Example in instantiating_binders.md does not compile

[smanilov]

I'm not sure it's supposed to compile, but this example:

fn foo<'a>(a: &'a u32) -> &'a u32 {
    a
}
fn bar<T>(a: fn(&u32) -> T) -> T {
    a(&10)
}

fn main() {
    let higher_ranked_fn_ptr = foo as for<'a> fn(&'a u32) -> &'a u32;
    let references_bound_vars = bar(higher_ranked_fn_ptr);
}

Gives me the following diagnostic:

error[E0308]: mismatched types
  --> src/main.rs:10:37
   |
10 |     let references_bound_vars = bar(higher_ranked_fn_ptr);
   |                                 --- ^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |                                 |
   |                                 arguments to this function are incorrect
   |
   = note: expected fn pointer `for<'a> fn(&'a _) -> _`
              found fn pointer `for<'a> fn(&'a _) -> &'a u32`
note: function defined here
  --> src/main.rs:4:4
   |
4  | fn bar<T>(a: fn(&u32) -> T) -> T {
   |    ^^^    ----------------

For more information about this error, try `rustc --explain E0308`.
error: could not compile `foo` (bin "foo") due to 1 previous error

Is the example supposed to compile? I don't grok the type system quite enough to answer this myself, sorry.

Page is here: https://rustc-dev-guide.rust-lang.org/ty_module/instantiating_binders.html

[smanilov]

I was poking at this and made a small modification: using a function item instead of a function pointer:

fn foo<'a>(a: &'a u32) -> &'a u32 {
    a
}
fn bar<T>(a: impl Fn(&u32) -> T) -> T {
    a(&10)
}

fn main() {
    // let higher_ranked_fn_ptr = foo as for<'a> fn(&'a u32) -> &'a u32;
    let _ = bar(&foo);
}

I now get an even stranger error:

error[E0308]: mismatched types
  --> src/main.rs:10:13
   |
10 |     let _ = bar(&foo);
   |             ^^^^^^^^^ one type is more general than the other
   |
   = note: expected reference `&_`
              found reference `&_`
note: the lifetime requirement is introduced here
  --> src/main.rs:4:31
   |
4  | fn bar<T>(a: impl Fn(&u32) -> T) -> T {
   |                               ^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `foo` (bin "foo") due to 1 previous error

Specifically, the note is very confusing:

   = note: expected reference `&_`
              found reference `&_`

But I am adding this comment, as it might help narrow down the issue. (If the example is indeed intended to compile. I saw no indications that it's not.)

[BoxyUwU]

This example isn't supposed to compile 👍 I'll edit the surrounding text to make that a little more clear. thank you for letting me know it's confusing

[smanilov]

Thanks! Could you please link the PR to this one, before I close this one?

[BoxyUwU]

When a PR fixes this issue it'll automatically close the issue so don't worry about closing this