cargo fix --edition does not make the code compilable in rust 2018 if uniform_paths are enabled

[orium]

Hi,

while testing cargo fix I believe I have found a bug: When using uniform_paths cargo fix --edition does not migrate paths in use, and those will fail to build on rust 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 now 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)]`
[...]

As this diagnostic suggests, cargo +nightly fix --edition should have changed line 4 to use ::criterion::Criterion.

[alexcrichton]

Thanks for the report! Currently not all suggestions in the compiler are marked as "machine applicable" which means that cargo fix will actually apply the fix. In that sense this is mostly an issue for upstream rust-lang/rust and related to rust-lang/rust#50723

[orium]

rustc doesn't even emit a warning. Opened issue rust-lang/rust#53797.

[alexcrichton]

Thanks! In that case I'll close this in favor of that