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Auto merge of #112272 - jieyouxu:issue-112269, r=compiler-errors
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Show note for type ascription on a local binding interpreted as a constant pattern and not a new variable

Given the code

```rust
pub fn main() {
    const y: i32 = 4;
    let y: i32 = 3;
}
```

`y` in the let binding is actually interpreted as a constant pattern and is not a new variable, causing confusing diagnostics about refutable patterns in local binding.

This PR extends the note for type ascription of a constant pattern to `AscribeUserType` patterns which have `Constant` subpatterns.

Fixes #112269.
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bors committed Jun 5, 2023
2 parents dcf3571 + 55b4549 commit e410606
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Showing 3 changed files with 46 additions and 1 deletion.
7 changes: 6 additions & 1 deletion compiler/rustc_mir_build/src/thir/pattern/check_match.rs
Original file line number Diff line number Diff line change
Expand Up @@ -443,7 +443,12 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
let mut let_suggestion = None;
let mut misc_suggestion = None;
let mut interpreted_as_const = None;
if let PatKind::Constant { .. } = pat.kind

if let PatKind::Constant { .. }
| PatKind::AscribeUserType {
subpattern: box Pat { kind: PatKind::Constant { .. }, .. },
..
} = pat.kind
&& let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(pat.span)
{
// If the pattern to match is an integer literal:
Expand Down
9 changes: 9 additions & 0 deletions tests/ui/mir/issue-112269.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
pub fn main() {
const x: i32 = 4;
let x: i32 = 3;
//~^ ERROR refutable pattern in local binding

const y: i32 = 3;
let y = 4;
//~^ ERROR refutable pattern in local binding
}
31 changes: 31 additions & 0 deletions tests/ui/mir/issue-112269.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,31 @@
error[E0005]: refutable pattern in local binding
--> $DIR/issue-112269.rs:3:9
|
LL | let x: i32 = 3;
| ^
| |
| patterns `i32::MIN..=3_i32` and `5_i32..=i32::MAX` not covered
| missing patterns are not covered because `x` is interpreted as a constant pattern, not a new variable
| help: introduce a variable instead: `x_var`
|
= note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
= note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
= note: the matched value is of type `i32`

error[E0005]: refutable pattern in local binding
--> $DIR/issue-112269.rs:7:9
|
LL | let y = 4;
| ^
| |
| patterns `i32::MIN..=2_i32` and `4_i32..=i32::MAX` not covered
| missing patterns are not covered because `y` is interpreted as a constant pattern, not a new variable
| help: introduce a variable instead: `y_var`
|
= note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
= note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
= note: the matched value is of type `i32`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0005`.

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