Rustc does not warn about `use` with paths incompatible with `uniform_paths` for edition 2018

[orium]

Updated Description:

This code generates no warnings when compiled with the 2015 edition:

#![deny(rust_2018_compatibility)]
#![allow(unused_imports)]

extern crate rayon;

mod foo {
    mod rayon {}
    
    use rayon::Scope;
}

but when compiled with the 2018 edition fails to compile:

error[E0432]: unresolved import `rayon::Scope`
 --> src/lib.rs:9:9
  |
9 |     use rayon::Scope;
  |         ^^^^^^^^^^^^ no `Scope` in `foo::rayon`

error[E0659]: `rayon` is ambiguous (name vs any other name during import resolution)
 --> src/lib.rs:9:9
  |
9 |     use rayon::Scope;
  |         ^^^^^ ambiguous name
  |
note: `rayon` could refer to the module defined here
 --> src/lib.rs:7:5
  |
7 |     mod rayon {}
  |     ^^^^^^^^^^^^
  = help: use `self::rayon` to refer to this module unambiguously
note: `rayon` could also refer to the extern crate imported here
 --> src/lib.rs:4:1
  |
4 | extern crate rayon;
  | ^^^^^^^^^^^^^^^^^^^
  = help: use `::rayon` to refer to this extern crate unambiguously

error: aborting due to 2 previous errors

Some errors occurred: E0432, E0659.
For more information about an error, try `rustc --explain E0432`.

Old Description

Rustc does not warn about use with paths incompatible with uniform_paths for edition 2018. This means that rustfix it will not fix your code for edition 2018.

To reproduce create the following project:

Cargo.toml:

[package]
name = "foobar"
version = "0.0.0"
authors = ["Foo <example@example.com>"]

[dependencies]
criterion = "0.2"

src/lib.rs:

#![allow(unused)]
#![feature(rust_2018_preview, uniform_paths)]
extern crate criterion;
use criterion::Criterion;

If you run cargo +nightly check you will not get any warning, which means that when you run cargo +nightly fix --edition nothing will change, but the code is not compilable in rust 2018. Change Cargo.toml to enable edition 2018 (by adding cargo-features = ["edition"] and package.edition = "2018") and run cargo +nightly build:

$ cargo +nightly build
[...]
error: `criterion` import is ambiguous
 --> src/lib.rs:4:5
  |
3 | extern crate criterion;
  | ----------------------- can refer to `self::criterion`
4 | use criterion::Criterion;
  |     ^^^^^^^^^ can refer to external crate `::criterion`
  |
  = help: write `::criterion` or `self::criterion` explicitly instead
  = note: relative `use` paths enabled by `#![feature(uniform_paths)]`
[...]

cargo +nightly fix --edition should have changed line 4 to use ::criterion::Criterion.

Ref: Bug in cargo: rust-lang/cargo#5905

[zackmdavis]

The error is emitted during name resolution:

rust/src/librustc_resolve/resolve_imports.rs

Lines 721 to 733 in ee73f80

	// Special-case the error when `self::x` finds its own `use x;`.
	if has_external_crate &&
	results.module_scope == Some(span) &&
	results.block_scopes.is_empty() {
	let msg = format!("`{}` import is redundant", name);
	this.session.struct_span_err(span, &msg)
	.span_label(span,
	format!("refers to external crate `::{}`", name))
	.span_label(span,
	format!("defines `self::{}`, shadowing itself", name))
	.help(&format!("remove or write `::{}` explicitly instead", name))
	.note("relative `use` paths enabled by `#![feature(uniform_paths)]`")
	.emit();

To make this rustfix-able, we would want to reformulate the .help note as a structured suggestion instead. (With the caveat that rustfix can't yet handle overlapping/alternative suggestions, so hopefully one of the "remove" or "::cratename" alternatives can be the canonical fix.)

[eddyb]

@zackmdavis That's not enough. What needs to happen is the canaries need to be injected in Rust 2015 before switching to Rust 2018, and that's where the correct suggestions can be generated, because we know what the user meant (in the working Rust 2015 code).

I've been meaning to do this ever since I realized it's actually plausible with the way the canaries are implemented, because we can inject them in more situations, and provide different diagnostics.