Suboptimal diagnostic on `Output()` type ascription

Code

fn foo(_: Option()) {}

Current output

error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
 --> src/lib.rs:1:11
  |
1 | fn foo(_: Option()) {}
  |           ^^^^^^^^ only `Fn` traits may use parentheses

error[E0107]: enum takes 1 generic argument but 0 generic arguments were supplied
   --> src/lib.rs:1:11
    |
  1 | fn foo(_: Option()) {}
    |           ^^^^^^ expected 1 generic argument
    |
note: enum defined here, with 1 generic parameter: `T`
   --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs:600:10
    |
600 | pub enum Option<T> {
    |          ^^^^^^ -
help: add missing generic argument
    |
  1 | fn foo(_: Option(T)) {}
    |                  +

Desired output

error[E0214]: parenthesized type parameters may only be used with a `Fn` trait
 --> src/lib.rs:1:11
    |
  1 | fn foo(_: Option()) {}
    |           ^^^^^^^^ only `Fn` traits may use parentheses
help: replace parentheses with angle brackets
    |
  1 | fn foo(_: Option<>) {}
    |                 ~~

error[E0107]: enum takes 1 generic argument but 0 generic arguments were supplied
   --> src/lib.rs:1:11
    |
  1 | fn foo(_: Option()) {}
    |           ^^^^^^ expected 1 generic argument
    |
note: enum defined here, with 1 generic parameter: `T`
   --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs:600:10
    |
600 | pub enum Option<T> {
    |          ^^^^^^ -
help: enclose `()` in angle brackets to make it the generic argument
    |
  1 | fn foo(_: Option<()>) {}
    |                 +  +
help: add missing generic argument
    |
  1 | fn foo(_: Option<T>) {}
    |                 ~+~

Rationale and extra context

What I intended in this case was Option<()> (I was prototyping :p). The suggestion Option(T) is obviously wrong just based on syntax, but I'm also a bit doubtful about Option<T>: that would assume that the user simultaneously used the incorrect syntax for generic arguments and forgot to specify the actual generic. Imo, it's more realistic that they made just one error -- forgot to put <> about the generic param.

So what I'm proposing is to:

1. below the first error, suggest ()-><> (maybe that would make sense in general)
2. below the second error:
   1. fix the existing suggestion ()->(T) to use angle brackets: <T>
   2. add a suggestion ()-><()>

Other cases

This particular error condition (in the diagnostic) is pretty difficult to achieve, since things like Optionu32 would just not resolve to a type.

Rust Version

rustc 1.94.0 (4a4ef493e 2026-03-02)
binary: rustc
commit-hash: 4a4ef493e3a1488c6e321570238084b38948f6db
commit-date: 2026-03-02
host: x86_64-unknown-linux-gnu
release: 1.94.0
LLVM version: 21.1.8

Anything else?

No response