Unhelpful diagnostic on `=` to `:` typo mentioning return type notation

[oriongonza]

Code

let x: Vec::new();

Current output

error: return type notation arguments must be elided with `..`
 --> src/main.rs:2:20
  |
2 |     let x: Vec::new();
  |                    ^^ help: add `..`: `(..)`

error: return type notation not allowed in this position yet
 --> src/main.rs:2:12
  |
2 |     let x: Vec::new();
  |            ^^^^^^^^^^

Rationale and extra context

This is a mistake where : was used instead of =. Instead of telling the user that it talks about rtn.

Rust Version

stable and latest nightly.
rustc 1.85.0-nightly (a2545fd6f 2024-11-28)

Anything else?

@rustbot label +D-invalid-suggestion

[RodBurman]

I got uneasy when I read the rationale here since given:

let x: Vec::new();

The rationale says the writer could only have "really" meant:

let x = Vec::new(); // inferred x's type

Which I agree is most likely but people (and IDE's) are not that simple so another possibility is:

let x: appropriateType = Vec::new();

Maybe not likely with Vec but in the wider case of x = someType::someFunction() maybe more so. There are probably many variations on "what was really meant", this is just one I came up with quickly.

So I think we need to be careful how we improve the diagnostic lest after the improvement's release another issue is raised "Rust said just change : to =" but that is a very poor diagnostic with 'new code example here'.