From e84efc4a4656e54a4f08b99592d5d98ac5726449 Mon Sep 17 00:00:00 2001 From: Dawer <7803845+iDawer@users.noreply.github.com> Date: Tue, 11 May 2021 17:18:16 +0500 Subject: Replace the old match checking algorithm --- .../src/diagnostics/match_check/usefulness.rs | 1180 ++++++++++++++++++++ 1 file changed, 1180 insertions(+) create mode 100644 crates/hir_ty/src/diagnostics/match_check/usefulness.rs (limited to 'crates/hir_ty/src/diagnostics/match_check/usefulness.rs') diff --git a/crates/hir_ty/src/diagnostics/match_check/usefulness.rs b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs new file mode 100644 index 000000000..b01e3557c --- /dev/null +++ b/crates/hir_ty/src/diagnostics/match_check/usefulness.rs @@ -0,0 +1,1180 @@ +//! 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 +//! +//! ----- +//! +//! This file includes the logic for exhaustiveness and reachability checking for pattern-matching. +//! Specifically, given a list of patterns for a type, we can tell whether: +//! (a) each pattern is reachable (reachability) +//! (b) the patterns cover every possible value for the type (exhaustiveness) +//! +//! The algorithm implemented here is a modified version of the one described in [this +//! paper](http://moscova.inria.fr/~maranget/papers/warn/index.html). We have however generalized +//! it to accommodate the variety of patterns that Rust supports. We thus explain our version here, +//! without being as rigorous. +//! +//! +//! # Summary +//! +//! The core of the algorithm is the notion of "usefulness". A pattern `q` is said to be *useful* +//! relative to another pattern `p` of the same type if there is a value that is matched by `q` and +//! not matched by `p`. This generalizes to many `p`s: `q` is useful w.r.t. a list of patterns +//! `p_1 .. p_n` if there is a value that is matched by `q` and by none of the `p_i`. We write +//! `usefulness(p_1 .. p_n, q)` for a function that returns a list of such values. The aim of this +//! file is to compute it efficiently. +//! +//! This is enough to compute reachability: a pattern in a `match` expression is reachable iff it +//! is useful w.r.t. the patterns above it: +//! ```rust +//! match x { +//! Some(_) => ..., +//! None => ..., // reachable: `None` is matched by this but not the branch above +//! Some(0) => ..., // unreachable: all the values this matches are already matched by +//! // `Some(_)` above +//! } +//! ``` +//! +//! This is also enough to compute exhaustiveness: a match is exhaustive iff the wildcard `_` +//! pattern is _not_ useful w.r.t. the patterns in the match. The values returned by `usefulness` +//! are used to tell the user which values are missing. +//! ```rust +//! match x { +//! Some(0) => ..., +//! None => ..., +//! // not exhaustive: `_` is useful because it matches `Some(1)` +//! } +//! ``` +//! +//! The entrypoint of this file is the [`compute_match_usefulness`] function, which computes +//! reachability for each match branch and exhaustiveness for the whole match. +//! +//! +//! # Constructors and fields +//! +//! Note: we will often abbreviate "constructor" as "ctor". +//! +//! The idea that powers everything that is done in this file is the following: a (matcheable) +//! value is made from a constructor applied to a number of subvalues. Examples of constructors are +//! `Some`, `None`, `(,)` (the 2-tuple constructor), `Foo {..}` (the constructor for a struct +//! `Foo`), and `2` (the constructor for the number `2`). This is natural when we think of +//! pattern-matching, and this is the basis for what follows. +//! +//! Some of the ctors listed above might feel weird: `None` and `2` don't take any arguments. +//! That's ok: those are ctors that take a list of 0 arguments; they are the simplest case of +//! ctors. We treat `2` as a ctor because `u64` and other number types behave exactly like a huge +//! `enum`, with one variant for each number. This allows us to see any matcheable value as made up +//! from a tree of ctors, each having a set number of children. For example: `Foo { bar: None, +//! baz: Ok(0) }` is made from 4 different ctors, namely `Foo{..}`, `None`, `Ok` and `0`. +//! +//! This idea can be extended to patterns: they are also made from constructors applied to fields. +//! A pattern for a given type is allowed to use all the ctors for values of that type (which we +//! call "value constructors"), but there are also pattern-only ctors. The most important one is +//! the wildcard (`_`), and the others are integer ranges (`0..=10`), variable-length slices (`[x, +//! ..]`), and or-patterns (`Ok(0) | Err(_)`). Examples of valid patterns are `42`, `Some(_)`, `Foo +//! { bar: Some(0) | None, baz: _ }`. Note that a binder in a pattern (e.g. `Some(x)`) matches the +//! same values as a wildcard (e.g. `Some(_)`), so we treat both as wildcards. +//! +//! From this deconstruction we can compute whether a given value matches a given pattern; we +//! simply look at ctors one at a time. Given a pattern `p` and a value `v`, we want to compute +//! `matches!(v, p)`. It's mostly straightforward: we compare the head ctors and when they match +//! we compare their fields recursively. A few representative examples: +//! +//! - `matches!(v, _) := true` +//! - `matches!((v0, v1), (p0, p1)) := matches!(v0, p0) && matches!(v1, p1)` +//! - `matches!(Foo { bar: v0, baz: v1 }, Foo { bar: p0, baz: p1 }) := matches!(v0, p0) && matches!(v1, p1)` +//! - `matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)` +//! - `matches!(Ok(v0), Err(p0)) := false` (incompatible variants) +//! - `matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)` +//! - `matches!([v0], [p0, .., p1]) := false` (incompatible lengths) +//! - `matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)` +//! - `matches!(v, p0 | p1) := matches!(v, p0) || matches!(v, p1)` +//! +//! Constructors, fields and relevant operations are defined in the [`super::deconstruct_pat`] module. +//! +//! Note: this constructors/fields distinction may not straightforwardly apply to every Rust type. +//! For example a value of type `Rc` can't be deconstructed that way, and `&str` has an +//! infinitude of constructors. There are also subtleties with visibility of fields and +//! uninhabitedness and various other things. The constructors idea can be extended to handle most +//! of these subtleties though; caveats are documented where relevant throughout the code. +//! +//! Whether constructors cover each other is computed by [`Constructor::is_covered_by`]. +//! +//! +//! # Specialization +//! +//! Recall that we wish to compute `usefulness(p_1 .. p_n, q)`: given a list of patterns `p_1 .. +//! p_n` and a pattern `q`, all of the same type, we want to find a list of values (called +//! "witnesses") that are matched by `q` and by none of the `p_i`. We obviously don't just +//! enumerate all possible values. From the discussion above we see that we can proceed +//! ctor-by-ctor: for each value ctor of the given type, we ask "is there a value that starts with +//! this constructor and matches `q` and none of the `p_i`?". As we saw above, there's a lot we can +//! say from knowing only the first constructor of our candidate value. +//! +//! Let's take the following example: +//! ``` +//! match x { +//! Enum::Variant1(_) => {} // `p1` +//! Enum::Variant2(None, 0) => {} // `p2` +//! Enum::Variant2(Some(_), 0) => {} // `q` +//! } +//! ``` +//! +//! We can easily see that if our candidate value `v` starts with `Variant1` it will not match `q`. +//! If `v = Variant2(v0, v1)` however, whether or not it matches `p2` and `q` will depend on `v0` +//! and `v1`. In fact, such a `v` will be a witness of usefulness of `q` exactly when the tuple +//! `(v0, v1)` is a witness of usefulness of `q'` in the following reduced match: +//! +//! ``` +//! match x { +//! (None, 0) => {} // `p2'` +//! (Some(_), 0) => {} // `q'` +//! } +//! ``` +//! +//! This motivates a new step in computing usefulness, that we call _specialization_. +//! Specialization consist of filtering a list of patterns for those that match a constructor, and +//! then looking into the constructor's fields. This enables usefulness to be computed recursively. +//! +//! Instead of acting on a single pattern in each row, we will consider a list of patterns for each +//! row, and we call such a list a _pattern-stack_. The idea is that we will specialize the +//! leftmost pattern, which amounts to popping the constructor and pushing its fields, which feels +//! like a stack. We note a pattern-stack simply with `[p_1 ... p_n]`. +//! Here's a sequence of specializations of a list of pattern-stacks, to illustrate what's +//! happening: +//! ``` +//! [Enum::Variant1(_)] +//! [Enum::Variant2(None, 0)] +//! [Enum::Variant2(Some(_), 0)] +//! //==>> specialize with `Variant2` +//! [None, 0] +//! [Some(_), 0] +//! //==>> specialize with `Some` +//! [_, 0] +//! //==>> specialize with `true` (say the type was `bool`) +//! [0] +//! //==>> specialize with `0` +//! [] +//! ``` +//! +//! The function `specialize(c, p)` takes a value constructor `c` and a pattern `p`, and returns 0 +//! or more pattern-stacks. If `c` does not match the head constructor of `p`, it returns nothing; +//! otherwise if returns the fields of the constructor. This only returns more than one +//! pattern-stack if `p` has a pattern-only constructor. +//! +//! - Specializing for the wrong constructor returns nothing +//! +//! `specialize(None, Some(p0)) := []` +//! +//! - Specializing for the correct constructor returns a single row with the fields +//! +//! `specialize(Variant1, Variant1(p0, p1, p2)) := [[p0, p1, p2]]` +//! +//! `specialize(Foo{..}, Foo { bar: p0, baz: p1 }) := [[p0, p1]]` +//! +//! - For or-patterns, we specialize each branch and concatenate the results +//! +//! `specialize(c, p0 | p1) := specialize(c, p0) ++ specialize(c, p1)` +//! +//! - We treat the other pattern constructors as if they were a large or-pattern of all the +//! possibilities: +//! +//! `specialize(c, _) := specialize(c, Variant1(_) | Variant2(_, _) | ...)` +//! +//! `specialize(c, 1..=100) := specialize(c, 1 | ... | 100)` +//! +//! `specialize(c, [p0, .., p1]) := specialize(c, [p0, p1] | [p0, _, p1] | [p0, _, _, p1] | ...)` +//! +//! - If `c` is a pattern-only constructor, `specialize` is defined on a case-by-case basis. See +//! the discussion about constructor splitting in [`super::deconstruct_pat`]. +//! +//! +//! We then extend this function to work with pattern-stacks as input, by acting on the first +//! column and keeping the other columns untouched. +//! +//! Specialization for the whole matrix is done in [`Matrix::specialize_constructor`]. Note that +//! or-patterns in the first column are expanded before being stored in the matrix. Specialization +//! for a single patstack is done from a combination of [`Constructor::is_covered_by`] and +//! [`PatStack::pop_head_constructor`]. The internals of how it's done mostly live in the +//! [`Fields`] struct. +//! +//! +//! # Computing usefulness +//! +//! We now have all we need to compute usefulness. The inputs to usefulness are a list of +//! pattern-stacks `p_1 ... p_n` (one per row), and a new pattern_stack `q`. The paper and this +//! file calls the list of patstacks a _matrix_. They must all have the same number of columns and +//! the patterns in a given column must all have the same type. `usefulness` returns a (possibly +//! empty) list of witnesses of usefulness. These witnesses will also be pattern-stacks. +//! +//! - base case: `n_columns == 0`. +//! Since a pattern-stack functions like a tuple of patterns, an empty one functions like the +//! unit type. Thus `q` is useful iff there are no rows above it, i.e. if `n == 0`. +//! +//! - inductive case: `n_columns > 0`. +//! We need a way to list the constructors we want to try. We will be more clever in the next +//! section but for now assume we list all value constructors for the type of the first column. +//! +//! - for each such ctor `c`: +//! +//! - for each `q'` returned by `specialize(c, q)`: +//! +//! - we compute `usefulness(specialize(c, p_1) ... specialize(c, p_n), q')` +//! +//! - for each witness found, we revert specialization by pushing the constructor `c` on top. +//! +//! - We return the concatenation of all the witnesses found, if any. +//! +//! Example: +//! ``` +//! [Some(true)] // p_1 +//! [None] // p_2 +//! [Some(_)] // q +//! //==>> try `None`: `specialize(None, q)` returns nothing +//! //==>> try `Some`: `specialize(Some, q)` returns a single row +//! [true] // p_1' +//! [_] // q' +//! //==>> try `true`: `specialize(true, q')` returns a single row +//! [] // p_1'' +//! [] // q'' +//! //==>> base case; `n != 0` so `q''` is not useful. +//! //==>> go back up a step +//! [true] // p_1' +//! [_] // q' +//! //==>> try `false`: `specialize(false, q')` returns a single row +//! [] // q'' +//! //==>> base case; `n == 0` so `q''` is useful. We return the single witness `[]` +//! witnesses: +//! [] +//! //==>> undo the specialization with `false` +//! witnesses: +//! [false] +//! //==>> undo the specialization with `Some` +//! witnesses: +//! [Some(false)] +//! //==>> we have tried all the constructors. The output is the single witness `[Some(false)]`. +//! ``` +//! +//! This computation is done in [`is_useful`]. In practice we don't care about the list of +//! witnesses when computing reachability; we only need to know whether any exist. We do keep the +//! witnesses when computing exhaustiveness to report them to the user. +//! +//! +//! # Making usefulness tractable: constructor splitting +//! +//! We're missing one last detail: which constructors do we list? Naively listing all value +//! constructors cannot work for types like `u64` or `&str`, so we need to be more clever. The +//! first obvious insight is that we only want to list constructors that are covered by the head +//! constructor of `q`. If it's a value constructor, we only try that one. If it's a pattern-only +//! constructor, we use the final clever idea for this algorithm: _constructor splitting_, where we +//! group together constructors that behave the same. +//! +//! The details are not necessary to understand this file, so we explain them in +//! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function. + +use std::{cell::RefCell, iter::FromIterator}; + +use hir_def::{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, Interner, Ty}; + +use super::{ + deconstruct_pat::{Constructor, Fields, SplitWildcard}, + Pat, PatId, PatKind, PatternFoldable, PatternFolder, +}; + +use self::{helper::PatIdExt, Usefulness::*, WitnessPreference::*}; + +pub(crate) struct MatchCheckCtx<'a> { + pub(crate) module: ModuleId, + pub(crate) match_expr: ExprId, + pub(crate) infer: &'a InferenceResult, + pub(crate) db: &'a dyn HirDatabase, + /// Lowered 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 { + LiteralExpander.fold_pattern(&pat) +} + +struct LiteralExpander; + +impl PatternFolder for LiteralExpander { + fn fold_pattern(&mut self, pat: &Pat) -> Pat { + match (pat.ty.kind(&Interner), pat.kind.as_ref()) { + (_, PatKind::Binding { subpattern: Some(s), .. }) => s.fold_with(self), + _ => pat.super_fold_with(self), + } + } +} + +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, pat_arena: &mut PatternArena) { + if let PatKind::Or { pats } = pat_arena[pat].kind.as_ref() { + let pats = pats.clone(); + for pat in pats { + // FIXME(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())) + } + + // 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). + 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: arm_usefulness, non_exhaustiveness_witnesses } +} + +pub(crate) type PatternArena = Arena; + +mod helper { + 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(crate) trait Captures<'a> {} + + impl<'a, T: ?Sized> Captures<'a> for T {} +} -- cgit v1.2.3