Patterns make parsing of optional parameter names kinda wonky

[ben0x539]

I'd expect these method declarations to work:

struct Welp<T>(pub T);

trait Bleh {
    fn foo(Welp(i): Welp<int>) {}
    //         ^ expected `` but found `(`
    fn bar(&&&x: &&&int) {}
    //           ^~ expected `,` but found `&&`
    fn baz((): ()) {}
    //       ^ expected `,` but found `:`
}

The manual says "As with let bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument." Yet these aren't, and it seems like it would require a ton of lookahead given that newtype struct constructors can be named with arbitrarily long paths and you can have lots of &s prefixed, or deeply nested tuples, ...

Edit: Rereading the parser code, it looks like currently the only valid patterns in trait method parameter position are a single identifier optionally prefixed with &, && or mut, and rereading the manual, the bit I quoted is in the description of functions, not trait methods, so I guess I don't have a real case here. ;)

[steveklabnik]

Given that this would be a language change, I'm going to give it a close. Feel free to start an RFC if you want to see this land in Rust!