Better "unresolved import" error message and "self::"

Importing names sometimes requires use self:: as shown here:

main.rs

use foo::Foo;
mod foo;

fn main() {}

foo.rs

use bar::Bar; //looks fine at first glance but is actually an error
mod bar;

pub struct Foo {
    bar: Bar
}

bar.rs

pub struct Bar;

Someone new to Rust will almost certainly make this mistake, and then see the following error message:

foo.rs:1:5: 1:8 error: unresolved import. maybe a missing `extern crate bar`?
foo.rs:1 use bar::Bar;
             ^~~
foo.rs:1:5: 1:13 error: failed to resolve import `bar::Bar`
foo.rs:1 use bar::Bar;
             ^~~~~~~~

Personally I found this was the main source of frustration while learning Rust's module system because of my ingrained thinking in absolute paths.

I think two changes would be helpful here:

The maybe a missing extern crate bar hint should only be given when appropriate (ie. when mod bar; cannot be resolved).

There should be a hint to maybe use self:: when use bar::Bar; cannot be resolved and the current module is not the crate root.

[steveklabnik]

Triage: no change, these errors are the same today

[ketsuban]

Triage: I get a different error now, which is a little more helpful.

src/foo.rs:2:5: 2:8 error: cannot declare a new module at this location
src/foo.rs:2 mod bar;
                 ^~~
src/foo.rs:2:5: 2:8 note: maybe move this module `foo` to its own directory via `foo/mod.rs`
src/foo.rs:2 mod bar;
                 ^~~
src/foo.rs:2:5: 2:8 note: ... or maybe `use` the module `bar` instead of possibly redeclaring it
src/foo.rs:2 mod bar;
                 ^~~

However, taking the first suggested course of action and moving src/{foo, bar}.rs to src/foo/{mod, bar}.rs gets me the unhelpful extern crate suggestion again, so the report stands.

[birkenfeld]

At least the long diagostics for E0432 now reference self:: and super::: #33320

[jseyfried]

Fixed in #36289.