1 //! Based on rust-lang/rust (last sync f31622a50 2021-11-12)
2 //! <https://github.com/rust-lang/rust/blob/f31622a50/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.
276 use hir_def::{AdtId, DefWithBodyId, HasModule, ModuleId};
277 use smallvec::{smallvec, SmallVec};
278 use typed_arena::Arena;
280 use crate::{db::HirDatabase, Ty, TyExt};
282 use super::deconstruct_pat::{Constructor, DeconstructedPat, Fields, SplitWildcard};
284 use self::{helper::Captures, ArmType::*, Usefulness::*};
286 pub(crate) struct MatchCheckCtx<'a, 'p> {
287 pub(crate) module: ModuleId,
288 pub(crate) body: DefWithBodyId,
289 pub(crate) db: &'a dyn HirDatabase,
290 /// Lowered patterns from arms plus generated by the check.
291 pub(crate) pattern_arena: &'p Arena<DeconstructedPat<'p>>,
294 impl<'a, 'p> MatchCheckCtx<'a, 'p> {
295 pub(super) fn is_uninhabited(&self, _ty: &Ty) -> bool {
296 // FIXME(iDawer) implement exhaustive_patterns feature. More info in:
297 // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085
301 /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
302 pub(super) fn is_foreign_non_exhaustive_enum(&self, ty: &Ty) -> bool {
304 Some((adt @ AdtId::EnumId(_), _)) => {
305 let has_non_exhaustive_attr =
306 self.db.attrs(adt.into()).by_key("non_exhaustive").exists();
307 let is_local = adt.module(self.db.upcast()).krate() == self.module.krate();
308 has_non_exhaustive_attr && !is_local
314 // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types."
315 pub(super) fn feature_exhaustive_patterns(&self) -> bool {
316 // FIXME see MatchCheckCtx::is_uninhabited
321 #[derive(Copy, Clone)]
322 pub(super) struct PatCtxt<'a, 'p> {
323 pub(super) cx: &'a MatchCheckCtx<'a, 'p>,
324 /// Type of the current column under investigation.
325 pub(super) ty: &'a Ty,
326 /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
328 pub(super) is_top_level: bool,
329 /// Wether the current pattern is from a `non_exhaustive` enum.
330 pub(super) is_non_exhaustive: bool,
333 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
336 pub(super) struct PatStack<'p> {
337 pats: SmallVec<[&'p DeconstructedPat<'p>; 2]>,
340 impl<'p> PatStack<'p> {
341 fn from_pattern(pat: &'p DeconstructedPat<'p>) -> Self {
342 Self::from_vec(smallvec![pat])
345 fn from_vec(vec: SmallVec<[&'p DeconstructedPat<'p>; 2]>) -> Self {
346 PatStack { pats: vec }
349 fn is_empty(&self) -> bool {
353 fn len(&self) -> usize {
357 fn head(&self) -> &'p DeconstructedPat<'p> {
361 // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an
362 // or-pattern. Panics if `self` is empty.
363 fn expand_or_pat(&self) -> impl Iterator<Item = PatStack<'p>> + Captures<'_> {
364 self.head().iter_fields().map(move |pat| {
365 let mut new_patstack = PatStack::from_pattern(pat);
366 new_patstack.pats.extend_from_slice(&self.pats[1..]);
371 /// This computes `S(self.head().ctor(), self)`. See top of the file for explanations.
373 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
374 /// fields filled with wild patterns.
376 /// This is roughly the inverse of `Constructor::apply`.
377 fn pop_head_constructor(&self, cx: &MatchCheckCtx<'_, 'p>, ctor: &Constructor) -> PatStack<'p> {
378 // We pop the head pattern and push the new fields extracted from the arguments of
380 let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(cx, ctor);
381 new_fields.extend_from_slice(&self.pats[1..]);
382 PatStack::from_vec(new_fields)
388 pub(super) struct Matrix<'p> {
389 patterns: Vec<PatStack<'p>>,
392 impl<'p> Matrix<'p> {
394 Matrix { patterns: vec![] }
397 /// Number of columns of this matrix. `None` is the matrix is empty.
398 pub(super) fn _column_count(&self) -> Option<usize> {
399 self.patterns.get(0).map(|r| r.len())
402 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively
404 fn push(&mut self, row: PatStack<'p>) {
405 if !row.is_empty() && row.head().is_or_pat() {
406 self.patterns.extend(row.expand_or_pat());
408 self.patterns.push(row);
412 /// Iterate over the first component of each row
413 fn heads(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p>> + Clone + Captures<'_> {
414 self.patterns.iter().map(|r| r.head())
417 /// This computes `S(constructor, self)`. See top of the file for explanations.
418 fn specialize_constructor(&self, pcx: PatCtxt<'_, 'p>, ctor: &Constructor) -> Matrix<'p> {
419 let mut matrix = Matrix::empty();
420 for row in &self.patterns {
421 if ctor.is_covered_by(pcx, row.head().ctor()) {
422 let new_row = row.pop_head_constructor(pcx.cx, ctor);
423 matrix.push(new_row);
430 /// This carries the results of computing usefulness, as described at the top of the file. When
431 /// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track
432 /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
433 /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
434 /// witnesses of non-exhaustiveness when there are any.
435 /// Which variant to use is dictated by `ArmType`.
436 enum Usefulness<'p> {
437 /// If we don't care about witnesses, simply remember if the pattern was useful.
438 NoWitnesses { useful: bool },
439 /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole
440 /// pattern is unreachable.
441 WithWitnesses(Vec<Witness<'p>>),
444 impl<'p> Usefulness<'p> {
445 fn new_useful(preference: ArmType) -> Self {
447 // A single (empty) witness of reachability.
448 FakeExtraWildcard => WithWitnesses(vec![Witness(vec![])]),
449 RealArm => NoWitnesses { useful: true },
452 fn new_not_useful(preference: ArmType) -> Self {
454 FakeExtraWildcard => WithWitnesses(vec![]),
455 RealArm => NoWitnesses { useful: false },
459 fn is_useful(&self) -> bool {
461 Usefulness::NoWitnesses { useful } => *useful,
462 Usefulness::WithWitnesses(witnesses) => !witnesses.is_empty(),
466 /// Combine usefulnesses from two branches. This is an associative operation.
467 fn extend(&mut self, other: Self) {
468 match (&mut *self, other) {
469 (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {}
470 (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o),
471 (WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
472 (NoWitnesses { useful: s_useful }, NoWitnesses { useful: o_useful }) => {
473 *s_useful = *s_useful || o_useful
479 /// After calculating usefulness after a specialization, call this to reconstruct a usefulness
480 /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
481 /// with the results of specializing with the other constructors.
482 fn apply_constructor(
484 pcx: PatCtxt<'_, 'p>,
489 NoWitnesses { .. } => self,
490 WithWitnesses(ref witnesses) if witnesses.is_empty() => self,
491 WithWitnesses(witnesses) => {
492 let new_witnesses = if let Constructor::Missing { .. } = ctor {
493 // We got the special `Missing` constructor, so each of the missing constructors
494 // gives a new pattern that is not caught by the match. We list those patterns.
495 let new_patterns = if pcx.is_non_exhaustive {
496 // Here we don't want the user to try to list all variants, we want them to add
497 // a wildcard, so we only suggest that.
498 vec![DeconstructedPat::wildcard(pcx.ty.clone())]
500 let mut split_wildcard = SplitWildcard::new(pcx);
501 split_wildcard.split(pcx, matrix.heads().map(DeconstructedPat::ctor));
503 // This lets us know if we skipped any variants because they are marked
504 // `doc(hidden)` or they are unstable feature gate (only stdlib types).
505 let mut hide_variant_show_wild = false;
506 // Construct for each missing constructor a "wild" version of this
507 // constructor, that matches everything that can be built with
508 // it. For example, if `ctor` is a `Constructor::Variant` for
509 // `Option::Some`, we get the pattern `Some(_)`.
510 let mut new: Vec<DeconstructedPat<'_>> = split_wildcard
512 .filter_map(|missing_ctor| {
513 // Check if this variant is marked `doc(hidden)`
514 if missing_ctor.is_doc_hidden_variant(pcx)
515 || missing_ctor.is_unstable_variant(pcx)
517 hide_variant_show_wild = true;
520 Some(DeconstructedPat::wild_from_ctor(pcx, missing_ctor.clone()))
524 if hide_variant_show_wild {
525 new.push(DeconstructedPat::wildcard(pcx.ty.clone()))
533 .flat_map(|witness| {
534 new_patterns.iter().map(move |pat| {
540 .map(DeconstructedPat::clone_and_forget_reachability)
549 .map(|witness| witness.apply_constructor(pcx, ctor))
552 WithWitnesses(new_witnesses)
558 #[derive(Copy, Clone, Debug)]
564 /// A witness of non-exhaustiveness for error reporting, represented
565 /// as a list of patterns (in reverse order of construction) with
566 /// wildcards inside to represent elements that can take any inhabitant
567 /// of the type as a value.
569 /// A witness against a list of patterns should have the same types
570 /// and length as the pattern matched against. Because Rust `match`
571 /// is always against a single pattern, at the end the witness will
572 /// have length 1, but in the middle of the algorithm, it can contain
573 /// multiple patterns.
575 /// For example, if we are constructing a witness for the match against
578 /// struct Pair(Option<(u32, u32)>, bool);
580 /// match (p: Pair) {
581 /// Pair(None, _) => {}
582 /// Pair(_, false) => {}
586 /// We'll perform the following steps:
587 /// 1. Start with an empty witness
588 /// `Witness(vec![])`
589 /// 2. Push a witness `true` against the `false`
590 /// `Witness(vec![true])`
591 /// 3. Push a witness `Some(_)` against the `None`
592 /// `Witness(vec![true, Some(_)])`
593 /// 4. Apply the `Pair` constructor to the witnesses
594 /// `Witness(vec![Pair(Some(_), true)])`
596 /// The final `Pair(Some(_), true)` is then the resulting witness.
597 pub(crate) struct Witness<'p>(Vec<DeconstructedPat<'p>>);
599 impl<'p> Witness<'p> {
600 /// Asserts that the witness contains a single pattern, and returns it.
601 fn single_pattern(self) -> DeconstructedPat<'p> {
602 assert_eq!(self.0.len(), 1);
603 self.0.into_iter().next().unwrap()
606 /// Constructs a partial witness for a pattern given a list of
607 /// patterns expanded by the specialization step.
609 /// When a pattern P is discovered to be useful, this function is used bottom-up
610 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
611 /// of values, V, where each value in that set is not covered by any previously
612 /// used patterns and is covered by the pattern P'. Examples:
614 /// left_ty: tuple of 3 elements
615 /// pats: [10, 20, _] => (10, 20, _)
617 /// left_ty: struct X { a: (bool, &'static str), b: usize}
618 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
619 fn apply_constructor(mut self, pcx: PatCtxt<'_, 'p>, ctor: &Constructor) -> Self {
621 let len = self.0.len();
622 let arity = ctor.arity(pcx);
623 let pats = self.0.drain((len - arity)..).rev();
624 let fields = Fields::from_iter(pcx.cx, pats);
625 DeconstructedPat::new(ctor.clone(), fields, pcx.ty.clone())
634 /// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
635 /// The algorithm from the paper has been modified to correctly handle empty
636 /// types. The changes are:
637 /// (0) We don't exit early if the pattern matrix has zero rows. We just
638 /// continue to recurse over columns.
639 /// (1) all_constructors will only return constructors that are statically
640 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
642 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
643 /// to a set of such vectors `m` - this is defined as there being a set of
644 /// inputs that will match `v` but not any of the sets in `m`.
646 /// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
648 /// This is used both for reachability checking (if a pattern isn't useful in
649 /// relation to preceding patterns, it is not reachable) and exhaustiveness
650 /// checking (if a wildcard pattern is useful in relation to a matrix, the
651 /// matrix isn't exhaustive).
653 /// `is_under_guard` is used to inform if the pattern has a guard. If it
654 /// has one it must not be inserted into the matrix. This shouldn't be
655 /// relied on for soundness.
657 cx: &MatchCheckCtx<'_, 'p>,
660 witness_preference: ArmType,
661 is_under_guard: bool,
663 ) -> Usefulness<'p> {
664 let Matrix { patterns: rows, .. } = matrix;
666 // The base case. We are pattern-matching on () and the return value is
667 // based on whether our matrix has a row or not.
668 // NOTE: This could potentially be optimized by checking rows.is_empty()
669 // first and then, if v is non-empty, the return value is based on whether
670 // the type of the tuple we're checking is inhabited or not.
672 let ret = if rows.is_empty() {
673 Usefulness::new_useful(witness_preference)
675 Usefulness::new_not_useful(witness_preference)
680 debug_assert!(rows.iter().all(|r| r.len() == v.len()));
682 let ty = v.head().ty();
683 let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
684 let pcx = PatCtxt { cx, ty, is_top_level, is_non_exhaustive };
686 // If the first pattern is an or-pattern, expand it.
687 let mut ret = Usefulness::new_not_useful(witness_preference);
688 if v.head().is_or_pat() {
689 // We try each or-pattern branch in turn.
690 let mut matrix = matrix.clone();
691 for v in v.expand_or_pat() {
692 let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false);
693 ret.extend(usefulness);
694 // If pattern has a guard don't add it to the matrix.
696 // We push the already-seen patterns into the matrix in order to detect redundant
697 // branches like `Some(_) | Some(0)`.
702 let v_ctor = v.head().ctor();
704 // FIXME: implement `overlapping_range_endpoints` lint
706 // We split the head constructor of `v`.
707 let split_ctors = v_ctor.split(pcx, matrix.heads().map(DeconstructedPat::ctor));
708 // For each constructor, we compute whether there's a value that starts with it that would
709 // witness the usefulness of `v`.
710 let start_matrix = matrix;
711 for ctor in split_ctors {
712 // We cache the result of `Fields::wildcards` because it is used a lot.
713 let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor);
714 let v = v.pop_head_constructor(cx, &ctor);
716 is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false);
717 let usefulness = usefulness.apply_constructor(pcx, start_matrix, &ctor);
719 // FIXME: implement `non_exhaustive_omitted_patterns` lint
721 ret.extend(usefulness);
726 v.head().set_reachable();
732 /// The arm of a match expression.
733 #[derive(Clone, Copy)]
734 pub(crate) struct MatchArm<'p> {
735 pub(crate) pat: &'p DeconstructedPat<'p>,
736 pub(crate) has_guard: bool,
739 /// Indicates whether or not a given arm is reachable.
740 #[derive(Clone, Debug)]
741 pub(crate) enum Reachability {
742 /// The arm is reachable. This additionally carries a set of or-pattern branches that have been
743 /// found to be unreachable despite the overall arm being reachable. Used only in the presence
744 /// of or-patterns, otherwise it stays empty.
745 // FIXME: store ureachable subpattern IDs
747 /// The arm is unreachable.
751 /// The output of checking a match for exhaustiveness and arm reachability.
752 pub(crate) struct UsefulnessReport<'p> {
753 /// For each arm of the input, whether that arm is reachable after the arms above it.
754 pub(crate) _arm_usefulness: Vec<(MatchArm<'p>, Reachability)>,
755 /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of
757 pub(crate) non_exhaustiveness_witnesses: Vec<DeconstructedPat<'p>>,
760 /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which
761 /// of its arms are reachable.
763 /// Note: the input patterns must have been lowered through
764 /// `check_match::MatchVisitor::lower_pattern`.
765 pub(crate) fn compute_match_usefulness<'p>(
766 cx: &MatchCheckCtx<'_, 'p>,
767 arms: &[MatchArm<'p>],
769 ) -> UsefulnessReport<'p> {
770 let mut matrix = Matrix::empty();
771 let arm_usefulness = arms
775 let v = PatStack::from_pattern(arm.pat);
776 is_useful(cx, &matrix, &v, RealArm, arm.has_guard, true);
780 let reachability = if arm.pat.is_reachable() {
781 Reachability::Reachable
783 Reachability::Unreachable
789 let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty.clone()));
790 let v = PatStack::from_pattern(wild_pattern);
791 let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, false, true);
792 let non_exhaustiveness_witnesses = match usefulness {
793 WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(),
794 NoWitnesses { .. } => panic!("bug"),
796 UsefulnessReport { _arm_usefulness: arm_usefulness, non_exhaustiveness_witnesses }
799 pub(crate) mod helper {
800 // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs
801 /// "Signaling" trait used in impl trait to tag lifetimes that you may
802 /// need to capture but don't really need for other reasons.
803 /// Basically a workaround; see [this comment] for details.
805 /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
806 // FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed.
807 #[allow(unused_lifetimes)]
808 pub(crate) trait Captures<'a> {}
810 impl<'a, T: ?Sized> Captures<'a> for T {}