Suggest _ for missing generic arguments in turbofish

[kornelski]

The compiler may suggest unusable generic type names for missing generic arguments in an expression context:

fn main() {
    (0..1).collect::<Vec>()
}

help: add missing generic argument

 (0..1).collect::<Vec<T>>()

but T is not a valid name in this context, and this suggestion won't compile.

I've changed it to use _ inside method calls (turbofish), so it will suggest (0..1).collect::<Vec<_>>() which may compile.

It's possible that the suggested _ will be ambiguous, but there is very extensive E0283 that will help resolve that, which is more helpful than a basic "cannot find type T in this scope" users would get otherwise.

Out of caution to limit scope of the change I've limited it to just turbofish, but I suspect _ could be the better choice in more cases. Perhaps in all expressions?

[rustbot]

r? @spastorino

rustbot has assigned @spastorino.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[compiler-errors]

Isn't this going to be true when we're inside a nested item? You should probably limit this search somehow so it doesn't trigger for like

fn hello() {
  x.foo({
    fn bar() {
        let x: Vec;
    }
  });

[kornelski]

find_map picks only the first expression, so it stops at the block around fn. I don't think it's possible to have an item directly as an argument.

I've considered checking whether MethodCall's args contain self.path_segment, but that requires digging a few layers deep, and the code for it is pretty involved.

BTW, let x: Vec<T> is not a great suggestion, and let x: Vec<_> would have been better. I'm wondering whether it should always use _ on non-items inside function bodies, and limit suggestions of parameter names to items where _ is not allowed.

[spastorino]

Seems good to me but r? @compiler-errors as they've already made the initial review.

[compiler-errors]

@bors r=spastorino,compiler-errors

[bors]

📌 Commit 3bbbe3c has been approved by spastorino,compiler-errors

It is now in the queue for this repository.