// Based on rust-lang/rust 1.52.0-nightly (25c15cdbe 2021-04-22) // https://github.com/rust-lang/rust/blob/25c15cdbe/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs use std::{cell::RefCell, iter::FromIterator, ops::Index, sync::Arc}; use hir_def::{body::Body, expr::ExprId, HasModule, ModuleId}; use la_arena::Arena; use once_cell::unsync::OnceCell; use rustc_hash::FxHashMap; use smallvec::{smallvec, SmallVec}; use crate::{db::HirDatabase, InferenceResult, Ty}; use super::{ deconstruct_pat::{Constructor, Fields, SplitWildcard}, Pat, PatId, PatKind, }; use self::{ helper::{Captures, PatIdExt}, Usefulness::*, WitnessPreference::*, }; pub(crate) struct MatchCheckCtx<'a> { pub(crate) module: ModuleId, pub(crate) match_expr: ExprId, pub(crate) body: Arc, pub(crate) infer: &'a InferenceResult, pub(crate) db: &'a dyn HirDatabase, /// Patterns from self.body.pats plus generated by the check. pub(crate) pattern_arena: &'a RefCell, } impl<'a> MatchCheckCtx<'a> { pub(super) fn is_uninhabited(&self, ty: &Ty) -> bool { // FIXME(iDawer) implement exhaustive_patterns feature. More info in: // Tracking issue for RFC 1872: exhaustive_patterns feature https://github.com/rust-lang/rust/issues/51085 false } /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`. pub(super) fn is_foreign_non_exhaustive_enum(&self, enum_id: hir_def::EnumId) -> bool { let has_non_exhaustive_attr = self.db.attrs(enum_id.into()).by_key("non_exhaustive").exists(); let is_local = hir_def::AdtId::from(enum_id).module(self.db.upcast()).krate() == self.module.krate(); has_non_exhaustive_attr && !is_local } // Rust feature described as "Allows exhaustive pattern matching on types that contain uninhabited types." pub(super) fn feature_exhaustive_patterns(&self) -> bool { // TODO false } pub(super) fn alloc_pat(&self, pat: Pat) -> PatId { self.pattern_arena.borrow_mut().alloc(pat) } /// Get type of a pattern. Handles expanded patterns. pub(super) fn type_of(&self, pat: PatId) -> Ty { self.pattern_arena.borrow()[pat].ty.clone() } } #[derive(Copy, Clone)] pub(super) struct PatCtxt<'a> { pub(super) cx: &'a MatchCheckCtx<'a>, /// Type of the current column under investigation. pub(super) ty: &'a Ty, /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a /// subpattern. pub(super) is_top_level: bool, } pub(crate) fn expand_pattern(pat: Pat) -> Pat { // TODO: LiteralExpander, it is about string literal patterns pat } impl Pat { fn is_wildcard(&self) -> bool { matches!(*self.kind, PatKind::Binding { subpattern: None, .. } | PatKind::Wild) } } impl PatIdExt for PatId { fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool { matches!(*cx.pattern_arena.borrow()[self].kind, PatKind::Or { .. }) } /// Recursively expand this pattern into its subpatterns. Only useful for or-patterns. fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec { fn expand(pat: PatId, vec: &mut Vec, mut pat_arena: &mut PatternArena) { if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() { let pats = pats.clone(); for pat in pats { // TODO(iDawer): Ugh, I want to go back to references (PatId -> &Pat) let pat = pat_arena.alloc(pat.clone()); expand(pat, vec, pat_arena); } } else { vec.push(pat) } } let mut pat_arena = cx.pattern_arena.borrow_mut(); let mut pats = Vec::new(); expand(self, &mut pats, &mut pat_arena); pats } } /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]` /// works well. #[derive(Clone)] pub(super) struct PatStack { pats: SmallVec<[PatId; 2]>, /// Cache for the constructor of the head head_ctor: OnceCell, } impl PatStack { fn from_pattern(pat: PatId) -> Self { Self::from_vec(smallvec![pat]) } fn from_vec(vec: SmallVec<[PatId; 2]>) -> Self { PatStack { pats: vec, head_ctor: OnceCell::new() } } fn is_empty(&self) -> bool { self.pats.is_empty() } fn len(&self) -> usize { self.pats.len() } fn head(&self) -> PatId { self.pats[0] } #[inline] fn head_ctor(&self, cx: &MatchCheckCtx<'_>) -> &Constructor { self.head_ctor.get_or_init(|| Constructor::from_pat(cx, self.head())) } fn iter(&self) -> impl Iterator + '_ { self.pats.iter().copied() } // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an // or-pattern. Panics if `self` is empty. fn expand_or_pat(&self, cx: &MatchCheckCtx<'_>) -> impl Iterator + '_ { self.head().expand_or_pat(cx).into_iter().map(move |pat| { let mut new_patstack = PatStack::from_pattern(pat); new_patstack.pats.extend_from_slice(&self.pats[1..]); new_patstack }) } /// This computes `S(self.head_ctor(), self)`. See top of the file for explanations. /// /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing /// fields filled with wild patterns. /// /// This is roughly the inverse of `Constructor::apply`. fn pop_head_constructor( &self, ctor_wild_subpatterns: &Fields, cx: &MatchCheckCtx<'_>, ) -> PatStack { // We pop the head pattern and push the new fields extracted from the arguments of // `self.head()`. let mut new_fields = ctor_wild_subpatterns.replace_with_pattern_arguments(self.head(), cx).into_patterns(); new_fields.extend_from_slice(&self.pats[1..]); PatStack::from_vec(new_fields) } } impl Default for PatStack { fn default() -> Self { Self::from_vec(smallvec![]) } } impl PartialEq for PatStack { fn eq(&self, other: &Self) -> bool { self.pats == other.pats } } impl FromIterator for PatStack { fn from_iter(iter: T) -> Self where T: IntoIterator, { Self::from_vec(iter.into_iter().collect()) } } /// A 2D matrix. #[derive(Clone)] pub(super) struct Matrix { patterns: Vec, } impl Matrix { fn empty() -> Self { Matrix { patterns: vec![] } } /// Number of columns of this matrix. `None` is the matrix is empty. pub(super) fn column_count(&self) -> Option { self.patterns.get(0).map(|r| r.len()) } /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively /// expands it. fn push(&mut self, row: PatStack, cx: &MatchCheckCtx<'_>) { if !row.is_empty() && row.head().is_or_pat(cx) { for row in row.expand_or_pat(cx) { self.patterns.push(row); } } else { self.patterns.push(row); } } /// Iterate over the first component of each row fn heads(&self) -> impl Iterator + '_ { self.patterns.iter().map(|r| r.head()) } /// Iterate over the first constructor of each row. fn head_ctors<'a>( &'a self, cx: &'a MatchCheckCtx<'_>, ) -> impl Iterator + Clone { self.patterns.iter().map(move |r| r.head_ctor(cx)) } /// This computes `S(constructor, self)`. See top of the file for explanations. fn specialize_constructor( &self, pcx: PatCtxt<'_>, ctor: &Constructor, ctor_wild_subpatterns: &Fields, ) -> Matrix { let rows = self .patterns .iter() .filter(|r| ctor.is_covered_by(pcx, r.head_ctor(pcx.cx))) .map(|r| r.pop_head_constructor(ctor_wild_subpatterns, pcx.cx)); Matrix::from_iter(rows, pcx.cx) } fn from_iter(rows: impl IntoIterator, cx: &MatchCheckCtx<'_>) -> Matrix { let mut matrix = Matrix::empty(); for x in rows { // Using `push` ensures we correctly expand or-patterns. matrix.push(x, cx); } matrix } } /// Given a pattern or a pattern-stack, this struct captures a set of its subpatterns. We use that /// to track reachable sub-patterns arising from or-patterns. In the absence of or-patterns this /// will always be either `Empty` (the whole pattern is unreachable) or `Full` (the whole pattern /// is reachable). When there are or-patterns, some subpatterns may be reachable while others /// aren't. In this case the whole pattern still counts as reachable, but we will lint the /// unreachable subpatterns. /// /// This supports a limited set of operations, so not all possible sets of subpatterns can be /// represented. That's ok, we only want the ones that make sense for our usage. /// /// What we're doing is illustrated by this: /// ``` /// match (true, 0) { /// (true, 0) => {} /// (_, 1) => {} /// (true | false, 0 | 1) => {} /// } /// ``` /// When we try the alternatives of the `true | false` or-pattern, the last `0` is reachable in the /// `false` alternative but not the `true`. So overall it is reachable. By contrast, the last `1` /// is not reachable in either alternative, so we want to signal this to the user. /// Therefore we take the union of sets of reachable patterns coming from different alternatives in /// order to figure out which subpatterns are overall reachable. /// /// Invariant: we try to construct the smallest representation we can. In particular if /// `self.is_empty()` we ensure that `self` is `Empty`, and same with `Full`. This is not important /// for correctness currently. #[derive(Debug, Clone)] enum SubPatSet { /// The empty set. This means the pattern is unreachable. Empty, /// The set containing the full pattern. Full, /// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each /// of its subpatterns. Missing entries in the map are implicitly full, because that's the /// common case. Seq { subpats: FxHashMap }, /// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing /// entries in the map are implicitly empty. Note: we always flatten nested or-patterns. Alt { subpats: FxHashMap, /// Counts the total number of alternatives in the pattern alt_count: usize, /// We keep the pattern around to retrieve spans. pat: PatId, }, } impl SubPatSet { fn full() -> Self { SubPatSet::Full } fn empty() -> Self { SubPatSet::Empty } fn is_empty(&self) -> bool { match self { SubPatSet::Empty => true, SubPatSet::Full => false, // If any subpattern in a sequence is unreachable, the whole pattern is unreachable. SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_empty()), // An or-pattern is reachable if any of its alternatives is. SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_empty()), } } fn is_full(&self) -> bool { match self { SubPatSet::Empty => false, SubPatSet::Full => true, // The whole pattern is reachable only when all its alternatives are. SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_full()), // The whole or-pattern is reachable only when all its alternatives are. SubPatSet::Alt { subpats, alt_count, .. } => { subpats.len() == *alt_count && subpats.values().all(|set| set.is_full()) } } } /// Union `self` with `other`, mutating `self`. fn union(&mut self, other: Self) { use SubPatSet::*; // Union with full stays full; union with empty changes nothing. if self.is_full() || other.is_empty() { return; } else if self.is_empty() { *self = other; return; } else if other.is_full() { *self = Full; return; } match (&mut *self, other) { (Seq { subpats: s_set }, Seq { subpats: mut o_set }) => { s_set.retain(|i, s_sub_set| { // Missing entries count as full. let o_sub_set = o_set.remove(&i).unwrap_or(Full); s_sub_set.union(o_sub_set); // We drop full entries. !s_sub_set.is_full() }); // Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since // unioning with full returns full, we can drop those entries. } (Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => { s_set.retain(|i, s_sub_set| { // Missing entries count as empty. let o_sub_set = o_set.remove(&i).unwrap_or(Empty); s_sub_set.union(o_sub_set); // We drop empty entries. !s_sub_set.is_empty() }); // Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since // unioning with empty changes nothing, we can take those entries as is. s_set.extend(o_set); } _ => panic!("bug"), } if self.is_full() { *self = Full; } } /// Returns a list of the unreachable subpatterns. If `self` is empty (i.e. the /// whole pattern is unreachable) we return `None`. fn list_unreachable_subpatterns(&self, cx: &MatchCheckCtx<'_>) -> Option> { /// Panics if `set.is_empty()`. fn fill_subpats( set: &SubPatSet, unreachable_pats: &mut Vec, cx: &MatchCheckCtx<'_>, ) { match set { SubPatSet::Empty => panic!("bug"), SubPatSet::Full => {} SubPatSet::Seq { subpats } => { for (_, sub_set) in subpats { fill_subpats(sub_set, unreachable_pats, cx); } } SubPatSet::Alt { subpats, pat, alt_count, .. } => { let expanded = pat.expand_or_pat(cx); for i in 0..*alt_count { let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Empty); if sub_set.is_empty() { // Found a unreachable subpattern. unreachable_pats.push(expanded[i]); } else { fill_subpats(sub_set, unreachable_pats, cx); } } } } } if self.is_empty() { return None; } if self.is_full() { // No subpatterns are unreachable. return Some(Vec::new()); } let mut unreachable_pats = Vec::new(); fill_subpats(self, &mut unreachable_pats, cx); Some(unreachable_pats) } /// When `self` refers to a patstack that was obtained from specialization, after running /// `unspecialize` it will refer to the original patstack before specialization. fn unspecialize(self, arity: usize) -> Self { use SubPatSet::*; match self { Full => Full, Empty => Empty, Seq { subpats } => { // We gather the first `arity` subpatterns together and shift the remaining ones. let mut new_subpats = FxHashMap::default(); let mut new_subpats_first_col = FxHashMap::default(); for (i, sub_set) in subpats { if i < arity { // The first `arity` indices are now part of the pattern in the first // column. new_subpats_first_col.insert(i, sub_set); } else { // Indices after `arity` are simply shifted new_subpats.insert(i - arity + 1, sub_set); } } // If `new_subpats_first_col` has no entries it counts as full, so we can omit it. if !new_subpats_first_col.is_empty() { new_subpats.insert(0, Seq { subpats: new_subpats_first_col }); } Seq { subpats: new_subpats } } Alt { .. } => panic!("bug"), } } /// When `self` refers to a patstack that was obtained from splitting an or-pattern, after /// running `unspecialize` it will refer to the original patstack before splitting. /// /// For example: /// ``` /// match Some(true) { /// Some(true) => {} /// None | Some(true | false) => {} /// } /// ``` /// Here `None` would return the full set and `Some(true | false)` would return the set /// containing `false`. After `unsplit_or_pat`, we want the set to contain `None` and `false`. /// This is what this function does. fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: PatId) -> Self { use SubPatSet::*; if self.is_empty() { return Empty; } // Subpatterns coming from inside the or-pattern alternative itself, e.g. in `None | Some(0 // | 1)`. let set_first_col = match &mut self { Full => Full, Seq { subpats } => subpats.remove(&0).unwrap_or(Full), Empty => unreachable!(), Alt { .. } => panic!("bug"), // `self` is a patstack }; let mut subpats_first_col = FxHashMap::default(); subpats_first_col.insert(alt_id, set_first_col); let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count }; let mut subpats = match self { Full => FxHashMap::default(), Seq { subpats } => subpats, Empty => unreachable!(), Alt { .. } => panic!("bug"), // `self` is a patstack }; subpats.insert(0, set_first_col); Seq { subpats } } } /// This carries the results of computing usefulness, as described at the top of the file. When /// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of /// witnesses of non-exhaustiveness when there are any. /// Which variant to use is dictated by `WitnessPreference`. #[derive(Clone, Debug)] enum Usefulness { /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates /// the whole pattern is unreachable. If not, this indicates that the pattern is reachable but /// that some sub-patterns may be unreachable (due to or-patterns). In the absence of /// or-patterns this will always be either `Empty` (the whole pattern is unreachable) or `Full` /// (the whole pattern is reachable). NoWitnesses(SubPatSet), /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole /// pattern is unreachable. WithWitnesses(Vec), } impl Usefulness { fn new_useful(preference: WitnessPreference) -> Self { match preference { ConstructWitness => WithWitnesses(vec![Witness(vec![])]), LeaveOutWitness => NoWitnesses(SubPatSet::full()), } } fn new_not_useful(preference: WitnessPreference) -> Self { match preference { ConstructWitness => WithWitnesses(vec![]), LeaveOutWitness => NoWitnesses(SubPatSet::empty()), } } /// Combine usefulnesses from two branches. This is an associative operation. fn extend(&mut self, other: Self) { match (&mut *self, other) { (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {} (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o), (WithWitnesses(s), WithWitnesses(o)) => s.extend(o), (NoWitnesses(s), NoWitnesses(o)) => s.union(o), _ => unreachable!(), } } /// When trying several branches and each returns a `Usefulness`, we need to combine the /// results together. fn merge(pref: WitnessPreference, usefulnesses: impl Iterator) -> Self { let mut ret = Self::new_not_useful(pref); for u in usefulnesses { ret.extend(u); if let NoWitnesses(subpats) = &ret { if subpats.is_full() { // Once we reach the full set, more unions won't change the result. return ret; } } } ret } /// After calculating the usefulness for a branch of an or-pattern, call this to make this /// usefulness mergeable with those from the other branches. fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: PatId) -> Self { match self { NoWitnesses(subpats) => NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat)), WithWitnesses(_) => panic!("bug"), } } /// After calculating usefulness after a specialization, call this to recontruct a usefulness /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged /// with the results of specializing with the other constructors. fn apply_constructor( self, pcx: PatCtxt<'_>, matrix: &Matrix, ctor: &Constructor, ctor_wild_subpatterns: &Fields, ) -> Self { match self { WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses), WithWitnesses(witnesses) => { let new_witnesses = if matches!(ctor, Constructor::Missing) { let mut split_wildcard = SplitWildcard::new(pcx); split_wildcard.split(pcx, matrix.head_ctors(pcx.cx)); // Construct for each missing constructor a "wild" version of this // constructor, that matches everything that can be built with // it. For example, if `ctor` is a `Constructor::Variant` for // `Option::Some`, we get the pattern `Some(_)`. let new_patterns: Vec<_> = split_wildcard .iter_missing(pcx) .map(|missing_ctor| { Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor) }) .collect(); witnesses .into_iter() .flat_map(|witness| { new_patterns.iter().map(move |pat| { let mut witness = witness.clone(); witness.0.push(pat.clone()); witness }) }) .collect() } else { witnesses .into_iter() .map(|witness| witness.apply_constructor(pcx, &ctor, ctor_wild_subpatterns)) .collect() }; WithWitnesses(new_witnesses) } NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())), } } } #[derive(Copy, Clone, Debug)] enum WitnessPreference { ConstructWitness, LeaveOutWitness, } /// A witness of non-exhaustiveness for error reporting, represented /// as a list of patterns (in reverse order of construction) with /// wildcards inside to represent elements that can take any inhabitant /// of the type as a value. /// /// A witness against a list of patterns should have the same types /// and length as the pattern matched against. Because Rust `match` /// is always against a single pattern, at the end the witness will /// have length 1, but in the middle of the algorithm, it can contain /// multiple patterns. /// /// For example, if we are constructing a witness for the match against /// /// ``` /// struct Pair(Option<(u32, u32)>, bool); /// /// match (p: Pair) { /// Pair(None, _) => {} /// Pair(_, false) => {} /// } /// ``` /// /// We'll perform the following steps: /// 1. Start with an empty witness /// `Witness(vec![])` /// 2. Push a witness `true` against the `false` /// `Witness(vec![true])` /// 3. Push a witness `Some(_)` against the `None` /// `Witness(vec![true, Some(_)])` /// 4. Apply the `Pair` constructor to the witnesses /// `Witness(vec![Pair(Some(_), true)])` /// /// The final `Pair(Some(_), true)` is then the resulting witness. #[derive(Clone, Debug)] pub(crate) struct Witness(Vec); impl Witness { /// Asserts that the witness contains a single pattern, and returns it. fn single_pattern(self) -> Pat { assert_eq!(self.0.len(), 1); self.0.into_iter().next().unwrap() } /// Constructs a partial witness for a pattern given a list of /// patterns expanded by the specialization step. /// /// When a pattern P is discovered to be useful, this function is used bottom-up /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset /// of values, V, where each value in that set is not covered by any previously /// used patterns and is covered by the pattern P'. Examples: /// /// left_ty: tuple of 3 elements /// pats: [10, 20, _] => (10, 20, _) /// /// left_ty: struct X { a: (bool, &'static str), b: usize} /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 } fn apply_constructor( mut self, pcx: PatCtxt<'_>, ctor: &Constructor, ctor_wild_subpatterns: &Fields, ) -> Self { let pat = { let len = self.0.len(); let arity = ctor_wild_subpatterns.len(); let pats = self.0.drain((len - arity)..).rev(); ctor_wild_subpatterns.replace_fields(pcx.cx, pats).apply(pcx, ctor) }; self.0.push(pat); self } } /// Algorithm from . /// The algorithm from the paper has been modified to correctly handle empty /// types. The changes are: /// (0) We don't exit early if the pattern matrix has zero rows. We just /// continue to recurse over columns. /// (1) all_constructors will only return constructors that are statically /// possible. E.g., it will only return `Ok` for `Result`. /// /// This finds whether a (row) vector `v` of patterns is 'useful' in relation /// to a set of such vectors `m` - this is defined as there being a set of /// inputs that will match `v` but not any of the sets in `m`. /// /// All the patterns at each column of the `matrix ++ v` matrix must have the same type. /// /// This is used both for reachability checking (if a pattern isn't useful in /// relation to preceding patterns, it is not reachable) and exhaustiveness /// checking (if a wildcard pattern is useful in relation to a matrix, the /// matrix isn't exhaustive). /// /// `is_under_guard` is used to inform if the pattern has a guard. If it /// has one it must not be inserted into the matrix. This shouldn't be /// relied on for soundness. fn is_useful( cx: &MatchCheckCtx<'_>, matrix: &Matrix, v: &PatStack, witness_preference: WitnessPreference, is_under_guard: bool, is_top_level: bool, ) -> Usefulness { let Matrix { patterns: rows, .. } = matrix; // The base case. We are pattern-matching on () and the return value is // based on whether our matrix has a row or not. // NOTE: This could potentially be optimized by checking rows.is_empty() // first and then, if v is non-empty, the return value is based on whether // the type of the tuple we're checking is inhabited or not. if v.is_empty() { let ret = if rows.is_empty() { Usefulness::new_useful(witness_preference) } else { Usefulness::new_not_useful(witness_preference) }; return ret; } assert!(rows.iter().all(|r| r.len() == v.len())); // FIXME(Nadrieril): Hack to work around type normalization issues (see rust-lang/rust#72476). // TODO(iDawer): ty.strip_references() ? let ty = matrix.heads().next().map_or(cx.type_of(v.head()), |r| cx.type_of(r)); let pcx = PatCtxt { cx, ty: &ty, is_top_level }; // If the first pattern is an or-pattern, expand it. let ret = if v.head().is_or_pat(cx) { //expanding or-pattern let v_head = v.head(); let vs: Vec<_> = v.expand_or_pat(cx).collect(); let alt_count = vs.len(); // We try each or-pattern branch in turn. let mut matrix = matrix.clone(); let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| { let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false); // If pattern has a guard don't add it to the matrix. if !is_under_guard { // We push the already-seen patterns into the matrix in order to detect redundant // branches like `Some(_) | Some(0)`. matrix.push(v, cx); } usefulness.unsplit_or_pat(i, alt_count, v_head) }); Usefulness::merge(witness_preference, usefulnesses) } else { let v_ctor = v.head_ctor(cx); // if let Constructor::IntRange(ctor_range) = v_ctor { // // Lint on likely incorrect range patterns (#63987) // ctor_range.lint_overlapping_range_endpoints( // pcx, // matrix.head_ctors_and_spans(cx), // matrix.column_count().unwrap_or(0), // hir_id, // ) // } // We split the head constructor of `v`. let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx)); // For each constructor, we compute whether there's a value that starts with it that would // witness the usefulness of `v`. let start_matrix = matrix; let usefulnesses = split_ctors.into_iter().map(|ctor| { // debug!("specialize({:?})", ctor); // We cache the result of `Fields::wildcards` because it is used a lot. let ctor_wild_subpatterns = Fields::wildcards(pcx, &ctor); let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor, &ctor_wild_subpatterns); let v = v.pop_head_constructor(&ctor_wild_subpatterns, cx); let usefulness = is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false); usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns) }); Usefulness::merge(witness_preference, usefulnesses) }; ret } /// The arm of a match expression. #[derive(Clone, Copy)] pub(crate) struct MatchArm { pub(crate) pat: PatId, pub(crate) has_guard: bool, } /// Indicates whether or not a given arm is reachable. #[derive(Clone, Debug)] pub(crate) enum Reachability { /// The arm is reachable. This additionally carries a set of or-pattern branches that have been /// found to be unreachable despite the overall arm being reachable. Used only in the presence /// of or-patterns, otherwise it stays empty. Reachable(Vec), /// The arm is unreachable. Unreachable, } /// The output of checking a match for exhaustiveness and arm reachability. pub(crate) struct UsefulnessReport { /// For each arm of the input, whether that arm is reachable after the arms above it. pub(crate) arm_usefulness: Vec<(MatchArm, Reachability)>, /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of /// exhaustiveness. pub(crate) non_exhaustiveness_witnesses: Vec, } /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which /// of its arms are reachable. /// /// Note: the input patterns must have been lowered through /// `check_match::MatchVisitor::lower_pattern`. pub(crate) fn compute_match_usefulness( cx: &MatchCheckCtx<'_>, arms: &[MatchArm], ) -> UsefulnessReport { let mut matrix = Matrix::empty(); let arm_usefulness: Vec<_> = arms .iter() .copied() .map(|arm| { let v = PatStack::from_pattern(arm.pat); let usefulness = is_useful(cx, &matrix, &v, LeaveOutWitness, arm.has_guard, true); if !arm.has_guard { matrix.push(v, cx); } let reachability = match usefulness { NoWitnesses(subpats) if subpats.is_empty() => Reachability::Unreachable, NoWitnesses(subpats) => { Reachability::Reachable(subpats.list_unreachable_subpatterns(cx).unwrap()) } WithWitnesses(..) => panic!("bug"), }; (arm, reachability) }) .collect(); let wild_pattern = cx.pattern_arena.borrow_mut().alloc(Pat::wildcard_from_ty(&cx.infer[cx.match_expr])); let v = PatStack::from_pattern(wild_pattern); let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, false, true); let non_exhaustiveness_witnesses = match usefulness { WithWitnesses(pats) => pats.into_iter().map(Witness::single_pattern).collect(), NoWitnesses(_) => panic!("bug"), }; UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses } } pub(crate) type PatternArena = Arena; mod helper { use hir_def::expr::{Pat, PatId}; use super::MatchCheckCtx; pub(super) trait PatIdExt: Sized { // fn is_wildcard(self, cx: &MatchCheckCtx<'_>) -> bool; fn is_or_pat(self, cx: &MatchCheckCtx<'_>) -> bool; fn expand_or_pat(self, cx: &MatchCheckCtx<'_>) -> Vec; } // Copy-pasted from rust/compiler/rustc_data_structures/src/captures.rs /// "Signaling" trait used in impl trait to tag lifetimes that you may /// need to capture but don't really need for other reasons. /// Basically a workaround; see [this comment] for details. /// /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999 // FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed. #[allow(unused_lifetimes)] pub trait Captures<'a> {} impl<'a, T: ?Sized> Captures<'a> for T {} } #[test] fn it_works() {}