Trait Solver 1 @BoxyUwU @WaffleLapkin

Warning: this quiz is still "work-in-progress", some questions might not have good explanations (or any at all), formatting/structure/titles/etc are not final, and so on. You might want to return here on a later date.

trait Trait {}

impl Trait for for<'a> fn(&'a u32) {}

fn f()
where
    for<'a> fn(&'a u32): Trait,
{
}

fn main() {
    f();
}
Solution 
error: implementation of `Trait` is not general enough
  --> examples/trait_solver_1.rs:12:5
   |
12 |     f();
   |     ^^^ implementation of `Trait` is not general enough
   |
   = note: `for<'a> fn(&'a u32)` must implement `Trait`, for any lifetime `'0`...
   = note: ...but `Trait` is actually implemented for the type `for<'a> fn(&'a u32)`

error: could not compile `code` (example "trait_solver_1") due to 1 previous error

The trait implementation is for a higher ranked function pointer (for<'a> fn). But the where clause is different, there the for<'a> is parsed as part of the bound, so the bound is on a not higher-ranked function pointer.

impl:

type: for<'a> fn(&'a u32)
trait: Trait

bound:

type: fn(&'a u32)
trait: Trait

Only higher-ranked function pointers implement the trait, so it fails to compile.