"Try invoking the function" hint

[Patryk27]

Hi,

Let's consider following code:

fn foo() -> usize {
    0
}

fn bar(i: usize) {
    
}

fn main() {
    bar(foo);
}

Right now it fails with the expected usize, found fn item message - I think it would be helpful if compiler bragged about missing parentheses when function's return type matches expected type:

fn main() {
    bar(foo);
//      ^^^ expected usize, found fn item
//          hint: try invoking the function: foo()
}

It may even help a little bit when dealing with async functions - recently I've written:

async fn foo() {
    
}

fn bar(f: impl Future<Output=()>) {
    
}

fn main() {
    bar(foo);
}

... and it took me a few minutes to notice that what I actually wanted was bar(foo()).

[cramertj]

@Centril why was this tagged async-await?

[Centril]

@Centril why was this tagged async-await?

It may even help a little bit when dealing with async functions - recently I've written:

[cramertj]

I don't think that makes this an async/await issue in any way, though. 🤷‍♀️

[gilescope]

It's a great suggestion either way. Going to take a crack at it if I may?
@rustbot claim

[gilescope]

Move to close this as PR #63337 now merged in nightly fixes this:

   |
2  | fn foo() -> usize {
   | ----------------- fn() -> usize {foo} defined here
...
11 |     bar(foo);
   |         ^^^
   |         |
   |         expected usize, found fn item
   |         help: use parentheses to call this function: `foo()`
   |
   = note: expected type `usize`
              found type `fn() -> usize {foo}`

[gilescope]

@Centril thoughts?

[Centril]

The message for async/await is still bad:

error[E0277]: the trait bound `fn() -> impl std::future::Future {foo}: std::future::Future` is not satisfied
  --> src/main.rs:10:5
   |
10 |     bar(foo);
   |     ^^^ the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
   |
note: required by `bar`
  --> src/main.rs:7:1
   |
7  | fn bar(f: impl Future<Output = ()>) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Leaving this up to @estebank

[Patryk27]

IMO the {foo} defined here part looks weird (why curly braces?) - how about:

2  | fn foo() -> usize {
   | ----------------- function `fn() -> usize` (named `foo`) defined here