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
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)
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.
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.
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:
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
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.
44 //! // not exhaustive: `_` is useful because it matches `Some(1)`
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.
52 //! # Constructors and fields
54 //! Note: we will often abbreviate "constructor" as "ctor".
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.
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`.
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.
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:
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)`
92 //! Constructors, fields and relevant operations are defined in the [`super::deconstruct_pat`] module.
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.
100 //! Whether constructors cover each other is computed by [`Constructor::is_covered_by`].
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.
113 //! Let's take the following example:
116 //! Enum::Variant1(_) => {} // `p1`
117 //! Enum::Variant2(None, 0) => {} // `p2`
118 //! Enum::Variant2(Some(_), 0) => {} // `q`
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:
129 //! (None, 0) => {} // `p2'`
130 //! (Some(_), 0) => {} // `q'`
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.
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
145 //! [Enum::Variant1(_)]
146 //! [Enum::Variant2(None, 0)]
147 //! [Enum::Variant2(Some(_), 0)]
148 //! //==>> specialize with `Variant2`
151 //! //==>> specialize with `Some`
153 //! //==>> specialize with `true` (say the type was `bool`)
155 //! //==>> specialize with `0`
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.
164 //! - Specializing for the wrong constructor returns nothing
166 //! `specialize(None, Some(p0)) := []`
168 //! - Specializing for the correct constructor returns a single row with the fields
170 //! `specialize(Variant1, Variant1(p0, p1, p2)) := [[p0, p1, p2]]`
172 //! `specialize(Foo{..}, Foo { bar: p0, baz: p1 }) := [[p0, p1]]`
174 //! - For or-patterns, we specialize each branch and concatenate the results
176 //! `specialize(c, p0 | p1) := specialize(c, p0) ++ specialize(c, p1)`
178 //! - We treat the other pattern constructors as if they were a large or-pattern of all the
181 //! `specialize(c, _) := specialize(c, Variant1(_) | Variant2(_, _) | ...)`
183 //! `specialize(c, 1..=100) := specialize(c, 1 | ... | 100)`
185 //! `specialize(c, [p0, .., p1]) := specialize(c, [p0, p1] | [p0, _, p1] | [p0, _, _, p1] | ...)`
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`].
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.
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.
201 //! # Computing usefulness
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.
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`.
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.
217 //! - for each such ctor `c`:
219 //! - for each `q'` returned by `specialize(c, q)`:
221 //! - we compute `usefulness(specialize(c, p_1) ... specialize(c, p_n), q')`
223 //! - for each witness found, we revert specialization by pushing the constructor `c` on top.
225 //! - We return the concatenation of all the witnesses found, if any.
229 //! [Some(true)] // p_1
232 //! //==>> try `None`: `specialize(None, q)` returns nothing
233 //! //==>> try `Some`: `specialize(Some, q)` returns a single row
236 //! //==>> try `true`: `specialize(true, q')` returns a single row
239 //! //==>> base case; `n != 0` so `q''` is not useful.
240 //! //==>> go back up a step
243 //! //==>> try `false`: `specialize(false, q')` returns a single row
245 //! //==>> base case; `n == 0` so `q''` is useful. We return the single witness `[]`
248 //! //==>> undo the specialization with `false`
251 //! //==>> undo the specialization with `Some`
254 //! //==>> we have tried all the constructors. The output is the single witness `[Some(false)]`.
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.
262 //! # Making usefulness tractable: constructor splitting
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.
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.
274 use std::{cell::RefCell, iter::FromIterator};
276 use hir_def::{expr::ExprId, HasModule, ModuleId};
278 use once_cell::unsync::OnceCell;
279 use rustc_hash::FxHashMap;
280 use smallvec::{smallvec, SmallVec};
282 use crate::{db::HirDatabase, InferenceResult, Interner, Ty};
285 deconstruct_pat::{Constructor, Fields, SplitWildcard},
286 Pat, PatId, PatKind, PatternFoldable, PatternFolder,
289 use self::{helper::PatIdExt, Usefulness::*, WitnessPreference::*};
291 pub(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(),
301 impl<'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
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();
313 hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
314 has_non_exhaustive_attr && !is_local
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
323 pub(super) fn alloc_pat(&self, pat: Pat) -> PatId {
324 self.pattern_arena.borrow_mut().alloc(pat)
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()
333 pub(super) fn bug(&self, info: &str) -> ! {
334 (self.eprint_panic_context)();
335 panic!("bug: {}", info);
339 #[derive(Copy, Clone)]
340 pub(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
346 pub(super) is_top_level: bool,
349 pub(crate) fn expand_pattern(pat: Pat) -> Pat {
350 LiteralExpander.fold_pattern(&pat)
353 struct LiteralExpander;
355 impl 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),
365 fn _is_wildcard(&self) -> bool {
366 matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild)
370 impl PatIdExt for PatId {
371 fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool {
372 matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. })
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();
383 let pat = pat_arena.alloc(pat.clone());
384 expand(pat, vec, pat_arena);
391 let mut pat_arena = cx.pattern_arena.borrow_mut();
392 let mut pats = Vec::new();
393 expand(self, &mut pats, &mut pat_arena);
398 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
401 pub(super) struct PatStack {
402 pats: SmallVec<[PatId; 2]>,
403 /// Cache for the constructor of the head
404 head_ctor: OnceCell<Constructor>,
408 fn from_pattern(pat: PatId) -> Self {
409 Self::from_vec(smallvec![pat])
412 fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self {
413 PatStack { pats: vec, head_ctor: OnceCell::new() }
416 fn is_empty(&self) -> bool {
420 fn len(&self) -> usize {
424 fn head(&self) -> PatId {
429 fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor {
430 self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head()))
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..]);
443 /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations.
445 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
446 /// fields filled with wild patterns.
448 /// This is roughly the inverse of `Constructor::apply`.
449 fn pop_head_constructor(
451 ctor_wild_subpatterns: &Fields,
452 cx: &MatchCheckCtx<'_>,
454 // We pop the head pattern and push the new fields extracted from the arguments of
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)
463 impl Default for PatStack {
464 fn default() -> Self {
465 Self::from_vec(smallvec![])
469 impl PartialEq for PatStack {
470 fn eq(&self, other: &Self) -> bool {
471 self.pats == other.pats
475 impl FromIterator<PatId> for PatStack {
476 fn from_iter<T>(iter: T) -> Self
478 T: IntoIterator<Item = PatId>,
480 Self::from_vec(iter.into_iter().collect())
486 pub(super) struct Matrix {
487 patterns: Vec<PatStack>,
492 Matrix { patterns: vec![] }
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())
500 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
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);
508 self.patterns.push(row);
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())
517 /// Iterate over the first constructor of each row.
520 cx: &'a MatchCheckCtx<'_>,
521 ) -> impl Iterator<Item = &'a Constructor> + Clone {
522 self.patterns.iter().map(move |r| r.head_ctor(cx))
525 /// This computes `S(constructor, self)`. See top of the file for explanations.
526 fn specialize_constructor(
530 ctor_wild_subpatterns: &Fields,
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)
540 fn from_iter(rows: impl IntoIterator<Item = PatStack>, cx: &MatchCheckCtx<'_>) -> Matrix {
541 let mut matrix = Matrix::empty();
543 // Using `push` ensures we correctly expand or-patterns.
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.
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.
560 /// What we're doing is illustrated by this:
562 /// match (true, 0) {
565 /// (true | false, 0 | 1) => {}
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.
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)]
579 /// The empty set. This means the pattern is unreachable.
581 /// The set containing the full pattern.
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
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.
590 subpats: FxHashMap<usize, SubPatSet>,
591 /// Counts the total number of alternatives in the pattern
593 /// We keep the pattern around to retrieve spans.
607 fn is_empty(&self) -> bool {
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()),
618 fn is_full(&self) -> bool {
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())
631 /// Union `self` with `other`, mutating `self`.
632 fn union(&mut self, other: Self) {
634 // Union with full stays full; union with empty changes nothing.
635 if self.is_full() || other.is_empty() {
637 } else if self.is_empty() {
640 } else if other.is_full() {
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.
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.
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()
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.
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()`.
683 unreachable_pats: &mut Vec<PatId>,
684 cx: &MatchCheckCtx<'_>,
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);
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]);
702 fill_subpats(sub_set, unreachable_pats, cx);
713 // No subpatterns are unreachable.
714 return Some(Vec::new());
716 let mut unreachable_pats = Vec::new();
717 fill_subpats(self, &mut unreachable_pats, cx);
718 Some(unreachable_pats)
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 {
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 {
734 // The first `arity` indices are now part of the pattern in the first
736 new_subpats_first_col.insert(i, sub_set);
738 // Indices after `arity` are simply shifted
739 new_subpats.insert(i - arity + 1, sub_set);
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 });
746 Seq { subpats: new_subpats }
748 Alt { .. } => panic!("bug"), // `self` is a patstack
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.
757 /// match Some(true) {
759 /// None | Some(true | false) => {}
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 {
771 // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0
773 let set_first_col = match &mut self {
775 Seq { subpats } => subpats.remove(&0).unwrap_or(Full),
776 Empty => unreachable!(),
777 Alt { .. } => panic!("bug"), // `self` is a patstack
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 };
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
789 subpats.insert(0, set_first_col);
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)]
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>),
814 fn new_useful(preference: WitnessPreference) -> Self {
816 ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
817 LeaveOutWitness => NoWitnesses(SubPatSet::full()),
820 fn new_not_useful(preference: WitnessPreference) -> Self {
822 ConstructWitness => WithWitnesses(vec![]),
823 LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
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),
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 {
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.
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 {
858 NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)),
859 WithWitnesses(_) => panic!("bug"),
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(
871 ctor_wild_subpatterns: &Fields,
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
885 .map(|missing_ctor| {
886 Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
891 .flat_map(|witness| {
892 new_patterns.iter().map(move |pat| {
893 let mut witness = witness.clone();
894 witness.0.push(pat.clone());
902 .map(|witness| witness.apply_constructor(pcx, &ctor, ctor_wild_subpatterns))
905 WithWitnesses(new_witnesses)
907 NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
912 #[derive(Copy, Clone, Debug)]
913 enum WitnessPreference {
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.
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.
929 /// For example, if we are constructing a witness for the match against
932 /// struct Pair(Option<(u32, u32)>, bool);
934 /// match (p: Pair) {
935 /// Pair(None, _) => {}
936 /// Pair(_, false) => {}
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)])`
950 /// The final `Pair(Some(_), true)` is then the resulting witness.
951 #[derive(Clone, Debug)]
952 pub(crate) struct Witness(Vec<Pat>);
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()
961 /// Constructs a partial witness for a pattern given a list of
962 /// patterns expanded by the specialization step.
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:
969 /// left_ty: tuple of 3 elements
970 /// pats: [10, 20, _] => (10, 20, _)
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(
978 ctor_wild_subpatterns: &Fields,
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)
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, !>`.
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`.
1005 /// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
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).
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.
1016 cx: &MatchCheckCtx<'_>,
1019 witness_preference: WitnessPreference,
1020 is_under_guard: bool,
1023 let Matrix { patterns: rows, .. } = matrix;
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.
1031 let ret = if rows.is_empty() {
1032 Usefulness::new_useful(witness_preference)
1034 Usefulness::new_not_useful(witness_preference)
1039 assert!(rows.iter().all(|r| r.len() == v.len()));
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 };
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)`.
1061 usefulness.unsplit_or_pat(i, alt_count, v_head)
1063 Usefulness::merge(witness_preference, usefulnesses)
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(
1070 // matrix.head_ctors_and_spans(cx),
1071 // matrix.column_count().unwrap_or(0),
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);
1086 start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns);
1087 let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx);
1089 is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
1090 usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
1092 Usefulness::merge(witness_preference, usefulnesses)
1098 /// The arm of a match expression.
1099 #[derive(Clone, Copy)]
1100 pub(crate) struct MatchArm {
1101 pub(crate) pat: PatId,
1102 pub(crate) has_guard: bool,
1105 /// Indicates whether or not a given arm is reachable.
1106 #[derive(Clone, Debug)]
1107 pub(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.
1116 /// The output of checking a match for exhaustiveness and arm reachability.
1117 pub(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
1122 pub(crate) non_exhaustiveness_witnesses: Vec<Pat>,
1125 /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
1126 /// of its arms are reachable.
1128 /// Note: the input patterns must have been lowered through
1129 /// `check_match::MatchVisitor::lower_pattern`.
1130 pub(crate) fn compute_match_usefulness(
1131 cx: &MatchCheckCtx<'_>,
1133 ) -> UsefulnessReport {
1134 let mut matrix = Matrix::empty();
1135 let arm_usefulness: Vec<_> = arms
1139 let v = PatStack::from_pattern(arm.pat);
1140 let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true);
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())
1149 WithWitnesses(..) => panic!("bug"),
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"),
1163 UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
1166 pub(crate) type PatternArena = Arena<Pat>;
1169 use super::MatchCheckCtx;
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>;
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.
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> {}
1187 impl<'a, T: ?Sized> Captures<'a> for T {}