Confusing E0425 error when referencing undefined name with ::foo

[joshtriplett]

Test program:

fn foo() { println!("foo") }

fn main() {
    ::foo()
}

This produces the following error messages:

error[E0425]: cannot find function `foo` in the crate root
 --> src/main.rs:4:7
  |
4 |     ::foo()
  |       ^^^ not found in the crate root
help: possible candidate is found in another module, you can import it into scope
  |
1 | use crate::foo;
  |

Both the error and the help seem entirely unhelpful. The error suggests that it looked for a function foo in the crate root, but in fact it looked for a crate foo. The whole "possible candidate" help seems incorrect, and adding that line would lead to a compilation failure.

[Centril]

cc @petrochenkov @estebank

[rylev]

This is still an issue, but at least the incorrect help message is no longer displayed:

error[E0425]: cannot find function `foo` in the crate root
 --> src/main.rs:4:7
  |
4 |     ::foo()
  |       ^^^ not found in the crate root