Rustc suggests both std and core imports at times

[leonardo-m]

rustc often offers a help to import something from std and core:

error[E0433]: failed to resolve: use of undeclared type `NonZeroU32`
  --> ...\test.rs:20:33
   |
20 | ...                   NonZeroU32::new(5).unwrap(),
   |                       ^^^^^^^^^^ not found in this scope
   |
help: consider importing one of these items
   |
6  | use core::num::NonZeroU32;
   |
6  | use std::num::NonZeroU32;
   |

Perhaps in such situations rustc could use an heuristic to suggest only one of the two imports.

[Manishearth]

cc @estebank feels like we should at least show std first here?

[leonardo-m]

feels like we should at least show std first here?

This is quite useful in situations like the rust playground because there sometimes there are many different sources for a function, and sometimes the std one (that's most times the one you really want) gets elided in the "others" sources.

[estebank]

We already have a couple of hacks for these kind of suggestions, so I'd be fine with just modifying the code here to push any use suggestion that starts with core:: to the end of the list, or at the very least, after std::.

rust/compiler/rustc_resolve/src/diagnostics.rs

Line 1640 in ee0357a

path_strings.sort();

As pointed above it might make sense to bias the sort to always put std:: first. We could also consider removing all references to core::, but that makes me uneasy.