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) => {
618 return Ok(Usefulness::NotUseful);
619 }
620 _ => (),
621 }
622
623 let head = match v.get_head() {
624 Some(head) => head,
625 None => {
626 let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
627
628 return Ok(result);
629 }
630 };
631
632 if let Pat::Or(pat_ids) = head.as_pat(cx) {
633 let mut found_unimplemented = false;
634 let any_useful = pat_ids.iter().any(|&pat_id| {
635 let v = PatStack::from_pattern(pat_id);
636
637 match is_useful(cx, matrix, &v) {
638 Ok(Usefulness::Useful) => true,
639 Ok(Usefulness::NotUseful) => false,
640 _ => {
641 found_unimplemented = true;
642 false
643 }
644 }
645 });
646
647 return if any_useful {
648 Ok(Usefulness::Useful)
649 } else if found_unimplemented {
650 Err(MatchCheckErr::NotImplemented)
651 } else {
652 Ok(Usefulness::NotUseful)
653 };
654 }
655
656 if let Some(constructor) = pat_constructor(cx, head)? {
657 let matrix = matrix.specialize_constructor(&cx, &constructor)?;
658 let v = v
659 .specialize_constructor(&cx, &constructor)?
660 .expect("we know this can't fail because we get the constructor from `v.head()` above");
661
662 is_useful(&cx, &matrix, &v)
663 } else {
664 // expanding wildcard
665 let mut used_constructors: Vec<Constructor> = vec![];
666 for pat in matrix.heads() {
667 if let Some(constructor) = pat_constructor(cx, pat)? {
668 used_constructors.push(constructor);
669 }
670 }
671
672 // We assume here that the first constructor is the "correct" type. Since we
673 // only care about the "type" of the constructor (i.e. if it is a bool we
674 // don't care about the value), this assumption should be valid as long as
675 // the match statement is well formed. We currently uphold this invariant by
676 // filtering match arms before calling `is_useful`, only passing in match arms
677 // whose type matches the type of the match expression.
678 match &used_constructors.first() {
679 Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
680 // If all constructors are covered, then we need to consider whether
681 // any values are covered by this wildcard.
682 //
683 // For example, with matrix '[[Some(true)], [None]]', all
684 // constructors are covered (`Some`/`None`), so we need
685 // to perform specialization to see that our wildcard will cover
686 // the `Some(false)` case.
687 //
688 // Here we create a constructor for each variant and then check
689 // usefulness after specializing for that constructor.
690 let mut found_unimplemented = false;
691 for constructor in constructor.all_constructors(cx) {
692 let matrix = matrix.specialize_constructor(&cx, &constructor)?;
693 let v = v.expand_wildcard(&cx, &constructor)?;
694
695 match is_useful(&cx, &matrix, &v) {
696 Ok(Usefulness::Useful) => return Ok(Usefulness::Useful),
697 Ok(Usefulness::NotUseful) => continue,
698 _ => found_unimplemented = true,
699 };
700 }
701
702 if found_unimplemented {
703 Err(MatchCheckErr::NotImplemented)
704 } else {
705 Ok(Usefulness::NotUseful)
706 }
707 }
708 _ => {
709 // Either not all constructors are covered, or the only other arms
710 // are wildcards. Either way, this pattern is useful if it is useful
711 // when compared to those arms with wildcards.
712 let matrix = matrix.specialize_wildcard(&cx);
713 let v = v.to_tail();
714
715 is_useful(&cx, &matrix, &v)
716 }
717 }
718 }
719}
720
721#[derive(Debug, Clone, Copy)]
722/// Similar to TypeCtor, but includes additional information about the specific
723/// value being instantiated. For example, TypeCtor::Bool doesn't contain the
724/// boolean value.
725enum Constructor {
726 Bool(bool),
727 Tuple { arity: usize },
728 Enum(EnumVariantId),
729}
730
731impl Constructor {
732 fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> {
733 let arity = match self {
734 Constructor::Bool(_) => 0,
735 Constructor::Tuple { arity } => *arity,
736 Constructor::Enum(e) => {
737 match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
738 VariantData::Tuple(struct_field_data) => struct_field_data.len(),
739 VariantData::Record(struct_field_data) => struct_field_data.len(),
740 VariantData::Unit => 0,
741 }
742 }
743 };
744
745 Ok(arity)
746 }
747
748 fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> {
749 match self {
750 Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)],
751 Constructor::Tuple { .. } => vec![*self],
752 Constructor::Enum(e) => cx
753 .db
754 .enum_data(e.parent)
755 .variants
756 .iter()
757 .map(|(local_id, _)| {
758 Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
759 })
760 .collect(),
761 }
762 }
763}
764
765/// Returns the constructor for the given pattern. Should only return None
766/// in the case of a Wild pattern.
767fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> {
768 let res = match pat.as_pat(cx) {
769 Pat::Wild => None,
770 // FIXME somehow create the Tuple constructor with the proper arity. If there are
771 // ellipsis, the arity is not equal to the number of patterns.
772 Pat::Tuple { args: pats, ellipsis } if ellipsis.is_none() => {
773 Some(Constructor::Tuple { arity: pats.len() })
774 }
775 Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
776 Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
777 _ => return Err(MatchCheckErr::NotImplemented),
778 },
779 Pat::TupleStruct { .. } | Pat::Path(_) | Pat::Record { .. } => {
780 let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
781 let variant_id =
782 cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckErr::Unknown)?;
783 match variant_id {
784 VariantId::EnumVariantId(enum_variant_id) => {
785 Some(Constructor::Enum(enum_variant_id))
786 }
787 _ => return Err(MatchCheckErr::NotImplemented),
788 }
789 }
790 _ => return Err(MatchCheckErr::NotImplemented),
791 };
792
793 Ok(res)
794}
795
796fn all_constructors_covered(
797 cx: &MatchCheckCtx,
798 constructor: &Constructor,
799 used_constructors: &[Constructor],
800) -> bool {
801 match constructor {
802 Constructor::Tuple { arity } => {
803 used_constructors.iter().any(|constructor| match constructor {
804 Constructor::Tuple { arity: used_arity } => arity == used_arity,
805 _ => false,
806 })
807 }
808 Constructor::Bool(_) => {
809 if used_constructors.is_empty() {
810 return false;
811 }
812
813 let covers_true =
814 used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true)));
815 let covers_false =
816 used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false)));
817
818 covers_true && covers_false
819 }
820 Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
821 for constructor in used_constructors {
822 if let Constructor::Enum(e) = constructor {
823 if id == e.local_id {
824 return true;
825 }
826 }
827 }
828
829 false
830 }),
831 }
832}
833
834fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool {
835 Some(enum_variant_id.into()) == cx.infer.variant_resolution_for_pat(pat_id)
836}
837
838#[cfg(test)]
839mod tests {
840 use crate::diagnostics::tests::check_diagnostics;
841
842 #[test]
843 fn empty_tuple() {
844 check_diagnostics(
845 r#"
846fn main() {
847 match () { }
848 //^^ Missing match arm
849 match (()) { }
850 //^^^^ Missing match arm
851
852 match () { _ => (), }
853 match () { () => (), }
854 match (()) { (()) => (), }
855}
856"#,
857 );
858 }
859
860 #[test]
861 fn tuple_of_two_empty_tuple() {
862 check_diagnostics(
863 r#"
864fn main() {
865 match ((), ()) { }
866 //^^^^^^^^ Missing match arm
867
868 match ((), ()) { ((), ()) => (), }
869}
870"#,
871 );
872 }
873
874 #[test]
875 fn boolean() {
876 check_diagnostics(
877 r#"
878fn test_main() {
879 match false { }
880 //^^^^^ Missing match arm
881 match false { true => (), }
882 //^^^^^ Missing match arm
883 match (false, true) {}
884 //^^^^^^^^^^^^^ Missing match arm
885 match (false, true) { (true, true) => (), }
886 //^^^^^^^^^^^^^ Missing match arm
887 match (false, true) {
888 //^^^^^^^^^^^^^ Missing match arm
889 (false, true) => (),
890 (false, false) => (),
891 (true, false) => (),
892 }
893 match (false, true) { (true, _x) => (), }
894 //^^^^^^^^^^^^^ Missing match arm
895
896 match false { true => (), false => (), }
897 match (false, true) {
898 (false, _) => (),
899 (true, false) => (),
900 (_, true) => (),
901 }
902 match (false, true) {
903 (true, true) => (),
904 (true, false) => (),
905 (false, true) => (),
906 (false, false) => (),
907 }
908 match (false, true) {
909 (true, _x) => (),
910 (false, true) => (),
911 (false, false) => (),
912 }
913 match (false, true, false) {
914 (false, ..) => (),
915 (true, ..) => (),
916 }
917 match (false, true, false) {
918 (.., false) => (),
919 (.., true) => (),
920 }
921 match (false, true, false) { (..) => (), }
922}
923"#,
924 );
925 }
926
927 #[test]
928 fn tuple_of_tuple_and_bools() {
929 check_diagnostics(
930 r#"
931fn main() {
932 match (false, ((), false)) {}
933 //^^^^^^^^^^^^^^^^^^^^ Missing match arm
934 match (false, ((), false)) { (true, ((), true)) => (), }
935 //^^^^^^^^^^^^^^^^^^^^ Missing match arm
936 match (false, ((), false)) { (true, _) => (), }
937 //^^^^^^^^^^^^^^^^^^^^ Missing match arm
938
939 match (false, ((), false)) {
940 (true, ((), true)) => (),
941 (true, ((), false)) => (),
942 (false, ((), true)) => (),
943 (false, ((), false)) => (),
944 }
945 match (false, ((), false)) {
946 (true, ((), true)) => (),
947 (true, ((), false)) => (),
948 (false, _) => (),
949 }
950}
951"#,
952 );
953 }
954
955 #[test]
956 fn enums() {
957 check_diagnostics(
958 r#"
959enum Either { A, B, }
960
961fn main() {
962 match Either::A { }
963 //^^^^^^^^^ Missing match arm
964 match Either::B { Either::A => (), }
965 //^^^^^^^^^ Missing match arm
966
967 match &Either::B {
968 //^^^^^^^^^^ Missing match arm
969 Either::A => (),
970 }
971
972 match Either::B {
973 Either::A => (), Either::B => (),
974 }
975 match &Either::B {
976 Either::A => (), Either::B => (),
977 }
978}
979"#,
980 );
981 }
982
983 #[test]
984 fn enum_containing_bool() {
985 check_diagnostics(
986 r#"
987enum Either { A(bool), B }
988
989fn main() {
990 match Either::B { }
991 //^^^^^^^^^ Missing match arm
992 match Either::B {
993 //^^^^^^^^^ Missing match arm
994 Either::A(true) => (), Either::B => ()
995 }
996
997 match Either::B {
998 Either::A(true) => (),
999 Either::A(false) => (),
1000 Either::B => (),
1001 }
1002 match Either::B {
1003 Either::B => (),
1004 _ => (),
1005 }
1006 match Either::B {
1007 Either::A(_) => (),
1008 Either::B => (),
1009 }
1010
1011}
1012 "#,
1013 );
1014 }
1015
1016 #[test]
1017 fn enum_different_sizes() {
1018 check_diagnostics(
1019 r#"
1020enum Either { A(bool), B(bool, bool) }
1021
1022fn main() {
1023 match Either::A(false) {
1024 //^^^^^^^^^^^^^^^^ Missing match arm
1025 Either::A(_) => (),
1026 Either::B(false, _) => (),
1027 }
1028
1029 match Either::A(false) {
1030 Either::A(_) => (),
1031 Either::B(true, _) => (),
1032 Either::B(false, _) => (),
1033 }
1034 match Either::A(false) {
1035 Either::A(true) | Either::A(false) => (),
1036 Either::B(true, _) => (),
1037 Either::B(false, _) => (),
1038 }
1039}
1040"#,
1041 );
1042 }
1043
1044 #[test]
1045 fn tuple_of_enum_no_diagnostic() {
1046 check_diagnostics(
1047 r#"
1048enum Either { A(bool), B(bool, bool) }
1049enum Either2 { C, D }
1050
1051fn main() {
1052 match (Either::A(false), Either2::C) {
1053 (Either::A(true), _) | (Either::A(false), _) => (),
1054 (Either::B(true, _), Either2::C) => (),
1055 (Either::B(false, _), Either2::C) => (),
1056 (Either::B(_, _), Either2::D) => (),
1057 }
1058}
1059"#,
1060 );
1061 }
1062
1063 #[test]
1064 fn mismatched_types() {
1065 // Match statements with arms that don't match the
1066 // expression pattern do not fire this diagnostic.
1067 check_diagnostics(
1068 r#"
1069enum Either { A, B }
1070enum Either2 { C, D }
1071
1072fn main() {
1073 match Either::A {
1074 Either2::C => (),
1075 Either2::D => (),
1076 }
1077 match (true, false) {
1078 (true, false, true) => (),
1079 (true) => (),
1080 }
1081 match (0) { () => () }
1082 match Unresolved::Bar { Unresolved::Baz => () }
1083}
1084 "#,
1085 );
1086 }
1087
1088 #[test]
1089 fn malformed_match_arm_tuple_enum_missing_pattern() {
1090 // We are testing to be sure we don't panic here when the match
1091 // arm `Either::B` is missing its pattern.
1092 check_diagnostics(
1093 r#"
1094enum Either { A, B(u32) }
1095
1096fn main() {
1097 match Either::A {
1098 Either::A => (),
1099 Either::B() => (),
1100 }
1101}
1102"#,
1103 );
1104 }
1105
1106 #[test]
1107 fn expr_diverges() {
1108 check_diagnostics(
1109 r#"
1110enum Either { A, B }
1111
1112fn main() {
1113 match loop {} {
1114 Either::A => (),
1115 Either::B => (),
1116 }
1117 match loop {} {
1118 Either::A => (),
1119 }
1120 match loop { break Foo::A } {
1121 //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
1122 Either::A => (),
1123 }
1124 match loop { break Foo::A } {
1125 Either::A => (),
1126 Either::B => (),
1127 }
1128}
1129"#,
1130 );
1131 }
1132
1133 #[test]
1134 fn expr_partially_diverges() {
1135 check_diagnostics(
1136 r#"
1137enum Either<T> { A(T), B }
1138
1139fn foo() -> Either<!> { Either::B }
1140fn main() -> u32 {
1141 match foo() {
1142 Either::A(val) => val,
1143 Either::B => 0,
1144 }
1145}
1146"#,
1147 );
1148 }
1149
1150 #[test]
1151 fn enum_record() {
1152 check_diagnostics(
1153 r#"
1154enum Either { A { foo: bool }, B }
1155
1156fn main() {
1157 let a = Either::A { foo: true };
1158 match a { }
1159 //^ Missing match arm
1160 match a { Either::A { foo: true } => () }
1161 //^ Missing match arm
1162 match a {
1163 Either::A { } => (),
1164 //^^^ Missing structure fields:
1165 // | - foo
1166 Either::B => (),
1167 }
1168 match a {
1169 //^ Missing match arm
1170 Either::A { } => (),
1171 } //^^^ Missing structure fields:
1172 // | - foo
1173
1174 match a {
1175 Either::A { foo: true } => (),
1176 Either::A { foo: false } => (),
1177 Either::B => (),
1178 }
1179 match a {
1180 Either::A { foo: _ } => (),
1181 Either::B => (),
1182 }
1183}
1184"#,
1185 );
1186 }
1187
1188 #[test]
1189 fn enum_record_fields_out_of_order() {
1190 check_diagnostics(
1191 r#"
1192enum Either {
1193 A { foo: bool, bar: () },
1194 B,
1195}
1196
1197fn main() {
1198 let a = Either::A { foo: true, bar: () };
1199 match a {
1200 //^ Missing match arm
1201 Either::A { bar: (), foo: false } => (),
1202 Either::A { foo: true, bar: () } => (),
1203 }
1204
1205 match a {
1206 Either::A { bar: (), foo: false } => (),
1207 Either::A { foo: true, bar: () } => (),
1208 Either::B => (),
1209 }
1210}
1211"#,
1212 );
1213 }
1214
1215 #[test]
1216 fn enum_record_ellipsis() {
1217 check_diagnostics(
1218 r#"
1219enum Either {
1220 A { foo: bool, bar: bool },
1221 B,
1222}
1223
1224fn main() {
1225 let a = Either::B;
1226 match a {
1227 //^ Missing match arm
1228 Either::A { foo: true, .. } => (),
1229 Either::B => (),
1230 }
1231 match a {
1232 //^ Missing match arm
1233 Either::A { .. } => (),
1234 }
1235
1236 match a {
1237 Either::A { foo: true, .. } => (),
1238 Either::A { foo: false, .. } => (),
1239 Either::B => (),
1240 }
1241
1242 match a {
1243 Either::A { .. } => (),
1244 Either::B => (),
1245 }
1246}
1247"#,
1248 );
1249 }
1250
1251 #[test]
1252 fn enum_tuple_partial_ellipsis() {
1253 check_diagnostics(
1254 r#"
1255enum Either {
1256 A(bool, bool, bool, bool),
1257 B,
1258}
1259
1260fn main() {
1261 match Either::B {
1262 //^^^^^^^^^ Missing match arm
1263 Either::A(true, .., true) => (),
1264 Either::A(true, .., false) => (),
1265 Either::A(false, .., false) => (),
1266 Either::B => (),
1267 }
1268 match Either::B {
1269 //^^^^^^^^^ Missing match arm
1270 Either::A(true, .., true) => (),
1271 Either::A(true, .., false) => (),
1272 Either::A(.., true) => (),
1273 Either::B => (),
1274 }
1275
1276 match Either::B {
1277 Either::A(true, .., true) => (),
1278 Either::A(true, .., false) => (),
1279 Either::A(false, .., true) => (),
1280 Either::A(false, .., false) => (),
1281 Either::B => (),
1282 }
1283 match Either::B {
1284 Either::A(true, .., true) => (),
1285 Either::A(true, .., false) => (),
1286 Either::A(.., true) => (),
1287 Either::A(.., false) => (),
1288 Either::B => (),
1289 }
1290}
1291"#,
1292 );
1293 }
1294
1295 #[test]
1296 fn never() {
1297 check_diagnostics(
1298 r#"
1299enum Never {}
1300
1301fn enum_(never: Never) {
1302 match never {}
1303}
1304fn enum_ref(never: &Never) {
1305 match never {}
1306}
1307fn bang(never: !) {
1308 match never {}
1309}
1310"#,
1311 );
1312 }
1313
1314 #[test]
1315 fn or_pattern_panic() {
1316 check_diagnostics(
1317 r#"
1318pub enum Category { Infinity, Zero }
1319
1320fn panic(a: Category, b: Category) {
1321 match (a, b) {
1322 (Category::Zero | Category::Infinity, _) => (),
1323 (_, Category::Zero | Category::Infinity) => (),
1324 }
1325
1326 // FIXME: This is a false positive, but the code used to cause a panic in the match checker,
1327 // so this acts as a regression test for that.
1328 match (a, b) {
1329 //^^^^^^ Missing match arm
1330 (Category::Infinity, Category::Infinity) | (Category::Zero, Category::Zero) => (),
1331 (Category::Infinity | Category::Zero, _) => (),
1332 }
1333}
1334"#,
1335 );
1336 }
1337
1338 mod false_negatives {
1339 //! The implementation of match checking here is a work in progress. As we roll this out, we
1340 //! prefer false negatives to false positives (ideally there would be no false positives). This
1341 //! test module should document known false negatives. Eventually we will have a complete
1342 //! implementation of match checking and this module will be empty.
1343 //!
1344 //! The reasons for documenting known false negatives:
1345 //!
1346 //! 1. It acts as a backlog of work that can be done to improve the behavior of the system.
1347 //! 2. It ensures the code doesn't panic when handling these cases.
1348 use super::*;
1349
1350 #[test]
1351 fn integers() {
1352 // We don't currently check integer exhaustiveness.
1353 check_diagnostics(
1354 r#"
1355fn main() {
1356 match 5 {
1357 10 => (),
1358 11..20 => (),
1359 }
1360}
1361"#,
1362 );
1363 }
1364
1365 #[test]
1366 fn internal_or() {
1367 // We do not currently handle patterns with internal `or`s.
1368 check_diagnostics(
1369 r#"
1370fn main() {
1371 enum Either { A(bool), B }
1372 match Either::B {
1373 Either::A(true | false) => (),
1374 }
1375}
1376"#,
1377 );
1378 }
1379
1380 #[test]
1381 fn tuple_of_bools_with_ellipsis_at_end_missing_arm() {
1382 // We don't currently handle tuple patterns with ellipsis.
1383 check_diagnostics(
1384 r#"
1385fn main() {
1386 match (false, true, false) {
1387 (false, ..) => (),
1388 }
1389}
1390"#,
1391 );
1392 }
1393
1394 #[test]
1395 fn tuple_of_bools_with_ellipsis_at_beginning_missing_arm() {
1396 // We don't currently handle tuple patterns with ellipsis.
1397 check_diagnostics(
1398 r#"
1399fn main() {
1400 match (false, true, false) {
1401 (.., false) => (),
1402 }
1403}
1404"#,
1405 );
1406 }
1407
1408 #[test]
1409 fn struct_missing_arm() {
1410 // We don't currently handle structs.
1411 check_diagnostics(
1412 r#"
1413struct Foo { a: bool }
1414fn main(f: Foo) {
1415 match f { Foo { a: true } => () }
1416}
1417"#,
1418 );
1419 }
1420 }
1421}