faster fails to migrate

[nikomatsakis]

@AdamNiederer helpfully reported that faster fails to migrate successfully:

Failing: https://github.com/AdamNiederer/faster/commits/rust-2018-migration

$ cargo build
   Compiling faster v0.4.3
error[E0463]: can't find crate for `core_or_std`
  --> src/intrin/eq.rs:58:25
   |
58 |                       use core_or_std::mem::transmute;
   |                           ^^^^^^^^^^^ can't find crate
...
79 | / rust_fallback_eq! {
80 | |     impl Eq for u8x16 where "sse2" {
81 | |         eq_mask, eq => u8x16, u8, _mm_cmpeq_epi8(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15];
82 | |     }
83 | | }
   | |_- in this macro invocation

error: aborting due to previous error

For more information about this error, try `rustc --explain E0463`.
error: Could not compile `faster`.

To learn more, run the command again with --verbose.

They write:

It looks like cargo fix isn't applying a crate:: prefix to my custom core/std module in a few cases. Manually fixing the imports fixes the build. It looks like cargo fix makes no attempt to fix imports in test modules, so the tests are still broken.

[Mark-Simulacrum]

Possibly a case of #48704.

[alexcrichton]

I've minimized this to the following:

#![feature(rust_2018_preview)]
#![warn(rust_2018_compatibility)]

macro_rules! foo {
    () => {
        {
            use myself::f;
            f();
        }
    }
}

mod myself {
    pub fn f() {}
}

fn main() {
    foo!();
    foo!();
}

which yields:

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
  --> foo.rs:7:17
   |
7  |             use myself::f;
   |                 ^^^^^^^^^ help: use `crate`: `crate::myself::f`
...
18 |     foo!();
   |     ------- in this macro invocation
   |
note: lint level defined here
  --> foo.rs:2:9
   |
2  | #![warn(rust_2018_compatibility)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^
   = note: #[warn(absolute_paths_not_starting_with_crate)] implied by #[warn(rust_2018_compatibility)]
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue TBD

warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
  --> foo.rs:7:17
   |
7  |             use myself::f;
   |                 ^^^^^^^^^ help: use `crate`: `crate::myself::f`
...
19 |     foo!();
   |     ------- in this macro invocation
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
   = note: for more information, see issue TBD

It's unclear to me whether this falls under the purview of #48704 or rustfix. The problem is that two duplicate suggestions are made and rustfix yields an error on this because it looks suspicious. I'm gonna send a fix to rustfix in the meantime as I've tested locally that it works and should at least help improve the situation.

[alexcrichton]

I've attempted a fix for this in rustfix with rust-lang/rustfix#131 which fixes this exact issue, and at least successfully got the faster crate to migrate!

[alexcrichton]

Actually, closing this in favor of #51205 as testing has shown that issue is the only thing that blocks faster