Misleading use foo::bar error message

[brson]

Here are two slightly different sources with slightly different errors:

use foo;

fn main() { }

error[E0432]: unresolved import `foo`
 --> src/main.rs:1:5
  |
1 | use foo;
  |     ^^^ no `foo` external crate

use foo::bar;

fn main() { }

error[E0432]: unresolved import `foo`
 --> src/main.rs:1:5
  |
1 | use foo::bar;
  |     ^^^ use of undeclared type or module `foo`

For use foo, the error is "no foo external crate"; for use foo::bar it is "undeclared type or module foo".

The first error I think is unambiguously correct - there's nothing foo could be but an external crate. The second though is failing to mention that foo could very will be a missing crate. And it may be more likely to be a missing crate (or a typo...) than a type or module that hasn't been imported.

I encountered this second error while writing some training material and had to explain that the error is misleading and that actually you need to list crate foo in your manifest.

A better error might be "use of undeclared crate, type, or module" (while also changing the first one to "use of undeclared crate" for consistency), or "no foo extern crate, or use of undeclared type or module foo". Even better might be to do some analysis to see if there is any crate with a name close to "foo", whether there are any internal names equal to or similar to "foo", and customize the error based on that.

This is using whatever nightly is current right now.

[Centril]

cc @estebank @petrochenkov

[petrochenkov]

The amount and inconsistency of resolve diagnostics make it hard to update them synchronously with the language.
In the uniform path PR I made sure diagnostics do not regress, but didn't update diagnostics that didn't change, and I didn't touch them since then.

[Spoonbender]

Triage: no change