Fixing bad suggestion for _ in const type when a function #81885

[kper]

Closes #81885

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> $DIR/typeck_type_placeholder_item_help.rs:13:22
   |
LL | const TEST4: fn() -> _ = 42;
   |                      ^
   |                      |
   |                      not allowed in type signatures
   |                      help: use type parameters instead: `T`

Do not show the suggestion help: use type parameters instead: T when fn

[rust-highfive]

r? @estebank

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[estebank]

I originally thought that a simple filter as you're doing here would be enough, but looking at the result I feel like we might have to do something a bit more involved to be accurate and remain helpful. Would you be interested in exploring alternatives we could have here?

[kper]

Thank you for your comments. I will look into it.

[kper]

I am currently thinking of ways to implement this filter. But, how can a function know that it is in a context of type?
Do I have to change something in typing context? https://github.com/rust-lang/rust/blob/master/compiler/rustc_typeck/src/collect.rs#L756

[kper]

I am currently thinking of ways to implement this filter. But, how can a function know that it is in a context of type?
Do I have to change something in typing context? https://github.com/rust-lang/rust/blob/master/compiler/rustc_typeck/src/collect.rs#L756

Nevermind that, I passed the hir::Ty which has a reference to the parent_node. This node can I check if it is const or static.

However, this is not working for impl yet

fn main() {}
struct MyStruct;

trait Test {
}

impl Test for MyStruct {
    const TEST: fn() -> _ = 42; 
}

error[E0438]: const `TEST` is not a member of trait `Test`
  --> main_test.rs:10:5
   |
10 |     const TEST: fn() -> _ = 42; 
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not a member of trait `Test`

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> main_test.rs:10:25
   |
10 |     const TEST: fn() -> _ = 42; 
   |                         ^
   |                         |
   |                         not allowed in type signatures
   |                         help: use type parameters instead: `T`

[estebank]

@kper that is ok about associated consts. The other improvements in the output are more than great.

@bors r+

[bors]

📌 Commit 1adc6be has been approved by estebank