Syntax error on trait fns with destructured args

[tobz1000]

Trait functions with destructured arguments do not parse successfully. Some examples:

struct S { a: usize, b: usize }
struct NewType(usize);

trait T {
    // Syntax Error: expected COMMA/expected type
    fn f1((a, b): (usize, usize)) {}

    // Syntax Error: expected COMMA/expected value parameter/expected R_PAREN...
    fn f2(S { a, b }: S) {}

    // Syntax Error: expected COMMA
    fn f4(NewType(a): NewType) {}

    // Syntax Error: expected COMMA
    fn f3(&&a: &&usize) {}
}

These all parse okay as free-standing functions, or in a type impl-block.

The double-deref example is mentioned here, so apologies if this is a known issue.

Running in VS Code, built from commit ea0ee8e.

[tobz1000]

It looks like the problem is within params::param_list_opt_patterns - because trait functions are parsed as having optional parameter bindings.

The parser looks ahead by 4 tokens for one of the following sequences before parsing a pattern:

• _:
• foo:
• mut foo:
• &foo:
• &mut foo:

Anything else, and it will just attempt to parse a type.

To detect anything more complex, it looks as though it needs to perform something close to the actual pattern::pattern(...) routine, but on encountering an error, backtracking and parsing just as a type instead.

I haven't seen any other examples of backtracking/"retrying" in this fashion, although I'm not too familiar with the project as a whole.

[matklad]

Lol, TIL that my first Rust RFC is actually implemented: rust-lang/rfcs#1685

How could I have missed that?

I think the best thing to do would be to support only 2018 edition and always parse patterns in arguments as required, removing the old lookahead special case.

[tobz1000]

Okay - does rust-analyzer generally not support 2015, or is it an just an acceptable lack of coverage in this case?

I'd be happy to put up a PR in the next day or so for this.

[matklad]

For now, we target 2018, and 2015 where it's not too hard to support. Long term, of course, we need to support all editions.