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Auto merge of #119610 - Nadrieril:never_pattern_bindings, r=compiler-errors

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never patterns: Check bindings wrt never patterns

Never patterns:
- Shouldn't contain bindings since they never match anything;
- Don't count when checking that or-patterns have consistent bindings.

r? `@compiler-errors`
This commit is contained in:
bors
2024-01-15 21:24:13 +00:00
parent bfcc027a75 5ccd29d6f0
commit 714b29a17f
13 changed files with 243 additions and 130 deletions
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126
compiler/rustc_resolve/src/late.rs
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@@ -65,6 +65,8 @@ enum IsRepeatExpr {
Yes,
}
struct IsNeverPattern;
/// Describes whether an `AnonConst` is a type level const arg or
/// some other form of anon const (i.e. inline consts or enum discriminants)
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -3246,12 +3248,31 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
self.resolve_pattern_top(&local.pat, PatternSource::Let);
}
/// build a map from pattern identifiers to binding-info's.
/// this is done hygienically. This could arise for a macro
/// that expands into an or-pattern where one 'x' was from the
/// user and one 'x' came from the macro.
fn binding_mode_map(&mut self, pat: &Pat) -> FxIndexMap<Ident, BindingInfo> {
/// Build a map from pattern identifiers to binding-info's, and check the bindings are
/// consistent when encountering or-patterns and never patterns.
/// This is done hygienically: this could arise for a macro that expands into an or-pattern
/// where one 'x' was from the user and one 'x' came from the macro.
///
/// A never pattern by definition indicates an unreachable case. For example, matching on
/// `Result<T, &!>` could look like:
/// ```rust
/// # #![feature(never_type)]
/// # #![feature(never_patterns)]
/// # fn bar(_x: u32) {}
/// let foo: Result<u32, &!> = Ok(0);
/// match foo {
/// Ok(x) => bar(x),
/// Err(&!),
/// }
/// ```
/// This extends to product types: `(x, !)` is likewise unreachable. So it doesn't make sense to
/// have a binding here, and we tell the user to use `_` instead.
fn compute_and_check_binding_map(
&mut self,
pat: &Pat,
) -> Result<FxIndexMap<Ident, BindingInfo>, IsNeverPattern> {
let mut binding_map = FxIndexMap::default();
let mut is_never_pat = false;
pat.walk(&mut |pat| {
match pat.kind {
@@ -3263,18 +3284,27 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
PatKind::Or(ref ps) => {
// Check the consistency of this or-pattern and
// then add all bindings to the larger map.
for bm in self.check_consistent_bindings(ps) {
binding_map.extend(bm);
match self.compute_and_check_or_pat_binding_map(ps) {
Ok(bm) => binding_map.extend(bm),
Err(IsNeverPattern) => is_never_pat = true,
}
return false;
}
PatKind::Never => is_never_pat = true,
_ => {}
}
true
});
binding_map
if is_never_pat {
for (_, binding) in binding_map {
self.report_error(binding.span, ResolutionError::BindingInNeverPattern);
}
Err(IsNeverPattern)
} else {
Ok(binding_map)
}
}
fn is_base_res_local(&self, nid: NodeId) -> bool {
@@ -3284,33 +3314,52 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
)
}
/// Checks that all of the arms in an or-pattern have exactly the
/// same set of bindings, with the same binding modes for each.
fn check_consistent_bindings(
/// Compute the binding map for an or-pattern. Checks that all of the arms in the or-pattern
/// have exactly the same set of bindings, with the same binding modes for each.
/// Returns the computed binding map and a boolean indicating whether the pattern is a never
/// pattern.
///
/// A never pattern by definition indicates an unreachable case. For example, destructuring a
/// `Result<T, &!>` could look like:
/// ```rust
/// # #![feature(never_type)]
/// # #![feature(never_patterns)]
/// # fn foo() -> Result<bool, &'static !> { Ok(true) }
/// let (Ok(x) | Err(&!)) = foo();
/// # let _ = x;
/// ```
/// Because the `Err(&!)` branch is never reached, it does not need to have the same bindings as
/// the other branches of the or-pattern. So we must ignore never pattern when checking the
/// bindings of an or-pattern.
/// Moreover, if all the subpatterns are never patterns (e.g. `Ok(!) | Err(!)`), then the
/// pattern as a whole counts as a never pattern (since it's definitionallly unreachable).
fn compute_and_check_or_pat_binding_map(
&mut self,
pats: &[P<Pat>],
) -> Vec<FxIndexMap<Ident, BindingInfo>> {
// pats is consistent.
) -> Result<FxIndexMap<Ident, BindingInfo>, IsNeverPattern> {
let mut missing_vars = FxIndexMap::default();
let mut inconsistent_vars = FxIndexMap::default();
// 1) Compute the binding maps of all arms.
let maps = pats.iter().map(|pat| self.binding_mode_map(pat)).collect::<Vec<_>>();
// 1) Compute the binding maps of all arms; we must ignore never patterns here.
let not_never_pats = pats
.iter()
.filter_map(|pat| {
let binding_map = self.compute_and_check_binding_map(pat).ok()?;
Some((binding_map, pat))
})
.collect::<Vec<_>>();
// 2) Record any missing bindings or binding mode inconsistencies.
for (map_outer, pat_outer) in maps.iter().zip(pats.iter()) {
for (map_outer, pat_outer) in not_never_pats.iter() {
// Check against all arms except for the same pattern which is always self-consistent.
let inners = maps
let inners = not_never_pats
.iter()
.zip(pats.iter())
.filter(|(_, pat)| pat.id != pat_outer.id)
.flat_map(|(map, _)| map)
.map(|(key, binding)| (key.name, map_outer.get(key), binding));
.flat_map(|(map, _)| map);
let inners = inners.collect::<Vec<_>>();
for (name, info, &binding_inner) in inners {
match info {
for (key, binding_inner) in inners {
let name = key.name;
match map_outer.get(key) {
None => {
// The inner binding is missing in the outer.
let binding_error =
@@ -3351,19 +3400,32 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
self.report_error(v.0, ResolutionError::VariableBoundWithDifferentMode(name, v.1));
}
// 5) Finally bubble up all the binding maps.
maps
// 5) Bubble up the final binding map.
if not_never_pats.is_empty() {
// All the patterns are never patterns, so the whole or-pattern is one too.
Err(IsNeverPattern)
} else {
let mut binding_map = FxIndexMap::default();
for (bm, _) in not_never_pats {
binding_map.extend(bm);
}
Ok(binding_map)
}
}
/// Check the consistency of the outermost or-patterns.
fn check_consistent_bindings_top(&mut self, pat: &'ast Pat) {
/// Check the consistency of bindings wrt or-patterns and never patterns.
fn check_consistent_bindings(&mut self, pat: &'ast Pat) {
let mut is_or_or_never = false;
pat.walk(&mut |pat| match pat.kind {
PatKind::Or(ref ps) => {
self.check_consistent_bindings(ps);
PatKind::Or(..) | PatKind::Never => {
is_or_or_never = true;
false
}
_ => true,
})
});
if is_or_or_never {
let _ = self.compute_and_check_binding_map(pat);
}
}
fn resolve_arm(&mut self, arm: &'ast Arm) {
@@ -3392,7 +3454,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
visit::walk_pat(self, pat);
self.resolve_pattern_inner(pat, pat_src, bindings);
// This has to happen *after* we determine which pat_idents are variants:
self.check_consistent_bindings_top(pat);
self.check_consistent_bindings(pat);
}
/// Resolve bindings in a pattern. This is a helper to `resolve_pattern`.