"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

[estebank]

@Patryk27 I feel like fn foo() -> usize could also work and be more natural. At the same time, I'd like to unify how we refer to functions and closures (currently [closure@file.rs:LL:CC]), and I feel that the braces style is would be suitable for them: fn(_) -> _ {file.rs:LL::CC}.

[Patryk27]

@estebank Thanks, I understand - I'm not certain about curly braces in this position, since it changes their meaning though (from denoting block to denoting location, depending on whether it is part of code block or message) and thus, to me, they are kinda confusing.

IMO the fn(_) -> _ [file.rs:LL:CC] construct could be more legible, since brackets are not usually set together with functions (contrary to curly braces), so they stand out more.

[estebank]

#63870:

error[E0277]: the trait bound `fn() -> impl std::future::Future {foo}: std::future::Future` is not satisfied
  --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:9:5
   |
LL | fn bar(f: impl Future<Output=()>) {}
   | --------------------------------- required by `bar`
...
LL |     bar(foo);
   |     ^^^ the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
   |
   = help: it looks like you forgot to use parentheses to call the function: `foo()`

😊