Allow destructuring parameters in trait function declarations

[algonomicon1]

The current syntax rules do not allow for trait functions to destructure parameters:

#![allow(unused)]

// This works
fn foo((x, y): (i32, i32)) {
}

trait Bar {
    // This does not work
    fn bar((x, y): (i32, i32));
}

Playground
Compiling the above outputs this:

error: expected one of `)` or `,`, found `:`
 --> src/main.rs:7:18
  |
7 |     fn bar((x, y): (i32, i32));
  |                  ^ expected one of `)` or `,` here

error: expected one of `!`, `&&`, `&`, `(`, `)`, `*`, `<`, `?`, `[`, `_`, `dyn`, `extern`, `fn`, `for`, `impl`, `unsafe`, or lifetime, found `:`
 --> src/main.rs:7:18
  |
7 |     fn bar((x, y): (i32, i32));
  |                  ^ expected one of 17 possible tokens here

error[E0601]: `main` function not found in crate `playground`
  |
  = note: consider adding a `main` function to `src/main.rs`

The destructuring of trait function parameters should be allowed for consistency and readability.

[varkor]

I think this is a consequence of trait method declarations not requiring argument names. Patterns as arguments are only accepted if names are required. I imagine there's some parsing issue here. Perhaps something like:

trait Trait {
	fn foo(&Bar); // is `&Bar` a pattern or type here?
}

However, in the presence of :, it might be possible to disambiguate (as : is not allowed in patterns).

Edit: see rust-lang/rfcs#1351 (comment) and #41686.

[varkor]

Interestingly, there's a better error message for this, E0642:

error[E0642]: patterns aren't allowed in methods without bodies
 --> fn_inconsistency.rs:6:12
  |
6 |     fn bar((x, y): (i32, i32));
  |            ^^^^^^

error: aborting due to previous error

However, it's unlisted on the Error Index and isn't emitted because the parser fails before that point.

Edit: Looks like it wasn't being type-checked properly before it was forbidden.

[steveklabnik]

Triage: there is a very small change now:

error[E0642]: patterns aren't allowed in functions without bodies
 --> src/lib.rs:9:12
  |
9 |     fn bar((x, y): (i32, i32));
  |            ^^^^^^ pattern not allowed in function without body

(now says function because it's not a method)