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Diffstat (limited to 'compiler/rustc_mir_build/src/thir/pattern/usefulness.rs')
| -rw-r--r-- | compiler/rustc_mir_build/src/thir/pattern/usefulness.rs | 1205 |
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diff --git a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs deleted file mode 100644 index da7b6587a..000000000 --- a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs +++ /dev/null @@ -1,1205 +0,0 @@ -//! Note: tests specific to this file can be found in: -//! -//! - `ui/pattern/usefulness` -//! - `ui/or-patterns` -//! - `ui/consts/const_in_pattern` -//! - `ui/rfc-2008-non-exhaustive` -//! - `ui/half-open-range-patterns` -//! - probably many others -//! -//! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific -//! reason not to, for example if they depend on a particular feature like `or_patterns`. -//! -//! ----- -//! -//! 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 -//! # fn foo(x: Option<i32>) { -//! 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. -//! ```compile_fail,E0004 -//! # fn foo(x: Option<i32>) { -//! 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 (matchable) -//! 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 matchable 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: -//! ```compile_fail,E0004 -//! # enum Enum { Variant1(()), Variant2(Option<bool>, u32)} -//! # fn foo(x: Enum) { -//! 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: -//! -//! ```compile_fail,E0004 -//! # fn foo(x: (Option<bool>, u32)) { -//! 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: -//! ```ignore (illustrative) -//! [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 -//! [`super::deconstruct_pat::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: -//! ```ignore (illustrative) -//! [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. -//! -//! # Constants in patterns -//! -//! There are two kinds of constants in patterns: -//! -//! * literals (`1`, `true`, `"foo"`) -//! * named or inline consts (`FOO`, `const { 5 + 6 }`) -//! -//! The latter are converted into other patterns with literals at the leaves. For example -//! `const_to_pat(const { [1, 2, 3] })` becomes an `Array(vec![Const(1), Const(2), Const(3)])` -//! pattern. This gets problematic when comparing the constant via `==` would behave differently -//! from matching on the constant converted to a pattern. Situations like that can occur, when -//! the user implements `PartialEq` manually, and thus could make `==` behave arbitrarily different. -//! In order to honor the `==` implementation, constants of types that implement `PartialEq` manually -//! stay as a full constant and become an `Opaque` pattern. These `Opaque` patterns do not participate -//! in exhaustiveness, specialization or overlap checking. - -use self::ArmType::*; -use self::Usefulness::*; -use super::deconstruct_pat::{ - Constructor, ConstructorSet, DeconstructedPat, IntRange, MaybeInfiniteInt, SplitConstructorSet, - WitnessPat, -}; -use crate::errors::{ - NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Overlap, - OverlappingRangeEndpoints, Uncovered, -}; - -use rustc_data_structures::captures::Captures; - -use rustc_arena::TypedArena; -use rustc_data_structures::stack::ensure_sufficient_stack; -use rustc_hir::def_id::DefId; -use rustc_hir::HirId; -use rustc_middle::ty::{self, Ty, TyCtxt}; -use rustc_session::lint; -use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS; -use rustc_span::{Span, DUMMY_SP}; - -use smallvec::{smallvec, SmallVec}; -use std::fmt; - -pub(crate) struct MatchCheckCtxt<'p, 'tcx> { - pub(crate) tcx: TyCtxt<'tcx>, - /// The module in which the match occurs. This is necessary for - /// checking inhabited-ness of types because whether a type is (visibly) - /// inhabited can depend on whether it was defined in the current module or - /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty - /// outside its module and should not be matchable with an empty match statement. - pub(crate) module: DefId, - pub(crate) param_env: ty::ParamEnv<'tcx>, - pub(crate) pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>, - /// The span of the whole match, if applicable. - pub(crate) match_span: Option<Span>, - /// Only produce `NON_EXHAUSTIVE_OMITTED_PATTERNS` lint on refutable patterns. - pub(crate) refutable: bool, -} - -impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> { - pub(super) fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool { - if self.tcx.features().exhaustive_patterns { - !ty.is_inhabited_from(self.tcx, self.module, self.param_env) - } 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<'tcx>) -> bool { - match ty.kind() { - ty::Adt(def, ..) => { - def.is_enum() && def.is_variant_list_non_exhaustive() && !def.did().is_local() - } - _ => false, - } - } -} - -#[derive(Copy, Clone)] -pub(super) struct PatCtxt<'a, 'p, 'tcx> { - pub(super) cx: &'a MatchCheckCtxt<'p, 'tcx>, - /// Type of the current column under investigation. - pub(super) ty: Ty<'tcx>, - /// Span of the current pattern under investigation. - pub(super) span: Span, - /// 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, -} - -impl<'a, 'p, 'tcx> fmt::Debug for PatCtxt<'a, 'p, 'tcx> { - fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - f.debug_struct("PatCtxt").field("ty", &self.ty).finish() - } -} - -/// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]` -/// works well. -#[derive(Clone)] -pub(crate) struct PatStack<'p, 'tcx> { - pub(crate) pats: SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]>, -} - -impl<'p, 'tcx> PatStack<'p, 'tcx> { - fn from_pattern(pat: &'p DeconstructedPat<'p, 'tcx>) -> Self { - Self::from_vec(smallvec![pat]) - } - - fn from_vec(vec: SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 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, 'tcx> { - self.pats[0] - } - - fn iter(&self) -> impl Iterator<Item = &DeconstructedPat<'p, 'tcx>> { - self.pats.iter().copied() - } - - // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an - // or-pattern. Panics if `self` is empty. - fn expand_or_pat<'a>(&'a self) -> impl Iterator<Item = PatStack<'p, 'tcx>> + Captures<'a> { - 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 - }) - } - - // Recursively expand all patterns into their subpatterns and push each `PatStack` to matrix. - fn expand_and_extend<'a>(&'a self, matrix: &mut Matrix<'p, 'tcx>) { - if !self.is_empty() && self.head().is_or_pat() { - for pat in self.head().iter_fields() { - let mut new_patstack = PatStack::from_pattern(pat); - new_patstack.pats.extend_from_slice(&self.pats[1..]); - if !new_patstack.is_empty() && new_patstack.head().is_or_pat() { - new_patstack.expand_and_extend(matrix); - } else if !new_patstack.is_empty() { - matrix.push(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, - pcx: &PatCtxt<'_, 'p, 'tcx>, - ctor: &Constructor<'tcx>, - ) -> PatStack<'p, 'tcx> { - // 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(pcx, ctor); - new_fields.extend_from_slice(&self.pats[1..]); - PatStack::from_vec(new_fields) - } -} - -/// Pretty-printing for matrix row. -impl<'p, 'tcx> fmt::Debug for PatStack<'p, 'tcx> { - fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - write!(f, "+")?; - for pat in self.iter() { - write!(f, " {pat:?} +")?; - } - Ok(()) - } -} - -/// A 2D matrix. -#[derive(Clone)] -pub(super) struct Matrix<'p, 'tcx> { - pub patterns: Vec<PatStack<'p, 'tcx>>, -} - -impl<'p, 'tcx> Matrix<'p, 'tcx> { - fn empty() -> Self { - Matrix { patterns: vec![] } - } - - /// 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, 'tcx>) { - if !row.is_empty() && row.head().is_or_pat() { - row.expand_and_extend(self); - } else { - self.patterns.push(row); - } - } - - /// Iterate over the first component of each row - fn heads<'a>( - &'a self, - ) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Clone + Captures<'a> { - 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, 'tcx>, - ctor: &Constructor<'tcx>, - ) -> Matrix<'p, 'tcx> { - 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, ctor); - matrix.push(new_row); - } - } - matrix - } -} - -/// Pretty-printer for matrices of patterns, example: -/// -/// ```text -/// + _ + [] + -/// + true + [First] + -/// + true + [Second(true)] + -/// + false + [_] + -/// + _ + [_, _, tail @ ..] + -/// ``` -impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> { - fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - write!(f, "\n")?; - - let Matrix { patterns: m, .. } = self; - let pretty_printed_matrix: Vec<Vec<String>> = - m.iter().map(|row| row.iter().map(|pat| format!("{pat:?}")).collect()).collect(); - - let column_count = m.iter().map(|row| row.len()).next().unwrap_or(0); - assert!(m.iter().all(|row| row.len() == column_count)); - let column_widths: Vec<usize> = (0..column_count) - .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)) - .collect(); - - for row in pretty_printed_matrix { - write!(f, "+")?; - for (column, pat_str) in row.into_iter().enumerate() { - write!(f, " ")?; - write!(f, "{:1$}", pat_str, column_widths[column])?; - write!(f, " +")?; - } - write!(f, "\n")?; - } - Ok(()) - } -} - -/// 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`. -#[derive(Debug, Clone)] -enum Usefulness<'tcx> { - /// 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<WitnessStack<'tcx>>), -} - -impl<'tcx> Usefulness<'tcx> { - fn new_useful(preference: ArmType) -> Self { - match preference { - // A single (empty) witness of reachability. - FakeExtraWildcard => WithWitnesses(vec![WitnessStack(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<'_, '_, 'tcx>, - matrix: &Matrix<'_, 'tcx>, // used to compute missing ctors - ctor: &Constructor<'tcx>, - ) -> Self { - match self { - NoWitnesses { .. } => self, - WithWitnesses(ref witnesses) if witnesses.is_empty() => self, - WithWitnesses(witnesses) => { - let new_witnesses = if let Constructor::Missing { .. } = ctor { - let mut missing = ConstructorSet::for_ty(pcx.cx, pcx.ty) - .compute_missing(pcx, matrix.heads().map(DeconstructedPat::ctor)); - if missing.iter().any(|c| c.is_non_exhaustive()) { - // We only report `_` here; listing other constructors would be redundant. - missing = vec![Constructor::NonExhaustive]; - } - - // We got the special `Missing` constructor, so each of the missing constructors - // gives a new pattern that is not caught by the match. - // We construct for each missing constructor a version of this constructor with - // wildcards for fields, i.e. 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<WitnessPat<'_>> = missing - .into_iter() - .map(|missing_ctor| WitnessPat::wild_from_ctor(pcx, missing_ctor.clone())) - .collect(); - - witnesses - .into_iter() - .flat_map(|witness| { - new_patterns.iter().map(move |pat| { - let mut stack = witness.clone(); - stack.0.push(pat.clone()); - stack - }) - }) - .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-tuple 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. -/// -/// This mirrors `PatStack`: they function similarly, except `PatStack` contains user patterns we -/// are inspecting, and `WitnessStack` contains witnesses we are constructing. -/// FIXME(Nadrieril): use the same order of patterns for both -/// -/// A `WitnessStack` should have the same types and length as the `PatStacks` we are inspecting -/// (except we store the patterns in reverse order). Because Rust `match` is always against a single -/// pattern, at the end the stack will have length 1. In the middle of the algorithm, it can contain -/// multiple patterns. -/// -/// For example, if we are constructing a witness for the match against -/// -/// ```compile_fail,E0004 -/// struct Pair(Option<(u32, u32)>, bool); -/// # fn foo(p: Pair) { -/// match p { -/// Pair(None, _) => {} -/// Pair(_, false) => {} -/// } -/// # } -/// ``` -/// -/// We'll perform the following steps (among others): -/// - Start with a matrix representing the match -/// `PatStack(vec![Pair(None, _)])` -/// `PatStack(vec![Pair(_, false)])` -/// - Specialize with `Pair` -/// `PatStack(vec![None, _])` -/// `PatStack(vec![_, false])` -/// - Specialize with `Some` -/// `PatStack(vec![_, false])` -/// - Specialize with `_` -/// `PatStack(vec![false])` -/// - Specialize with `true` -/// // no patstacks left -/// - This is a non-exhaustive match: we have the empty witness stack as a witness. -/// `WitnessStack(vec![])` -/// - Apply `true` -/// `WitnessStack(vec![true])` -/// - Apply `_` -/// `WitnessStack(vec![true, _])` -/// - Apply `Some` -/// `WitnessStack(vec![true, Some(_)])` -/// - Apply `Pair` -/// `WitnessStack(vec![Pair(Some(_), true)])` -/// -/// The final `Pair(Some(_), true)` is then the resulting witness. -#[derive(Debug, Clone)] -pub(crate) struct WitnessStack<'tcx>(Vec<WitnessPat<'tcx>>); - -impl<'tcx> WitnessStack<'tcx> { - /// Asserts that the witness contains a single pattern, and returns it. - fn single_pattern(self) -> WitnessPat<'tcx> { - 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<'_, '_, 'tcx>, ctor: &Constructor<'tcx>) -> Self { - let pat = { - let len = self.0.len(); - let arity = ctor.arity(pcx); - let fields = self.0.drain((len - arity)..).rev().collect(); - WitnessPat::new(ctor.clone(), fields, pcx.ty) - }; - - 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. -#[instrument(level = "debug", skip(cx, matrix, lint_root), ret)] -fn is_useful<'p, 'tcx>( - cx: &MatchCheckCtxt<'p, 'tcx>, - matrix: &Matrix<'p, 'tcx>, - v: &PatStack<'p, 'tcx>, - witness_preference: ArmType, - lint_root: HirId, - is_under_guard: bool, - is_top_level: bool, -) -> Usefulness<'tcx> { - debug!(?matrix, ?v); - 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) - }; - debug!(?ret); - return ret; - } - - debug_assert!(rows.iter().all(|r| r.len() == v.len())); - - // 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() { - debug!("expanding or-pattern"); - // We try each or-pattern branch in turn. - let mut matrix = matrix.clone(); - for v in v.expand_or_pat() { - debug!(?v); - let usefulness = ensure_sufficient_stack(|| { - is_useful(cx, &matrix, &v, witness_preference, lint_root, is_under_guard, false) - }); - debug!(?usefulness); - 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 mut ty = v.head().ty(); - - // Opaque types can't get destructured/split, but the patterns can - // actually hint at hidden types, so we use the patterns' types instead. - if let ty::Alias(ty::Opaque, ..) = ty.kind() { - if let Some(row) = rows.first() { - ty = row.head().ty(); - } - } - debug!("v.head: {:?}, v.span: {:?}", v.head(), v.head().span()); - let pcx = &PatCtxt { cx, ty, span: v.head().span(), is_top_level }; - - let v_ctor = v.head().ctor(); - debug!(?v_ctor); - // 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 { - debug!("specialize({:?})", ctor); - // 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(pcx, &ctor); - let usefulness = ensure_sufficient_stack(|| { - is_useful( - cx, - &spec_matrix, - &v, - witness_preference, - lint_root, - is_under_guard, - false, - ) - }); - let usefulness = usefulness.apply_constructor(pcx, start_matrix, &ctor); - ret.extend(usefulness); - } - } - - if ret.is_useful() { - v.head().set_reachable(); - } - - ret -} - -/// A column of patterns in the matrix, where a column is the intuitive notion of "subpatterns that -/// inspect the same subvalue". -/// This is used to traverse patterns column-by-column for lints. Despite similarities with -/// `is_useful`, this is a different traversal. Notably this is linear in the depth of patterns, -/// whereas `is_useful` is worst-case exponential (exhaustiveness is NP-complete). -#[derive(Debug)] -struct PatternColumn<'p, 'tcx> { - patterns: Vec<&'p DeconstructedPat<'p, 'tcx>>, -} - -impl<'p, 'tcx> PatternColumn<'p, 'tcx> { - fn new(patterns: Vec<&'p DeconstructedPat<'p, 'tcx>>) -> Self { - Self { patterns } - } - - fn is_empty(&self) -> bool { - self.patterns.is_empty() - } - fn head_ty(&self) -> Option<Ty<'tcx>> { - if self.patterns.len() == 0 { - return None; - } - // If the type is opaque and it is revealed anywhere in the column, we take the revealed - // version. Otherwise we could encounter constructors for the revealed type and crash. - let is_opaque = |ty: Ty<'tcx>| matches!(ty.kind(), ty::Alias(ty::Opaque, ..)); - let first_ty = self.patterns[0].ty(); - if is_opaque(first_ty) { - for pat in &self.patterns { - let ty = pat.ty(); - if !is_opaque(ty) { - return Some(ty); - } - } - } - Some(first_ty) - } - - fn analyze_ctors(&self, pcx: &PatCtxt<'_, 'p, 'tcx>) -> SplitConstructorSet<'tcx> { - let column_ctors = self.patterns.iter().map(|p| p.ctor()); - ConstructorSet::for_ty(pcx.cx, pcx.ty).split(pcx, column_ctors) - } - fn iter<'a>(&'a self) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Captures<'a> { - self.patterns.iter().copied() - } - - /// Does specialization: given a constructor, this takes the patterns from the column that match - /// the constructor, and outputs their fields. - /// This returns one column per field of the constructor. The normally all have the same length - /// (the number of patterns in `self` that matched `ctor`), except that we expand or-patterns - /// which may change the lengths. - fn specialize(&self, pcx: &PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Vec<Self> { - let arity = ctor.arity(pcx); - if arity == 0 { - return Vec::new(); - } - - // We specialize the column by `ctor`. This gives us `arity`-many columns of patterns. These - // columns may have different lengths in the presence of or-patterns (this is why we can't - // reuse `Matrix`). - let mut specialized_columns: Vec<_> = - (0..arity).map(|_| Self { patterns: Vec::new() }).collect(); - let relevant_patterns = - self.patterns.iter().filter(|pat| ctor.is_covered_by(pcx, pat.ctor())); - for pat in relevant_patterns { - let specialized = pat.specialize(pcx, &ctor); - for (subpat, column) in specialized.iter().zip(&mut specialized_columns) { - if subpat.is_or_pat() { - column.patterns.extend(subpat.flatten_or_pat()) - } else { - column.patterns.push(subpat) - } - } - } - - assert!( - !specialized_columns[0].is_empty(), - "ctor {ctor:?} was listed as present but isn't; - there is an inconsistency between `Constructor::is_covered_by` and `ConstructorSet::split`" - ); - specialized_columns - } -} - -/// Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned -/// in a given column. -#[instrument(level = "debug", skip(cx), ret)] -fn collect_nonexhaustive_missing_variants<'p, 'tcx>( - cx: &MatchCheckCtxt<'p, 'tcx>, - column: &PatternColumn<'p, 'tcx>, -) -> Vec<WitnessPat<'tcx>> { - let Some(ty) = column.head_ty() else { - return Vec::new(); - }; - let pcx = &PatCtxt { cx, ty, span: DUMMY_SP, is_top_level: false }; - - let set = column.analyze_ctors(pcx); - if set.present.is_empty() { - // We can't consistently handle the case where no constructors are present (since this would - // require digging deep through any type in case there's a non_exhaustive enum somewhere), - // so for consistency we refuse to handle the top-level case, where we could handle it. - return vec![]; - } - - let mut witnesses = Vec::new(); - if cx.is_foreign_non_exhaustive_enum(ty) { - witnesses.extend( - set.missing - .into_iter() - // This will list missing visible variants. - .filter(|c| !matches!(c, Constructor::Hidden | Constructor::NonExhaustive)) - .map(|missing_ctor| WitnessPat::wild_from_ctor(pcx, missing_ctor)), - ) - } - - // Recurse into the fields. - for ctor in set.present { - let specialized_columns = column.specialize(pcx, &ctor); - let wild_pat = WitnessPat::wild_from_ctor(pcx, ctor); - for (i, col_i) in specialized_columns.iter().enumerate() { - // Compute witnesses for each column. - let wits_for_col_i = collect_nonexhaustive_missing_variants(cx, col_i); - // For each witness, we build a new pattern in the shape of `ctor(_, _, wit, _, _)`, - // adding enough wildcards to match `arity`. - for wit in wits_for_col_i { - let mut pat = wild_pat.clone(); - pat.fields[i] = wit; - witnesses.push(pat); - } - } - } - witnesses -} - -/// Traverse the patterns to warn the user about ranges that overlap on their endpoints. -#[instrument(level = "debug", skip(cx, lint_root))] -fn lint_overlapping_range_endpoints<'p, 'tcx>( - cx: &MatchCheckCtxt<'p, 'tcx>, - column: &PatternColumn<'p, 'tcx>, - lint_root: HirId, -) { - let Some(ty) = column.head_ty() else { - return; - }; - let pcx = &PatCtxt { cx, ty, span: DUMMY_SP, is_top_level: false }; - - let set = column.analyze_ctors(pcx); - - if IntRange::is_integral(ty) { - let emit_lint = |overlap: &IntRange, this_span: Span, overlapped_spans: &[Span]| { - let overlap_as_pat = overlap.to_diagnostic_pat(ty, cx.tcx); - let overlaps: Vec<_> = overlapped_spans - .iter() - .copied() - .map(|span| Overlap { range: overlap_as_pat.clone(), span }) - .collect(); - cx.tcx.emit_spanned_lint( - lint::builtin::OVERLAPPING_RANGE_ENDPOINTS, - lint_root, - this_span, - OverlappingRangeEndpoints { overlap: overlaps, range: this_span }, - ); - }; - - // If two ranges overlapped, the split set will contain their intersection as a singleton. - let split_int_ranges = set.present.iter().filter_map(|c| c.as_int_range()); - for overlap_range in split_int_ranges.clone() { - if overlap_range.is_singleton() { - let overlap: MaybeInfiniteInt = overlap_range.lo; - // Ranges that look like `lo..=overlap`. - let mut prefixes: SmallVec<[_; 1]> = Default::default(); - // Ranges that look like `overlap..=hi`. - let mut suffixes: SmallVec<[_; 1]> = Default::default(); - // Iterate on patterns that contained `overlap`. - for pat in column.iter() { - let this_span = pat.span(); - let Constructor::IntRange(this_range) = pat.ctor() else { continue }; - if this_range.is_singleton() { - // Don't lint when one of the ranges is a singleton. - continue; - } - if this_range.lo == overlap { - // `this_range` looks like `overlap..=this_range.hi`; it overlaps with any - // ranges that look like `lo..=overlap`. - if !prefixes.is_empty() { - emit_lint(overlap_range, this_span, &prefixes); - } - suffixes.push(this_span) - } else if this_range.hi == overlap.plus_one() { - // `this_range` looks like `this_range.lo..=overlap`; it overlaps with any - // ranges that look like `overlap..=hi`. - if !suffixes.is_empty() { - emit_lint(overlap_range, this_span, &suffixes); - } - prefixes.push(this_span) - } - } - } - } - } else { - // Recurse into the fields. - for ctor in set.present { - for col in column.specialize(pcx, &ctor) { - lint_overlapping_range_endpoints(cx, &col, lint_root); - } - } - } -} - -/// The arm of a match expression. -#[derive(Clone, Copy, Debug)] -pub(crate) struct MatchArm<'p, 'tcx> { - /// The pattern must have been lowered through `check_match::MatchVisitor::lower_pattern`. - pub(crate) pat: &'p DeconstructedPat<'p, 'tcx>, - pub(crate) hir_id: HirId, - 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<Span>), - /// The arm is unreachable. - Unreachable, -} - -/// The output of checking a match for exhaustiveness and arm reachability. -pub(crate) struct UsefulnessReport<'p, 'tcx> { - /// For each arm of the input, whether that arm is reachable after the arms above it. - pub(crate) arm_usefulness: Vec<(MatchArm<'p, 'tcx>, 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<WitnessPat<'tcx>>, -} - -/// 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`. -#[instrument(skip(cx, arms), level = "debug")] -pub(crate) fn compute_match_usefulness<'p, 'tcx>( - cx: &MatchCheckCtxt<'p, 'tcx>, - arms: &[MatchArm<'p, 'tcx>], - lint_root: HirId, - scrut_ty: Ty<'tcx>, - scrut_span: Span, -) -> UsefulnessReport<'p, 'tcx> { - let mut matrix = Matrix::empty(); - let arm_usefulness: Vec<_> = arms - .iter() - .copied() - .map(|arm| { - debug!(?arm); - let v = PatStack::from_pattern(arm.pat); - is_useful(cx, &matrix, &v, RealArm, arm.hir_id, arm.has_guard, true); - if !arm.has_guard { - matrix.push(v); - } - let reachability = if arm.pat.is_reachable() { - Reachability::Reachable(arm.pat.unreachable_spans()) - } else { - Reachability::Unreachable - }; - (arm, reachability) - }) - .collect(); - - let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty, DUMMY_SP)); - let v = PatStack::from_pattern(wild_pattern); - let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, lint_root, false, true); - let non_exhaustiveness_witnesses: Vec<_> = match usefulness { - WithWitnesses(pats) => pats.into_iter().map(|w| w.single_pattern()).collect(), - NoWitnesses { .. } => bug!(), - }; - - let pat_column = arms.iter().flat_map(|arm| arm.pat.flatten_or_pat()).collect::<Vec<_>>(); - let pat_column = PatternColumn::new(pat_column); - lint_overlapping_range_endpoints(cx, &pat_column, lint_root); - - // Run the non_exhaustive_omitted_patterns lint. Only run on refutable patterns to avoid hitting - // `if let`s. Only run if the match is exhaustive otherwise the error is redundant. - if cx.refutable && non_exhaustiveness_witnesses.is_empty() { - if !matches!( - cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, lint_root).0, - rustc_session::lint::Level::Allow - ) { - let witnesses = collect_nonexhaustive_missing_variants(cx, &pat_column); - - if !witnesses.is_empty() { - // Report that a match of a `non_exhaustive` enum marked with `non_exhaustive_omitted_patterns` - // is not exhaustive enough. - // - // NB: The partner lint for structs lives in `compiler/rustc_hir_analysis/src/check/pat.rs`. - cx.tcx.emit_spanned_lint( - NON_EXHAUSTIVE_OMITTED_PATTERNS, - lint_root, - scrut_span, - NonExhaustiveOmittedPattern { - scrut_ty, - uncovered: Uncovered::new(scrut_span, cx, witnesses), - }, - ); - } - } else { - // We used to allow putting the `#[allow(non_exhaustive_omitted_patterns)]` on a match - // arm. This no longer makes sense so we warn users, to avoid silently breaking their - // usage of the lint. - for arm in arms { - let (lint_level, lint_level_source) = - cx.tcx.lint_level_at_node(NON_EXHAUSTIVE_OMITTED_PATTERNS, arm.hir_id); - if !matches!(lint_level, rustc_session::lint::Level::Allow) { - let decorator = NonExhaustiveOmittedPatternLintOnArm { - lint_span: lint_level_source.span(), - suggest_lint_on_match: cx.match_span.map(|span| span.shrink_to_lo()), - lint_level: lint_level.as_str(), - lint_name: "non_exhaustive_omitted_patterns", - }; - - use rustc_errors::DecorateLint; - let mut err = cx.tcx.sess.struct_span_warn(arm.pat.span(), ""); - err.set_primary_message(decorator.msg()); - decorator.decorate_lint(&mut err); - err.emit(); - } - } - } - } - - UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses } -} |