Confusing error with integer identifers

[Ezrashaw]

I tried this code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d53daa961c3b462df261d1e774a9329f

fn main() {
    let 6 = 5;
}

I expected to see an error explaining the limits on identifiers (identifiers can't start with digits).

Instead, I got a pattern-matching error:

error[E0005]: refutable pattern in local binding: `i32::MIN..=5_i32` and `7_i32..=i32::MAX` not covered
 --> src/main.rs:4:5
  |
4 | let 6 = 5;
  |     ^ patterns `i32::MIN..=5_i32` and `7_i32..=i32::MAX` 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 `i32`
help: you might want to use `if let` to ignore the variants that aren't matched
  |
4 | if let 6 = 5 { todo!() }
  | ++           ~~~~~~~~~~~

While of course this error is correct, perhaps a note should be added explaining that you can't use a number as an identifier.

Meta

Occurs on latest stable and nightly.

@rustbot claim

[ibraheemdev]

This isn't a bug, 6 is a pattern there. For example, this compiles:

let 6 = 5 else { panic!("6 != 5") };

[Ezrashaw]

Yes, but does a new contributor know that? Typically you would use a if statement here, if we see this then most likely a user has mistyped let _6 = 5; or something to that effect. A non-pattern binding here would be more common.