diagnostics: suggest Result instead of Option if inner arguments would match

[matthiaskrgr]

pub struct Error {}

pub fn want_result() -> Option<(), Error> {
     unimplemented!()
}

rust fails to compile this:

error[E0107]: wrong number of type arguments: expected 1, found 2
 --> src/lib.rs:3:36
  |
3 | pub fn want_result() -> Option<(), Error> {
  |                                    ^^^^^ unexpected type argument

error: aborting due to previous error

Since <x,y> matches the Result signature, it would be super neat if rustc could suggest something like Did you mean to use Result<(), Error> instead of Option<(), Error> here?

[matthiaskrgr]

@rustbot label: +A-diagnostics

[rylev]

I believe Option and Result are both library types. This would require special casing in order to provide this diagnostic. Is there precedence for doing such a thing with library types?

[camelid]

We already special-case Option and Result: they are what's called diagnostic items. Some, None, Ok, and Err are also language items.

[camelid]

I labeled this as P-low since I don't think it would provide that much benefit.