the type placeholder `_` is not allowed within types on item signatures should hint at generics

[oli-obk]

The following signature yields the mentioned error

fn shuffle(args: Vec<_>) -> Vec<_>

it should be replaced by

fn shuffle<T>(args: Vec<T>) -> Vec<T>

There should be an error hint suggesting this change.

[aatxe]

👍

I've actually wanted this to be sugar for a while. (As well as sugar to make Vec equivalent to Vec<_>, but I imagine that might be unpopular.)

[arielb1]

@aatxe

Generic Elision isn't going to be a supported feature (you do need to be able to specify type parameters in paths - e.g. foo::<T>).

[cyplo]

Hi ! @arielb1, @oli-obk: does it mean this issue can be closed ?

[estebank]

Current output:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:1:22
  |
1 | fn shuffle(args: Vec<_>) -> Vec<_> {}
  |                      ^ not allowed in type signatures

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:1:33
  |
1 | fn shuffle(args: Vec<_>) -> Vec<_> {}
  |                                 ^ not allowed in type signatures

I agree that it should teach the generic type argument syntax on a suggestion.

Proposed output:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
 --> src/main.rs:1:22
  |
1 | fn shuffle(args: Vec<_>) -> Vec<_> {}
  |                      ^          ^ not allowed in type signatures
  |                      |
  |                      not allowed in type signatures
help: instead use a generic type argument
  |
1 | fn shuffle<T>(args: Vec<T>) -> Vec<T> {}
  |           ^^^           ^          ^