| //! 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.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<'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 {} |
| } |
