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>,
300 impl<'a> MatchCheckCtx<'a> {
301 pub(super) fn is_uninhabited(&self, _ty: &Ty) -> bool {
302 // FIXME(iDawer) implement exhaustive_patterns feature. More info in:
303 // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085
307 /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
308 pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool {
309 let has_non_exhaustive_attr =
310 self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists();
312 hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate();
313 has_non_exhaustive_attr && !is_local
316 // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
317 pub(super) fn feature_exhaustive_patterns(&self) -> bool {
318 // FIXME see MatchCheckCtx::is_uninhabited
322 pub(super) fn alloc_pat(&self, pat: Pat) -> PatId {
323 self.pattern_arena.borrow_mut().alloc(pat)
326 /// Get type of a pattern. Handles expanded patterns.
327 pub(super) fn type_of(&self, pat: PatId) -> Ty {
328 self.pattern_arena.borrow()[pat].ty.clone()
332 #[derive(Copy, Clone)]
333 pub(super) struct PatCtxt<'a> {
334 pub(super) cx: &'a MatchCheckCtx<'a>,
335 /// Type of the current column under investigation.
336 pub(super) ty: &'a Ty,
337 /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
339 pub(super) is_top_level: bool,
342 pub(crate) fn expand_pattern(pat: Pat) -> Pat {
343 LiteralExpander.fold_pattern(&pat)
346 struct LiteralExpander;
348 impl PatternFolder for LiteralExpander {
349 fn fold_pattern(&mut self, pat: &Pat) -> Pat {
350 match (pat.ty.kind(&Interner), pat.kind.as_ref()) {
351 (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self),
352 _ => pat.super_fold_with(self),
358 fn _is_wildcard(&self) -> bool {
359 matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild)
363 impl PatIdExt for PatId {
364 fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool {
365 matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. })
368 /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns.
369 fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self> {
370 fn expand(pat: PatId, vec: &mut Vec<PatId>, pat_arena: &mut PatternArena) {
371 if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() {
372 // FIXME(iDawer): Factor out pattern deep cloning. See discussion:
373 // https://github.com/rust-analyzer/rust-analyzer/pull/8717#discussion_r633086640
374 let pats = pats.clone();
376 let pat = pat_arena.alloc(pat.clone());
377 expand(pat, vec, pat_arena);
384 let mut pat_arena = cx.pattern_arena.borrow_mut();
385 let mut pats = Vec::new();
386 expand(self, &mut pats, &mut pat_arena);
391 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
394 pub(super) struct PatStack {
395 pats: SmallVec<[PatId; 2]>,
396 /// Cache for the constructor of the head
397 head_ctor: OnceCell<Constructor>,
401 fn from_pattern(pat: PatId) -> Self {
402 Self::from_vec(smallvec![pat])
405 fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self {
406 PatStack { pats: vec, head_ctor: OnceCell::new() }
409 fn is_empty(&self) -> bool {
413 fn len(&self) -> usize {
417 fn head(&self) -> PatId {
422 fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor {
423 self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head()))
426 // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an
427 // or-pattern. Panics if `self` is empty.
428 fn expand_or_pat(&self, cx: &MatchCheckCtx<'_>) -> impl Iterator<Item = PatStack> + '_ {
429 self.head().expand_or_pat(cx).into_iter().map(move |pat| {
430 let mut new_patstack = PatStack::from_pattern(pat);
431 new_patstack.pats.extend_from_slice(&self.pats[1..]);
436 /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations.
438 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
439 /// fields filled with wild patterns.
441 /// This is roughly the inverse of `Constructor::apply`.
442 fn pop_head_constructor(
444 ctor_wild_subpatterns: &Fields,
445 cx: &MatchCheckCtx<'_>,
447 // We pop the head pattern and push the new fields extracted from the arguments of
450 ctor_wild_subpatterns.replace_with_pattern_arguments(self.head(), cx).into_patterns();
451 new_fields.extend_from_slice(&self.pats[1..]);
452 PatStack::from_vec(new_fields)
456 impl Default for PatStack {
457 fn default() -> Self {
458 Self::from_vec(smallvec![])
462 impl PartialEq for PatStack {
463 fn eq(&self, other: &Self) -> bool {
464 self.pats == other.pats
468 impl FromIterator<PatId> for PatStack {
469 fn from_iter<T>(iter: T) -> Self
471 T: IntoIterator<Item = PatId>,
473 Self::from_vec(iter.into_iter().collect())
479 pub(super) struct Matrix {
480 patterns: Vec<PatStack>,
485 Matrix { patterns: vec![] }
488 /// Number of columns of this matrix. `None` is the matrix is empty.
489 pub(super) fn _column_count(&self) -> Option<usize> {
490 self.patterns.get(0).map(|r| r.len())
493 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
495 fn push(&mut self, row: PatStack, cx: &MatchCheckCtx<'_>) {
496 if !row.is_empty() && row.head().is_or_pat(cx) {
497 for row in row.expand_or_pat(cx) {
498 self.patterns.push(row);
501 self.patterns.push(row);
505 /// Iterate over the first component of each row
506 fn heads(&self) -> impl Iterator<Item = PatId> + '_ {
507 self.patterns.iter().map(|r| r.head())
510 /// Iterate over the first constructor of each row.
513 cx: &'a MatchCheckCtx<'_>,
514 ) -> impl Iterator<Item = &'a Constructor> + Clone {
515 self.patterns.iter().map(move |r| r.head_ctor(cx))
518 /// This computes `S(constructor, self)`. See top of the file for explanations.
519 fn specialize_constructor(
523 ctor_wild_subpatterns: &Fields,
528 .filter(|r| ctor.is_covered_by(pcx, r.head_ctor(pcx.cx)))
529 .map(|r| r.pop_head_constructor(ctor_wild_subpatterns, pcx.cx));
530 Matrix::from_iter(rows, pcx.cx)
533 fn from_iter(rows: impl IntoIterator<Item = PatStack>, cx: &MatchCheckCtx<'_>) -> Matrix {
534 let mut matrix = Matrix::empty();
536 // Using `push` ensures we correctly expand or-patterns.
543 /// Given a pattern or a pattern-stack, this struct captures a set of its subpatterns. We use that
544 /// to track reachable sub-patterns arising from or-patterns. In the absence of or-patterns this
545 /// will always be either `Empty` (the whole pattern is unreachable) or `Full` (the whole pattern
546 /// is reachable). When there are or-patterns, some subpatterns may be reachable while others
547 /// aren't. In this case the whole pattern still counts as reachable, but we will lint the
548 /// unreachable subpatterns.
550 /// This supports a limited set of operations, so not all possible sets of subpatterns can be
551 /// represented. That's ok, we only want the ones that make sense for our usage.
553 /// What we're doing is illustrated by this:
555 /// match (true, 0) {
558 /// (true | false, 0 | 1) => {}
561 /// When we try the alternatives of the `true | false` or-pattern, the last `0` is reachable in the
562 /// `false` alternative but not the `true`. So overall it is reachable. By contrast, the last `1`
563 /// is not reachable in either alternative, so we want to signal this to the user.
564 /// Therefore we take the union of sets of reachable patterns coming from different alternatives in
565 /// order to figure out which subpatterns are overall reachable.
567 /// Invariant: we try to construct the smallest representation we can. In particular if
568 /// `self.is_empty()` we ensure that `self` is `Empty`, and same with `Full`. This is not important
569 /// for correctness currently.
570 #[derive(Debug, Clone)]
572 /// The empty set. This means the pattern is unreachable.
574 /// The set containing the full pattern.
576 /// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each
577 /// of its subpatterns. Missing entries in the map are implicitly full, because that's the
579 Seq { subpats: FxHashMap<usize, SubPatSet> },
580 /// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing
581 /// entries in the map are implicitly empty. Note: we always flatten nested or-patterns.
583 subpats: FxHashMap<usize, SubPatSet>,
584 /// Counts the total number of alternatives in the pattern
586 /// We keep the pattern around to retrieve spans.
600 fn is_empty(&self) -> bool {
602 SubPatSet::Empty => true,
603 SubPatSet::Full => false,
604 // If any subpattern in a sequence is unreachable, the whole pattern is unreachable.
605 SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_empty()),
606 // An or-pattern is reachable if any of its alternatives is.
607 SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_empty()),
611 fn is_full(&self) -> bool {
613 SubPatSet::Empty => false,
614 SubPatSet::Full => true,
615 // The whole pattern is reachable only when all its alternatives are.
616 SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_full()),
617 // The whole or-pattern is reachable only when all its alternatives are.
618 SubPatSet::Alt { subpats, alt_count, .. } => {
619 subpats.len() == *alt_count && subpats.values().all(|set| set.is_full())
624 /// Union `self` with `other`, mutating `self`.
625 fn union(&mut self, other: Self) {
627 // Union with full stays full; union with empty changes nothing.
628 if self.is_full() || other.is_empty() {
630 } else if self.is_empty() {
633 } else if other.is_full() {
638 match (&mut *self, other) {
639 (Seq { subpats: s_set }, Seq { subpats: mut o_set }) => {
640 s_set.retain(|i, s_sub_set| {
641 // Missing entries count as full.
642 let o_sub_set = o_set.remove(i).unwrap_or(Full);
643 s_sub_set.union(o_sub_set);
644 // We drop full entries.
647 // Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since
648 // unioning with full returns full, we can drop those entries.
650 (Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => {
651 s_set.retain(|i, s_sub_set| {
652 // Missing entries count as empty.
653 let o_sub_set = o_set.remove(i).unwrap_or(Empty);
654 s_sub_set.union(o_sub_set);
655 // We drop empty entries.
656 !s_sub_set.is_empty()
658 // Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since
659 // unioning with empty changes nothing, we can take those entries as is.
670 /// Returns a list of the unreachable subpatterns. If `self` is empty (i.e. the
671 /// whole pattern is unreachable) we return `None`.
672 fn list_unreachable_subpatterns(&self, cx: &MatchCheckCtx<'_>) -> Option<Vec<PatId>> {
673 /// Panics if `set.is_empty()`.
676 unreachable_pats: &mut Vec<PatId>,
677 cx: &MatchCheckCtx<'_>,
680 SubPatSet::Empty => panic!("bug"),
681 SubPatSet::Full => {}
682 SubPatSet::Seq { subpats } => {
683 for sub_set in subpats.values() {
684 fill_subpats(sub_set, unreachable_pats, cx);
687 SubPatSet::Alt { subpats, pat, alt_count, .. } => {
688 let expanded = pat.expand_or_pat(cx);
689 for (i, &expanded) in expanded.iter().enumerate().take(*alt_count) {
690 let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Empty);
691 if sub_set.is_empty() {
692 // Found an unreachable subpattern.
693 unreachable_pats.push(expanded);
695 fill_subpats(sub_set, unreachable_pats, cx);
706 // No subpatterns are unreachable.
707 return Some(Vec::new());
709 let mut unreachable_pats = Vec::new();
710 fill_subpats(self, &mut unreachable_pats, cx);
711 Some(unreachable_pats)
714 /// When `self` refers to a patstack that was obtained from specialization, after running
715 /// `unspecialize` it will refer to the original patstack before specialization.
716 fn unspecialize(self, arity: usize) -> Self {
722 // We gather the first `arity` subpatterns together and shift the remaining ones.
723 let mut new_subpats = FxHashMap::default();
724 let mut new_subpats_first_col = FxHashMap::default();
725 for (i, sub_set) in subpats {
727 // The first `arity` indices are now part of the pattern in the first
729 new_subpats_first_col.insert(i, sub_set);
731 // Indices after `arity` are simply shifted
732 new_subpats.insert(i - arity + 1, sub_set);
735 // If `new_subpats_first_col` has no entries it counts as full, so we can omit it.
736 if !new_subpats_first_col.is_empty() {
737 new_subpats.insert(0, Seq { subpats: new_subpats_first_col });
739 Seq { subpats: new_subpats }
741 Alt { .. } => panic!("bug"), // `self` is a patstack
745 /// When `self` refers to a patstack that was obtained from splitting an or-pattern, after
746 /// running `unspecialize` it will refer to the original patstack before splitting.
750 /// match Some(true) {
752 /// None | Some(true | false) => {}
755 /// Here `None` would return the full set and `Some(true | false)` would return the set
756 /// containing `false`. After `unsplit_or_pat`, we want the set to contain `None` and `false`.
757 /// This is what this function does.
758 fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
764 // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0
766 let set_first_col = match &mut self {
768 Seq { subpats } => subpats.remove(&0).unwrap_or(Full),
769 Empty => unreachable!(),
770 Alt { .. } => panic!("bug"), // `self` is a patstack
772 let mut subpats_first_col = FxHashMap::default();
773 subpats_first_col.insert(alt_id, set_first_col);
774 let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count };
776 let mut subpats = match self {
777 Full => FxHashMap::default(),
778 Seq { subpats } => subpats,
779 Empty => unreachable!(),
780 Alt { .. } => panic!("bug"), // `self` is a patstack
782 subpats.insert(0, set_first_col);
787 /// This carries the results of computing usefulness, as described at the top of the file. When
788 /// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track
789 /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
790 /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
791 /// witnesses of non-exhaustiveness when there are any.
792 /// Which variant to use is dictated by `WitnessPreference`.
793 #[derive(Clone, Debug)]
795 /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates
796 /// the whole pattern is unreachable. If not, this indicates that the pattern is reachable but
797 /// that some sub-patterns may be unreachable (due to or-patterns). In the absence of
798 /// or-patterns this will always be either `Empty` (the whole pattern is unreachable) or `Full`
799 /// (the whole pattern is reachable).
800 NoWitnesses(SubPatSet),
801 /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole
802 /// pattern is unreachable.
803 WithWitnesses(Vec<Witness>),
807 fn new_useful(preference: WitnessPreference) -> Self {
809 ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
810 LeaveOutWitness => NoWitnesses(SubPatSet::full()),
813 fn new_not_useful(preference: WitnessPreference) -> Self {
815 ConstructWitness => WithWitnesses(vec![]),
816 LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
820 /// Combine usefulnesses from two branches. This is an associative operation.
821 fn extend(&mut self, other: Self) {
822 match (&mut *self, other) {
823 (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {}
824 (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o),
825 (WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
826 (NoWitnesses(s), NoWitnesses(o)) => s.union(o),
831 /// When trying several branches and each returns a `Usefulness`, we need to combine the
832 /// results together.
833 fn merge(pref: WitnessPreference, usefulnesses: impl Iterator<Item = Self>) -> Self {
834 let mut ret = Self::new_not_useful(pref);
835 for u in usefulnesses {
837 if let NoWitnesses(subpats) = &ret {
838 if subpats.is_full() {
839 // Once we reach the full set, more unions won't change the result.
847 /// After calculating the usefulness for a branch of an or-pattern, call this to make this
848 /// usefulness mergeable with those from the other branches.
849 fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: PatId) -> Self {
851 NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)),
852 WithWitnesses(_) => panic!("bug"),
856 /// After calculating usefulness after a specialization, call this to recontruct a usefulness
857 /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
858 /// with the results of specializing with the other constructors.
859 fn apply_constructor(
864 ctor_wild_subpatterns: &Fields,
867 WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses),
868 WithWitnesses(witnesses) => {
869 let new_witnesses = if matches!(ctor, Constructor::Missing) {
870 let mut split_wildcard = SplitWildcard::new(pcx);
871 split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
872 // Construct for each missing constructor a "wild" version of this
873 // constructor, that matches everything that can be built with
874 // it. For example, if `ctor` is a `Constructor::Variant` for
875 // `Option::Some`, we get the pattern `Some(_)`.
876 let new_patterns: Vec<_> = split_wildcard
878 .map(|missing_ctor| {
879 Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
884 .flat_map(|witness| {
885 new_patterns.iter().map(move |pat| {
886 let mut witness = witness.clone();
887 witness.0.push(pat.clone());
895 .map(|witness| witness.apply_constructor(pcx, ctor, ctor_wild_subpatterns))
898 WithWitnesses(new_witnesses)
900 NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
905 #[derive(Copy, Clone, Debug)]
906 enum WitnessPreference {
911 /// A witness of non-exhaustiveness for error reporting, represented
912 /// as a list of patterns (in reverse order of construction) with
913 /// wildcards inside to represent elements that can take any inhabitant
914 /// of the type as a value.
916 /// A witness against a list of patterns should have the same types
917 /// and length as the pattern matched against. Because Rust `match`
918 /// is always against a single pattern, at the end the witness will
919 /// have length 1, but in the middle of the algorithm, it can contain
920 /// multiple patterns.
922 /// For example, if we are constructing a witness for the match against
925 /// struct Pair(Option<(u32, u32)>, bool);
927 /// match (p: Pair) {
928 /// Pair(None, _) => {}
929 /// Pair(_, false) => {}
933 /// We'll perform the following steps:
934 /// 1. Start with an empty witness
935 /// `Witness(vec![])`
936 /// 2. Push a witness `true` against the `false`
937 /// `Witness(vec![true])`
938 /// 3. Push a witness `Some(_)` against the `None`
939 /// `Witness(vec![true, Some(_)])`
940 /// 4. Apply the `Pair` constructor to the witnesses
941 /// `Witness(vec![Pair(Some(_), true)])`
943 /// The final `Pair(Some(_), true)` is then the resulting witness.
944 #[derive(Clone, Debug)]
945 pub(crate) struct Witness(Vec<Pat>);
948 /// Asserts that the witness contains a single pattern, and returns it.
949 fn single_pattern(self) -> Pat {
950 assert_eq!(self.0.len(), 1);
951 self.0.into_iter().next().unwrap()
954 /// Constructs a partial witness for a pattern given a list of
955 /// patterns expanded by the specialization step.
957 /// When a pattern P is discovered to be useful, this function is used bottom-up
958 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
959 /// of values, V, where each value in that set is not covered by any previously
960 /// used patterns and is covered by the pattern P'. Examples:
962 /// left_ty: tuple of 3 elements
963 /// pats: [10, 20, _] => (10, 20, _)
965 /// left_ty: struct X { a: (bool, &'static str), b: usize}
966 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
967 fn apply_constructor(
971 ctor_wild_subpatterns: &Fields,
974 let len = self.0.len();
975 let arity = ctor_wild_subpatterns.len();
976 let pats = self.0.drain((len - arity)..).rev();
977 ctor_wild_subpatterns.replace_fields(pcx.cx, pats).apply(pcx, ctor)
986 /// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
987 /// The algorithm from the paper has been modified to correctly handle empty
988 /// types. The changes are:
989 /// (0) We don't exit early if the pattern matrix has zero rows. We just
990 /// continue to recurse over columns.
991 /// (1) all_constructors will only return constructors that are statically
992 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
994 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
995 /// to a set of such vectors `m` - this is defined as there being a set of
996 /// inputs that will match `v` but not any of the sets in `m`.
998 /// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
1000 /// This is used both for reachability checking (if a pattern isn't useful in
1001 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1002 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1003 /// matrix isn't exhaustive).
1005 /// `is_under_guard` is used to inform if the pattern has a guard. If it
1006 /// has one it must not be inserted into the matrix. This shouldn't be
1007 /// relied on for soundness.
1009 cx: &MatchCheckCtx<'_>,
1012 witness_preference: WitnessPreference,
1013 is_under_guard: bool,
1016 let Matrix { patterns: rows, .. } = matrix;
1018 // The base case. We are pattern-matching on () and the return value is
1019 // based on whether our matrix has a row or not.
1020 // NOTE: This could potentially be optimized by checking rows.is_empty()
1021 // first and then, if v is non-empty, the return value is based on whether
1022 // the type of the tuple we're checking is inhabited or not.
1024 let ret = if rows.is_empty() {
1025 Usefulness::new_useful(witness_preference)
1027 Usefulness::new_not_useful(witness_preference)
1032 assert!(rows.iter().all(|r| r.len() == v.len()));
1034 // FIXME(Nadrieril): Hack to work around type normalization issues (see rust-lang/rust#72476).
1035 let ty = matrix.heads().next().map_or(cx.type_of(v.head()), |r| cx.type_of(r));
1036 let pcx = PatCtxt { cx, ty: &ty, is_top_level };
1038 // If the first pattern is an or-pattern, expand it.
1039 let ret = if v.head().is_or_pat(cx) {
1040 //expanding or-pattern
1041 let v_head = v.head();
1042 let vs: Vec<_> = v.expand_or_pat(cx).collect();
1043 let alt_count = vs.len();
1044 // We try each or-pattern branch in turn.
1045 let mut matrix = matrix.clone();
1046 let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| {
1047 let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false);
1048 // If pattern has a guard don't add it to the matrix.
1049 if !is_under_guard {
1050 // We push the already-seen patterns into the matrix in order to detect redundant
1051 // branches like `Some(_) | Some(0)`.
1054 usefulness.unsplit_or_pat(i, alt_count, v_head)
1056 Usefulness::merge(witness_preference, usefulnesses)
1058 let v_ctor = v.head_ctor(cx);
1059 // if let Constructor::IntRange(ctor_range) = v_ctor {
1060 // // Lint on likely incorrect range patterns (#63987)
1061 // ctor_range.lint_overlapping_range_endpoints(
1063 // matrix.head_ctors_and_spans(cx),
1064 // matrix.column_count().unwrap_or(0),
1069 // We split the head constructor of `v`.
1070 let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx));
1071 // For each constructor, we compute whether there's a value that starts with it that would
1072 // witness the usefulness of `v`.
1073 let start_matrix = matrix;
1074 let usefulnesses = split_ctors.into_iter().map(|ctor| {
1075 // debug!("specialize({:?})", ctor);
1076 // We cache the result of `Fields::wildcards` because it is used a lot.
1077 let ctor_wild_subpatterns = Fields::wildcards(pcx, &ctor);
1079 start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns);
1080 let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx);
1082 is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
1083 usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
1085 Usefulness::merge(witness_preference, usefulnesses)
1091 /// The arm of a match expression.
1092 #[derive(Clone, Copy)]
1093 pub(crate) struct MatchArm {
1094 pub(crate) pat: PatId,
1095 pub(crate) has_guard: bool,
1098 /// Indicates whether or not a given arm is reachable.
1099 #[derive(Clone, Debug)]
1100 pub(crate) enum Reachability {
1101 /// The arm is reachable. This additionally carries a set of or-pattern branches that have been
1102 /// found to be unreachable despite the overall arm being reachable. Used only in the presence
1103 /// of or-patterns, otherwise it stays empty.
1104 Reachable(Vec<PatId>),
1105 /// The arm is unreachable.
1109 /// The output of checking a match for exhaustiveness and arm reachability.
1110 pub(crate) struct UsefulnessReport {
1111 /// For each arm of the input, whether that arm is reachable after the arms above it.
1112 pub(crate) _arm_usefulness: Vec<(MatchArm, Reachability)>,
1113 /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
1115 pub(crate) non_exhaustiveness_witnesses: Vec<Pat>,
1118 /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
1119 /// of its arms are reachable.
1121 /// Note: the input patterns must have been lowered through
1122 /// `check_match::MatchVisitor::lower_pattern`.
1123 pub(crate) fn compute_match_usefulness(
1124 cx: &MatchCheckCtx<'_>,
1126 ) -> UsefulnessReport {
1127 let mut matrix = Matrix::empty();
1128 let arm_usefulness = arms
1132 let v = PatStack::from_pattern(arm.pat);
1133 let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true);
1137 let reachability = match usefulness {
1138 NoWitnesses(subpats) if subpats.is_empty() => Reachability::Unreachable,
1139 NoWitnesses(subpats) => {
1140 Reachability::Reachable(subpats.list_unreachable_subpatterns(cx).unwrap())
1142 WithWitnesses(..) => panic!("bug"),
1149 cx.pattern_arena.borrow_mut().alloc(Pat::wildcard_from_ty(cx.infer[cx.match_expr].clone()));
1150 let v = PatStack::from_pattern(wild_pattern);
1151 let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, false, true);
1152 let non_exhaustiveness_witnesses = match usefulness {
1153 WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(),
1154 NoWitnesses(_) => panic!("bug"),
1156 UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
1159 pub(crate) type PatternArena = Arena<Pat>;
1162 use super::MatchCheckCtx;
1164 pub(super) trait PatIdExt: Sized {
1165 // fn is_wildcard(self, cx: &MatchCheckCtx<'_>) -> bool;
1166 fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool;
1167 fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec<Self>;
1170 // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs
1171 /// "Signaling" trait used in impl trait to tag lifetimes that you may
1172 /// need to capture but don't really need for other reasons.
1173 /// Basically a workaround; see [this comment] for details.
1175 /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
1176 // FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed.
1177 #[allow(unused_lifetimes)]
1178 pub(crate) trait Captures<'a> {}
1180 impl<'a, T: ?Sized> Captures<'a> for T {}