Confusing "arbitrary expressions aren't allowed in patterns" error

[est31]

Take the following code:

macro_rules! foo {
    ($p:expr) => {
        if let $p = Some(42) {
            return;
        }
    };
}

fn main() {
    foo!(Some(3));
}

Currently it fails with:

error: arbitrary expressions aren't allowed in patterns
  --> src/main.rs:10:10
   |
10 |     foo!(Some(3));
   |          ^^^^^^^

If you expanded the macro manually, Some(3) would be parsed as a pattern, but the $p:expr above changes it to be parsed as an expression. So what you need to do here is to change the $p:expr above to $p:pat. But this is very non-obvious.

Ideally, the compiler would try to check if the input is a valid pattern, and if so, add a note like "Some(3) is a valid pattern but the metavariable $p is specified to be an expression", and maybe also point to the metavariable's matcher.

@rustbot label A-macros

[alexpetros]

Finding this issue saved me a lot of time, and I think adding this error to the compiler would help!

[est31]

I've looked into this and adding an error message is harder than I thought. The issue is that the span provided to the lower_expr_within_pat function is already resolved to the original location of the Some(3) so calling .from_expansion() isn't going to help much.

I can add a check like is_approximately_pattern or something, but it then causes basically anything to be recognized, even if it has never been from a macro matcher, so usually the hint would be a false positive.