Function pointer should not allow full patterns.

[ehuss]

For example, the following should be rejected as a type:

fn((a, b): (i32, i32))

Instead of "pat", I think it should be IDENT | _.

Beware, don't alter FnSigInput for FnArg.

[Centril]

This seems like a semantic restriction, not a syntactic one (this is checked in AST validation).

[CAD97]

Rustc does disallow this with #[cfg(FALSE)], though, emitting E642.

error[E0642]: patterns aren't allowed in methods without bodies
 --> src/lib.rs:2:13
  |
2 | type x = fn((a, b): (i32, i32));
  |             ^^^^^^
  |
help: give this argument a name or use an underscore to ignore it
  |
2 | type x = fn(_: (i32, i32));
  |             ^

[Centril]

@CAD97 Oh yes. That's a question of precedence though in the parser, not disallowing per se; see rust-lang/reference#781 (comment) for the actual grammar here.

[Centril]

Actual logic is defined in https://doc.rust-lang.org/nightly/nightly-rustc/rustc_parse/parser/struct.Parser.html#method.parse_param_general.