Trait bound not satisfied should specify "fn item" vs "fn ptr" kind

[CAD97]

Given the following code: [playground]

struct Argument;
struct Return;

fn function(_: Argument) -> Return { todo!() }

trait Trait {}
impl Trait for fn(Argument) -> Return {}

fn takes(_: impl Trait) {}

fn main() {
    takes(function);
    takes(|_: Argument| -> Return { todo!() });
}

The current output is:

error[E0277]: the trait bound `fn(Argument) -> Return {function}: Trait` is not satisfied
  --> src/main.rs:12:11
   |
12 |     takes(function);
   |     ----- ^^^^^^^^ the trait `Trait` is not implemented for `fn(Argument) -> Return {function}`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Trait` is implemented for `fn(Argument) -> Return`
note: required by a bound in `takes`
  --> src/main.rs:9:18
   |
9  | fn takes(_: impl Trait) {}
   |                  ^^^^^ required by this bound in `takes`

error[E0277]: the trait bound `[closure@src/main.rs:13:11: 13:34]: Trait` is not satisfied
  --> src/main.rs:13:5
   |
13 |     takes(|_: Argument| -> Return { todo!() });
   |     ^^^^^ the trait `Trait` is not implemented for `[closure@src/main.rs:13:11: 13:34]`
   |
   = help: the trait `Trait` is implemented for `fn(Argument) -> Return`
note: required by a bound in `takes`
  --> src/main.rs:9:18
   |
9  | fn takes(_: impl Trait) {}
   |                  ^^^^^ required by this bound in `takes`

Compare E0308 mismatched types: [playground]

error[E0308]: mismatched types
  --> src/main.rs:12:21
   |
12 |     let _: fn(()) = function;
   |            ------   ^^^^^^^^ expected `()`, found struct `Argument`
   |            |
   |            expected due to this
   |
   = note: expected fn pointer `fn(())`
                 found fn item `fn(Argument) -> Return {function}`

error[E0308]: mismatched types
  --> src/main.rs:13:21
   |
13 |     let _: fn(()) = |_: Argument| -> Return { todo!() };
   |            ------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found struct `Argument`
   |            |
   |            expected due to this
   |
   = note: expected fn pointer `fn(())`
                 found closure `[closure@src/main.rs:13:21: 13:44]`

Ideally the output (of the reported example) should look like:

error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `fn(Argument) -> Return {function}: Trait` is not satisfied
  --> src/main.rs:12:11
   |
12 |     takes(function);
   |     ----- ^^^^^^^^ the trait `Trait` is not implemented for fn item `fn(Argument) -> Return {function}`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the trait `Trait` is implemented for fn pointer `fn(Argument) -> Return`
note: required by a bound in `takes`
  --> src/main.rs:9:18
   |
9  | fn takes(_: impl Trait) {}
   |                  ^^^^^ required by this bound in `takes`

error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): the trait bound `[closure@src/main.rs:13:11: 13:34]: Trait` is not satisfied
  --> src/main.rs:13:5
   |
13 |     takes(|_: Argument| -> Return { todo!() });
   |     ^^^^^ the trait `Trait` is not implemented for closure `[closure@src/main.rs:13:11: 13:34]`
   |
   = help: the trait `Trait` is implemented for fn pointer `fn(Argument) -> Return`
note: required by a bound in `takes`
  --> src/main.rs:9:18
   |
9  | fn takes(_: impl Trait) {}
   |                  ^^^^^ required by this bound in `takes`

[steffahn]

I believe instead of "fixing" the error message, there's an underlying problem to be solved that function item types and function pointer types are rendered wayyy too similar. That should be changed, instead of working around it everywhere. I've expressed some thoughts on an alternative way to represent them in this post on IRLO.

[CAD97]

I agree that's the better the long-term fix, but this is a fairly simple incremental change in theory (just copy what E0308 is doing to E0277). And probably that makes it a good

@rustbot label E-easy

for entering in to diagnostics hacking.

[obeis]

@rustbot claim