async makes pattern matching errors in function signatures worse

[BaerLKR]

Code

struct A(pub i32);

async fn example(Some(A(_)): Option<A>) {
}

Current output

error[E0005]: refutable pattern in local binding
 --> src/main.rs:7:12
  |
7 | async fn example(Some(A(_)): Option<A>) {}
  |            ^^^^^^^^^^ pattern `None` not covered
  |
  = 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 `Option<A>`
help: you might want to use `let else` to handle the variant that isn't matched
  |
7 | async fn example(Some(A(_)) else { todo!() }: Option<A>) {}
  |                       ++++++++++++++++

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

Desired output

error[E0005]: refutable pattern in function argument
 --> src/main.rs:7:6
  |
7 | fn example(Some(A(_)): Option<A>) {}
  |      ^^^^^^^^^^ pattern `None` not covered
  |
  = note: the matched value is of type `Option<A>`

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

Rationale and extra context

The desired output is what I get when I don't make the funtion async. So this

struct A(pub i32);
fn example(Some(A(_)): Option<A>) {}

gives the desired output

Other cases

Rust Version

rustc 1.84.1 (e71f9a9a9 2025-01-27)
binary: rustc
commit-hash: e71f9a9a98b0faf423844bf0ba7438f29dc27d58
commit-date: 2025-01-27
host: x86_64-unknown-linux-gnu
release: 1.84.1
LLVM version: 19.1.5

Anything else?

No response

[joculatrix]

It looks like this has to do with the way async functions are represented internally. If I input the following code into the Playground:

struct A(pub i32);

fn foo(Some(A(_)): Option<A>) {}

async fn bar(Some(A(_)): Option<A>) {}

Telling it to generate the HIR gives the following:

// --(prelude/std imports and attributes)--

struct A(i32);

fn foo(Some(A(_)): Option<A>) { }

async fn bar(__arg0: Option<A>)
    ->
            /*impl Trait*/ |mut _task_context: ResumeTy|
    { let mut __arg0 = __arg0; let Some(A(_)) = __arg0; { let _t = {}; _t } }

...which explains why the error message says "local binding" instead of "function argument" and why it thinks there's a let there. I don't know how to fix this, but I figured I'd share my investigating in case it saves someone else time.

(Also, in both the async and non-async case, the error is being emitted in <rustc_mir_build::thir::pattern::check_match::MatchVisitor>::check_binding_is_irrefutable.)

[moulins]

Interestingly, one can get this exact bug without any async:

fn main() {
    let a;
    // confusing error with broken suggestion that doesn't compile
    Some(a) = Some(3);
}

Output

error[E0005]: refutable pattern in local binding
 --> src/main.rs:6:5
  |
6 |     Some(a) = Some(3);
  |     ^^^^^^^ pattern `None` not covered
  |
  = 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/ch19-02-refutability.html
  = note: the matched value is of type `Option<i32>`
help: you might want to use `let else` to handle the variant that isn't matched
  |
6 |     Some(a) = Some(3) else { todo!() };
  |                       ++++++++++++++++

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

(Aside: actually allowing else { ... } would be natural here, but alas, it doesn't work. Future language feature? 😄)