From a208de15b7846911856e6c069f7df03676c18a03 Mon Sep 17 00:00:00 2001 From: Josh Mcguigan Date: Mon, 6 Apr 2020 15:38:20 -0700 Subject: PR feedback implementation --- crates/ra_hir_ty/src/_match.rs | 425 ++++++++++++++++++++++++++++++++--------- 1 file changed, 331 insertions(+), 94 deletions(-) diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs index 02472e0c0..f29a25505 100644 --- a/crates/ra_hir_ty/src/_match.rs +++ b/crates/ra_hir_ty/src/_match.rs @@ -2,7 +2,191 @@ //! for match arms. //! //! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which -//! contains very detailed documentation about the algorithms used here. +//! contains very detailed documentation about the algorithms used here. I've duplicated +//! most of that documentation below. +//! +//! This file includes the logic for exhaustiveness and usefulness checking for +//! pattern-matching. Specifically, given a list of patterns for a type, we can +//! tell whether: +//! (a) the patterns cover every possible constructor for the type [exhaustiveness] +//! (b) each pattern is necessary [usefulness] +//! +//! The algorithm implemented here is a modified version of the one described in: +//! http://moscova.inria.fr/~maranget/papers/warn/index.html +//! However, to save future implementors from reading the original paper, we +//! summarise the algorithm here to hopefully save time and be a little clearer +//! (without being so rigorous). +//! +//! The core of the algorithm revolves about a "usefulness" check. In particular, we +//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as +//! a matrix). `U(P, p)` represents whether, given an existing list of patterns +//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously- +//! uncovered values of the type). +//! +//! If we have this predicate, then we can easily compute both exhaustiveness of an +//! entire set of patterns and the individual usefulness of each one. +//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard +//! match doesn't increase the number of values we're matching) +//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a +//! pattern to those that have come before it doesn't increase the number of values +//! we're matching). +//! +//! During the course of the algorithm, the rows of the matrix won't just be individual patterns, +//! but rather partially-deconstructed patterns in the form of a list of patterns. The paper +//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the +//! new pattern `p`. +//! +//! For example, say we have the following: +//! ``` +//! // x: (Option, Result<()>) +//! match x { +//! (Some(true), _) => {} +//! (None, Err(())) => {} +//! (None, Err(_)) => {} +//! } +//! ``` +//! Here, the matrix `P` starts as: +//! [ +//! [(Some(true), _)], +//! [(None, Err(()))], +//! [(None, Err(_))], +//! ] +//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering +//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because +//! all the values it covers are already covered by row 2. +//! +//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of +//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks. +//! To match the paper, the top of the stack is at the beginning / on the left. +//! +//! There are two important operations on pattern-stacks necessary to understand the algorithm: +//! 1. We can pop a given constructor off the top of a stack. This operation is called +//! `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or +//! `None`) and `p` a pattern-stack. +//! If the pattern on top of the stack can cover `c`, this removes the constructor and +//! pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns. +//! Otherwise the pattern-stack is discarded. +//! This essentially filters those pattern-stacks whose top covers the constructor `c` and +//! discards the others. +//! +//! For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we +//! pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the +//! `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get +//! nothing back. +//! +//! This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1` +//! on top of the stack, and we have four cases: +//! 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We +//! push onto the stack the arguments of this constructor, and return the result: +//! r_1, .., r_a, p_2, .., p_n +//! 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and +//! return nothing. +//! 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has +//! arguments (its arity), and return the resulting stack: +//! _, .., _, p_2, .., p_n +//! 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting +//! stack: +//! S(c, (r_1, p_2, .., p_n)) +//! S(c, (r_2, p_2, .., p_n)) +//! +//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is +//! a pattern-stack. +//! This is used when we know there are missing constructor cases, but there might be +//! existing wildcard patterns, so to check the usefulness of the matrix, we have to check +//! all its *other* components. +//! +//! It is computed as follows. We look at the pattern `p_1` on top of the stack, +//! and we have three cases: +//! 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing. +//! 1.2. `p_1 = _`. We return the rest of the stack: +//! p_2, .., p_n +//! 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting +//! stack. +//! D((r_1, p_2, .., p_n)) +//! D((r_2, p_2, .., p_n)) +//! +//! Note that the OR-patterns are not always used directly in Rust, but are used to derive the +//! exhaustive integer matching rules, so they're written here for posterity. +//! +//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by +//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with +//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard. +//! +//! +//! The algorithm for computing `U` +//! ------------------------------- +//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns). +//! That means we're going to check the components from left-to-right, so the algorithm +//! operates principally on the first component of the matrix and new pattern-stack `p`. +//! This algorithm is realised in the `is_useful` function. +//! +//! Base case. (`n = 0`, i.e., an empty tuple pattern) +//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), +//! then `U(P, p)` is false. +//! - Otherwise, `P` must be empty, so `U(P, p)` is true. +//! +//! Inductive step. (`n > 0`, i.e., whether there's at least one column +//! [which may then be expanded into further columns later]) +//! We're going to match on the top of the new pattern-stack, `p_1`. +//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern. +//! Then, the usefulness of `p_1` can be reduced to whether it is useful when +//! we ignore all the patterns in the first column of `P` that involve other constructors. +//! This is where `S(c, P)` comes in: +//! `U(P, p) := U(S(c, P), S(c, p))` +//! This special case is handled in `is_useful_specialized`. +//! +//! For example, if `P` is: +//! [ +//! [Some(true), _], +//! [None, 0], +//! ] +//! and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only +//! matches values that row 2 doesn't. For row 1 however, we need to dig into the +//! arguments of `Some` to know whether some new value is covered. So we compute +//! `U([[true, _]], [false, 0])`. +//! +//! - If `p_1 == _`, then we look at the list of constructors that appear in the first +//! component of the rows of `P`: +//! + If there are some constructors that aren't present, then we might think that the +//! wildcard `_` is useful, since it covers those constructors that weren't covered +//! before. +//! That's almost correct, but only works if there were no wildcards in those first +//! components. So we need to check that `p` is useful with respect to the rows that +//! start with a wildcard, if there are any. This is where `D` comes in: +//! `U(P, p) := U(D(P), D(p))` +//! +//! For example, if `P` is: +//! [ +//! [_, true, _], +//! [None, false, 1], +//! ] +//! and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we +//! only had row 2, we'd know that `p` is useful. However row 1 starts with a +//! wildcard, so we need to check whether `U([[true, _]], [false, 1])`. +//! +//! + Otherwise, all possible constructors (for the relevant type) are present. In this +//! case we must check whether the wildcard pattern covers any unmatched value. For +//! that, we can think of the `_` pattern as a big OR-pattern that covers all +//! possible constructors. For `Option`, that would mean `_ = None | Some(_)` for +//! example. The wildcard pattern is useful in this case if it is useful when +//! specialized to one of the possible constructors. So we compute: +//! `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))` +//! +//! For example, if `P` is: +//! [ +//! [Some(true), _], +//! [None, false], +//! ] +//! and `p` is [_, false], both `None` and `Some` constructors appear in the first +//! components of `P`. We will therefore try popping both constructors in turn: we +//! compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]], +//! [false]) for the `None` constructor. The first case returns true, so we know that +//! `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched +//! before. +//! +//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately: +//! `U(P, p) := U(P, (r_1, p_2, .., p_n)) +//! || U(P, (r_2, p_2, .., p_n))` use std::sync::Arc; use smallvec::{smallvec, SmallVec}; @@ -134,16 +318,26 @@ impl PatStack { ) -> MatchCheckResult> { let result = match (self.head().as_pat(cx), constructor) { (Pat::Tuple(ref pat_ids), Constructor::Tuple { arity }) => { - if pat_ids.len() != *arity { - None - } else { - Some(self.replace_head_with(pat_ids)) + debug_assert_eq!( + pat_ids.len(), + *arity, + "we type check before calling this code, so we should never hit this case", + ); + + Some(self.replace_head_with(pat_ids)) + } + (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => { + match cx.body.exprs[lit_expr] { + Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => { + Some(self.to_tail()) + } + // it was a bool but the value doesn't match + Expr::Literal(Literal::Bool(_)) => None, + // perhaps this is actually unreachable given we have + // already checked that these match arms have the appropriate type? + _ => return Err(MatchCheckNotImplemented), } } - (Pat::Lit(_), Constructor::Bool(_)) => { - // for now we only support bool literals - Some(self.to_tail()) - } (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?), (Pat::Path(_), Constructor::Enum(constructor)) => { // enums with no associated data become `Pat::Path` @@ -162,7 +356,7 @@ impl PatStack { Some(self.replace_head_with(pat_ids)) } } - (Pat::Or(_), _) => unreachable!("we desugar or patterns so this should never happen"), + (Pat::Or(_), _) => return Err(MatchCheckNotImplemented), (_, _) => return Err(MatchCheckNotImplemented), }; @@ -186,19 +380,8 @@ impl PatStack { ); let mut patterns: PatStackInner = smallvec![]; - let arity = match constructor { - Constructor::Bool(_) => 0, - Constructor::Tuple { arity } => *arity, - Constructor::Enum(e) => { - match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() { - VariantData::Tuple(struct_field_data) => struct_field_data.len(), - VariantData::Unit => 0, - _ => return Err(MatchCheckNotImplemented), - } - } - }; - for _ in 0..arity { + for _ in 0..constructor.arity(cx)? { patterns.push(PatIdOrWild::Wild); } @@ -368,46 +551,23 @@ pub(crate) fn is_useful( // constructors are covered (`Some`/`None`), so we need // to perform specialization to see that our wildcard will cover // the `Some(false)` case. - let mut constructor = None; - for pat in matrix.heads() { - if let Some(c) = pat_constructor(cx, pat)? { - constructor = Some(c); - break; - } + // + // Here we create a constructor for each variant and then check + // usefulness after specializing for that constructor. + let mut found_unimplemented = false; + for constructor in constructor.all_constructors(cx) { + let matrix = matrix.specialize_constructor(&cx, &constructor)?; + let v = v.expand_wildcard(&cx, &constructor)?; + + match is_useful(&cx, &matrix, &v) { + Ok(Usefulness::Useful) => return Ok(Usefulness::Useful), + Ok(Usefulness::NotUseful) => continue, + _ => found_unimplemented = true, + }; } - if let Some(constructor) = constructor { - if let Constructor::Enum(e) = constructor { - // For enums we handle each variant as a distinct constructor, so - // here we create a constructor for each variant and then check - // usefulness after specializing for that constructor. - let mut found_unimplemented = false; - for constructor in - cx.db.enum_data(e.parent).variants.iter().map(|(local_id, _)| { - Constructor::Enum(EnumVariantId { parent: e.parent, local_id }) - }) - { - let matrix = matrix.specialize_constructor(&cx, &constructor)?; - let v = v.expand_wildcard(&cx, &constructor)?; - - match is_useful(&cx, &matrix, &v) { - Ok(Usefulness::Useful) => return Ok(Usefulness::Useful), - Ok(Usefulness::NotUseful) => continue, - _ => found_unimplemented = true, - }; - } - - if found_unimplemented { - Err(MatchCheckNotImplemented) - } else { - Ok(Usefulness::NotUseful) - } - } else { - let matrix = matrix.specialize_constructor(&cx, &constructor)?; - let v = v.expand_wildcard(&cx, &constructor)?; - - is_useful(&cx, &matrix, &v) - } + if found_unimplemented { + Err(MatchCheckNotImplemented) } else { Ok(Usefulness::NotUseful) } @@ -425,7 +585,7 @@ pub(crate) fn is_useful( } } -#[derive(Debug)] +#[derive(Debug, Clone, Copy)] /// Similar to TypeCtor, but includes additional information about the specific /// value being instantiated. For example, TypeCtor::Bool doesn't contain the /// boolean value. @@ -435,6 +595,40 @@ enum Constructor { Enum(EnumVariantId), } +impl Constructor { + fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult { + let arity = match self { + Constructor::Bool(_) => 0, + Constructor::Tuple { arity } => *arity, + Constructor::Enum(e) => { + match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() { + VariantData::Tuple(struct_field_data) => struct_field_data.len(), + VariantData::Unit => 0, + _ => return Err(MatchCheckNotImplemented), + } + } + }; + + Ok(arity) + } + + fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec { + match self { + Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)], + Constructor::Tuple { .. } => vec![*self], + Constructor::Enum(e) => cx + .db + .enum_data(e.parent) + .variants + .iter() + .map(|(local_id, _)| { + Constructor::Enum(EnumVariantId { parent: e.parent, local_id }) + }) + .collect(), + } + } +} + /// Returns the constructor for the given pattern. Should only return None /// in the case of a Wild pattern. fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult> { @@ -501,14 +695,7 @@ fn all_constructors_covered( } fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool { - if let Some(VariantId::EnumVariantId(pat_variant_id)) = - cx.infer.variant_resolution_for_pat(pat_id) - { - if pat_variant_id.local_id == enum_variant_id.local_id { - return true; - } - } - false + Some(enum_variant_id.into()) == cx.infer.variant_resolution_for_pat(pat_id) } #[cfg(test)] @@ -522,10 +709,10 @@ mod tests { TestDB::with_single_file(content).0.diagnostics().0 } - pub(super) fn check_diagnostic_with_no_fix(content: &str) { + pub(super) fn check_diagnostic(content: &str) { let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1; - assert_eq!(1, diagnostic_count, "no diagnotic reported"); + assert_eq!(1, diagnostic_count, "no diagnostic reported"); } pub(super) fn check_no_diagnostic(content: &str) { @@ -558,7 +745,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -596,7 +783,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -621,7 +808,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -646,7 +833,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -659,7 +846,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -685,7 +872,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -698,7 +885,37 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); + } + + #[test] + fn tuple_of_bools_missing_arm() { + let content = r" + fn test_fn() { + match (false, true) { + (false, true) => {}, + (false, false) => {}, + (true, false) => {}, + } + } + "; + + check_diagnostic(content); + } + + #[test] + fn tuple_of_bools_with_wilds() { + let content = r" + fn test_fn() { + match (false, true) { + (false, _) => {}, + (true, false) => {}, + (_, true) => {}, + } + } + "; + + check_no_diagnostic(content); } #[test] @@ -727,7 +944,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -754,7 +971,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -767,7 +984,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -796,7 +1013,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -827,7 +1044,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -844,7 +1061,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -879,7 +1096,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -913,7 +1130,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -931,7 +1148,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -1004,7 +1221,7 @@ mod tests { } "; - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -1089,7 +1306,7 @@ mod tests { "; // Match arms with the incorrect type are filtered out. - check_diagnostic_with_no_fix(content); + check_diagnostic(content); } #[test] @@ -1104,7 +1321,23 @@ mod tests { "; // Match arms with the incorrect type are filtered out. - check_diagnostic_with_no_fix(content); + check_diagnostic(content); + } + + #[test] + fn enum_not_in_scope() { + let content = r" + fn test_fn() { + match Foo::Bar { + Foo::Baz => (), + } + } + "; + + // The enum is not in scope so we don't perform exhaustiveness + // checking, but we want to be sure we don't panic here (and + // we don't create a diagnostic). + check_no_diagnostic(content); } } @@ -1158,17 +1391,21 @@ mod false_negatives { } #[test] - fn enum_not_in_scope() { + fn internal_or() { let content = r" fn test_fn() { - match Foo::Bar { - Foo::Baz => (), + enum Either { + A(bool), + B, + } + match Either::B { + Either::A(true | false) => (), } } "; // This is a false negative. - // The enum is not in scope so we don't perform exhaustiveness checking. + // We do not currently handle patterns with internal `or`s. check_no_diagnostic(content); } } -- cgit v1.2.3