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diff --git a/crates/hir-ty/src/diagnostics/match_check/usefulness.rs b/crates/hir-ty/src/diagnostics/match_check/usefulness.rs deleted file mode 100644 index 1b1a5ff269..0000000000 --- a/crates/hir-ty/src/diagnostics/match_check/usefulness.rs +++ /dev/null @@ -1,824 +0,0 @@ -//! Based on rust-lang/rust (last sync f31622a50 2021-11-12) -//! <https://github.com/rust-lang/rust/blob/f31622a50/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<u64>` 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::iter::once; - -use hir_def::{AdtId, DefWithBodyId, HasModule, ModuleId}; -use smallvec::{smallvec, SmallVec}; -use typed_arena::Arena; - -use crate::{db::HirDatabase, inhabitedness::is_ty_uninhabited_from, Ty, TyExt}; - -use super::deconstruct_pat::{Constructor, DeconstructedPat, Fields, SplitWildcard}; - -use self::{helper::Captures, ArmType::*, Usefulness::*}; - -pub(crate) struct MatchCheckCtx<'a, 'p> { - pub(crate) module: ModuleId, - pub(crate) body: DefWithBodyId, - pub(crate) db: &'a dyn HirDatabase, - /// Lowered patterns from arms plus generated by the check. - pub(crate) pattern_arena: &'p Arena<DeconstructedPat<'p>>, - exhaustive_patterns: bool, -} - -impl<'a, 'p> MatchCheckCtx<'a, 'p> { - pub(crate) fn new( - module: ModuleId, - body: DefWithBodyId, - db: &'a dyn HirDatabase, - pattern_arena: &'p Arena<DeconstructedPat<'p>>, - ) -> Self { - let def_map = db.crate_def_map(module.krate()); - let exhaustive_patterns = def_map.is_unstable_feature_enabled("exhaustive_patterns"); - Self { module, body, db, pattern_arena, exhaustive_patterns } - } - - pub(super) fn is_uninhabited(&self, ty: &Ty) -> bool { - if self.feature_exhaustive_patterns() { - is_ty_uninhabited_from(ty, self.module, self.db) - } else { - false - } - } - - /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`. - pub(super) fn is_foreign_non_exhaustive_enum(&self, ty: &Ty) -> bool { - match ty.as_adt() { - Some((adt @ AdtId::EnumId(_), _)) => { - let has_non_exhaustive_attr = - self.db.attrs(adt.into()).by_key("non_exhaustive").exists(); - let is_local = adt.module(self.db.upcast()).krate() == self.module.krate(); - has_non_exhaustive_attr && !is_local - } - _ => false, - } - } - - // Rust's unstable feature described as "Allows exhaustive pattern matching on types that contain uninhabited types." - pub(super) fn feature_exhaustive_patterns(&self) -> bool { - self.exhaustive_patterns - } -} - -#[derive(Copy, Clone)] -pub(super) struct PatCtxt<'a, 'p> { - pub(super) cx: &'a MatchCheckCtx<'a, 'p>, - /// 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, - /// Whether the current pattern is from a `non_exhaustive` enum. - pub(super) is_non_exhaustive: bool, -} - -/// 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<'p> { - pats: SmallVec<[&'p DeconstructedPat<'p>; 2]>, -} - -impl<'p> PatStack<'p> { - fn from_pattern(pat: &'p DeconstructedPat<'p>) -> Self { - Self::from_vec(smallvec![pat]) - } - - fn from_vec(vec: SmallVec<[&'p DeconstructedPat<'p>; 2]>) -> Self { - PatStack { pats: vec } - } - - fn is_empty(&self) -> bool { - self.pats.is_empty() - } - - fn len(&self) -> usize { - self.pats.len() - } - - fn head(&self) -> &'p DeconstructedPat<'p> { - self.pats[0] - } - - // 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) -> impl Iterator<Item = PatStack<'p>> + Captures<'_> { - self.head().iter_fields().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, cx: &MatchCheckCtx<'_, 'p>, ctor: &Constructor) -> PatStack<'p> { - // We pop the head pattern and push the new fields extracted from the arguments of - // `self.head()`. - let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(cx, ctor); - new_fields.extend_from_slice(&self.pats[1..]); - PatStack::from_vec(new_fields) - } -} - -/// A 2D matrix. -#[derive(Clone)] -pub(super) struct Matrix<'p> { - patterns: Vec<PatStack<'p>>, -} - -impl<'p> Matrix<'p> { - 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<usize> { - self.patterns.first().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<'p>) { - if !row.is_empty() && row.head().is_or_pat() { - self.patterns.extend(row.expand_or_pat()); - } else { - self.patterns.push(row); - } - } - - /// Iterate over the first component of each row - fn heads(&self) -> impl Iterator<Item = &'p DeconstructedPat<'p>> + Clone + Captures<'_> { - self.patterns.iter().map(|r| r.head()) - } - - /// This computes `S(constructor, self)`. See top of the file for explanations. - fn specialize_constructor(&self, pcx: PatCtxt<'_, 'p>, ctor: &Constructor) -> Matrix<'p> { - let mut matrix = Matrix::empty(); - for row in &self.patterns { - if ctor.is_covered_by(pcx, row.head().ctor()) { - let new_row = row.pop_head_constructor(pcx.cx, ctor); - matrix.push(new_row); - } - } - matrix - } -} - -/// 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 `ArmType`. -enum Usefulness<'p> { - /// If we don't care about witnesses, simply remember if the pattern was useful. - NoWitnesses { useful: bool }, - /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole - /// pattern is unreachable. - WithWitnesses(Vec<Witness<'p>>), -} - -impl<'p> Usefulness<'p> { - fn new_useful(preference: ArmType) -> Self { - match preference { - // A single (empty) witness of reachability. - FakeExtraWildcard => WithWitnesses(vec![Witness(vec![])]), - RealArm => NoWitnesses { useful: true }, - } - } - fn new_not_useful(preference: ArmType) -> Self { - match preference { - FakeExtraWildcard => WithWitnesses(vec![]), - RealArm => NoWitnesses { useful: false }, - } - } - - fn is_useful(&self) -> bool { - match self { - Usefulness::NoWitnesses { useful } => *useful, - Usefulness::WithWitnesses(witnesses) => !witnesses.is_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 { useful: s_useful }, NoWitnesses { useful: o_useful }) => { - *s_useful = *s_useful || o_useful - } - _ => unreachable!(), - } - } - - /// After calculating usefulness after a specialization, call this to reconstruct 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<'_, 'p>, - matrix: &Matrix<'p>, - ctor: &Constructor, - ) -> Self { - match self { - NoWitnesses { .. } => self, - WithWitnesses(ref witnesses) if witnesses.is_empty() => self, - WithWitnesses(witnesses) => { - let new_witnesses = if let Constructor::Missing { .. } = ctor { - // We got the special `Missing` constructor, so each of the missing constructors - // gives a new pattern that is not caught by the match. We list those patterns. - let new_patterns = if pcx.is_non_exhaustive { - // Here we don't want the user to try to list all variants, we want them to add - // a wildcard, so we only suggest that. - vec![DeconstructedPat::wildcard(pcx.ty.clone())] - } else { - let mut split_wildcard = SplitWildcard::new(pcx); - split_wildcard.split(pcx, matrix.heads().map(DeconstructedPat::ctor)); - - // This lets us know if we skipped any variants because they are marked - // `doc(hidden)` or they are unstable feature gate (only stdlib types). - let mut hide_variant_show_wild = false; - // 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 mut new: Vec<DeconstructedPat<'_>> = split_wildcard - .iter_missing(pcx) - .filter_map(|missing_ctor| { - // Check if this variant is marked `doc(hidden)` - if missing_ctor.is_doc_hidden_variant(pcx) - || missing_ctor.is_unstable_variant(pcx) - { - hide_variant_show_wild = true; - return None; - } - Some(DeconstructedPat::wild_from_ctor(pcx, missing_ctor.clone())) - }) - .collect(); - - if hide_variant_show_wild { - new.push(DeconstructedPat::wildcard(pcx.ty.clone())) - } - - new - }; - - witnesses - .into_iter() - .flat_map(|witness| { - new_patterns.iter().map(move |pat| { - Witness( - witness - .0 - .iter() - .chain(once(pat)) - .map(DeconstructedPat::clone_and_forget_reachability) - .collect(), - ) - }) - }) - .collect() - } else { - witnesses - .into_iter() - .map(|witness| witness.apply_constructor(pcx, ctor)) - .collect() - }; - WithWitnesses(new_witnesses) - } - } - } -} - -#[derive(Copy, Clone, Debug)] -enum ArmType { - FakeExtraWildcard, - RealArm, -} - -/// 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. -pub(crate) struct Witness<'p>(Vec<DeconstructedPat<'p>>); - -impl<'p> Witness<'p> { - /// Asserts that the witness contains a single pattern, and returns it. - fn single_pattern(self) -> DeconstructedPat<'p> { - 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<'_, 'p>, ctor: &Constructor) -> Self { - let pat = { - let len = self.0.len(); - let arity = ctor.arity(pcx); - let pats = self.0.drain((len - arity)..).rev(); - let fields = Fields::from_iter(pcx.cx, pats); - DeconstructedPat::new(ctor.clone(), fields, pcx.ty.clone()) - }; - - self.0.push(pat); - - self - } -} - -/// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>. -/// 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<T, !>`. -/// -/// 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<'p>( - cx: &MatchCheckCtx<'_, 'p>, - matrix: &Matrix<'p>, - v: &PatStack<'p>, - witness_preference: ArmType, - is_under_guard: bool, - is_top_level: bool, -) -> Usefulness<'p> { - 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; - } - - debug_assert!(rows.iter().all(|r| r.len() == v.len())); - - let ty = v.head().ty(); - let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty); - let pcx = PatCtxt { cx, ty, is_top_level, is_non_exhaustive }; - - // If the first pattern is an or-pattern, expand it. - let mut ret = Usefulness::new_not_useful(witness_preference); - if v.head().is_or_pat() { - // We try each or-pattern branch in turn. - let mut matrix = matrix.clone(); - for v in v.expand_or_pat() { - let usefulness = is_useful(cx, &matrix, &v, witness_preference, is_under_guard, false); - ret.extend(usefulness); - // 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); - } - } - } else { - let v_ctor = v.head().ctor(); - - // FIXME: implement `overlapping_range_endpoints` lint - - // We split the head constructor of `v`. - let split_ctors = v_ctor.split(pcx, matrix.heads().map(DeconstructedPat::ctor)); - // 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; - for ctor in split_ctors { - // We cache the result of `Fields::wildcards` because it is used a lot. - let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor); - let v = v.pop_head_constructor(cx, &ctor); - let usefulness = - is_useful(cx, &spec_matrix, &v, witness_preference, is_under_guard, false); - let usefulness = usefulness.apply_constructor(pcx, start_matrix, &ctor); - - // FIXME: implement `non_exhaustive_omitted_patterns` lint - - ret.extend(usefulness); - } - }; - - if ret.is_useful() { - v.head().set_reachable(); - } - - ret -} - -/// The arm of a match expression. -#[derive(Clone, Copy)] -pub(crate) struct MatchArm<'p> { - pub(crate) pat: &'p DeconstructedPat<'p>, - 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. - // FIXME: store unreachable subpattern IDs - Reachable, - /// The arm is unreachable. - Unreachable, -} - -/// The output of checking a match for exhaustiveness and arm reachability. -pub(crate) struct UsefulnessReport<'p> { - /// For each arm of the input, whether that arm is reachable after the arms above it. - pub(crate) _arm_usefulness: Vec<(MatchArm<'p>, 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<DeconstructedPat<'p>>, -} - -/// 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<'p>( - cx: &MatchCheckCtx<'_, 'p>, - arms: &[MatchArm<'p>], - scrut_ty: &Ty, -) -> UsefulnessReport<'p> { - let mut matrix = Matrix::empty(); - let arm_usefulness = arms - .iter() - .copied() - .map(|arm| { - let v = PatStack::from_pattern(arm.pat); - is_useful(cx, &matrix, &v, RealArm, arm.has_guard, true); - if !arm.has_guard { - matrix.push(v); - } - let reachability = if arm.pat.is_reachable() { - Reachability::Reachable - } else { - Reachability::Unreachable - }; - (arm, reachability) - }) - .collect(); - - let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty.clone())); - let v = PatStack::from_pattern(wild_pattern); - let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, 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) mod helper { - // 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 {} -} |