Slice patterns cannot be used in trait methods

[cake4289]

The following code works on beta:

fn foo([a, b]: [i32; 2]) {}

But when used in a trait like so:

trait Foo {
    fn foo([a, b]: [i32; 2]) {}
}

rustc reports the following error:

error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `]`, found `,`
 --> src/main.rs:2:14
  |
2 |     fn foo([a, b]: [i32; 2]) {}
  |              ^ expected one of 7 possible tokens here

error: aborting due to previous error

[cake4289]

Possibly related to rustc expecting possibly a type in this scenario? As you can declare trait methods with no parameter name

[petrochenkov]

@cake4289

Possibly related to rustc expecting possibly a type in this scenario?

Exactly.
When #41686 becomes an error, we'll be able to accept arbitrary patterns in parameters of trait methods, but for now only very limited subset is allowed.

[varkor]

The example in the issue now compiles. If we don't have existing tests for this, we ought to add one.