cargo fix --edition-idioms fails on extern crate ignore

[illicitonion]

Problem
A module with extern crate ignore; causes auto-fixes to fail.
Expect cargo fix --edition-idioms to succeed. Instead, it errors and asks me to file a bug.

Steps

$ git clone https://github.com/illicitonion/cargo-fix-2018-repro.git
$ cd cargo-fix-2018-repro
$ rustc +beta --version
rustc 1.31.0-beta.17 (1a4f1f398 2018-11-25)
$ cargo +beta --version
cargo 1.31.0-beta (339d9f9c8 2018-11-16)
$ cargo +beta fix --edition-idioms
    Checking rust-ignore-repro v0.0.1 (/home/dwh/tmp/rustplay)                                                                                                                                                                                 
warning: failed to automatically apply fixes suggested by rustc to crate `rust_ignore_repro`                                                                                                                                                   

after fixes were automatically applied the compiler reported errors within these files:

  * src/main.rs

This likely indicates a bug in either rustc or cargo itself,
and we would appreciate a bug report! You're likely to see 
a number of compiler warnings after this message which cargo
attempted to fix but failed. If you could open an issue at
https://github.com/rust-lang/cargo/issues
quoting the full output of this command we'd be very appreciative!

warning: `extern crate` is not idiomatic in the new edition                                                                                                                                                                                    
 --> src/main.rs:1:1                                                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
1 | extern crate ignore;                                                                                                                                                                                                                       
  | ^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`                                                                                                                                                                           
                                                                                                                                                                                                                                               
warning: `extern crate` is not idiomatic in the new edition                                                                                                                                                                                    
 --> src/main.rs:1:1                                                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
1 | extern crate ignore;                                                                                                                                                                                                                       
  | ^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
  = note: `-W unused-extern-crates` implied by `-W rust-2018-idioms`                                                                                                                                                                           
                                                                                                                                                                                                                                               
    Finished dev [unoptimized + debuginfo] target(s) in 0.43s
$ sed -e 's#extern crate#use#g' -i src/main.rs 
$ cargo +beta build
   Compiling rust-ignore-repro v0.0.1 (/home/dwh/tmp/rustplay)                                                                                                                                                                                 
error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)                                                                                                                         
 --> src/main.rs:1:5                                                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
1 | use ignore;                                                                                                                                                                                                                                
  |     ^^^^^^ not an extern crate passed with `--extern`                                                                                                                                                                                      
  |                                                                                                                                                                                                                                            
note: this import refers to the built-in attribute imported here                                                                                                                                                                               
 --> src/main.rs:1:5                                                                                                                                                                                                                           
  |                                                                                                                                                                                                                                            
1 | use ignore;                                                                                                                                                                                                                                
  |     ^^^^^^                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                               
error: aborting due to previous error                                                                                                                                                                                                          
                                                                                                                                                                                                                                               
For more information about this error, try `rustc --explain E0658`.                                                                                                                                                                            
error: Could not compile `rust-ignore-repro`.                                                                                                                                                                                                  

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

Output of cargo version:

$ rustc +beta --version
rustc 1.31.0-beta.17 (1a4f1f398 2018-11-25)
$ cargo +beta --version
cargo 1.31.0-beta (339d9f9c8 2018-11-16)

If I instead replace with use ::ignore; it seems to work fine.

[ehuss]

That's interesting. It seems to fail with any crate with a name that matches any item in the standard prelude (including attributes, macros, functions, types, etc.).

[illicitonion]

Probably related: Depending on a crate which has use ::ignore; in it gives an error about ignore being defined multiple times: https://github.com/illicitonion/cargo-2018-multiple-defines

Can spin off a separate bug if this turns out to be unrelated...

[alexcrichton]

cc @petrochenkov, is this a bug or expected behavior? Specifically it looks like with --extern ignore=... this fails to compile:

use ignore;

but this succeeds:

use ::ignore;

(and @ehuss indicates that this applies to anything in the prelude!)

[petrochenkov]

It's a deficiency of the migration lint.

extern crate foo; imports foo only in type namespace.
use ::foo; imports foo only in type namespace.

use foo; however imports foo in all three namespaces, and some of the extra imported names may cause issues, e.g. be feature gated (like with the ignore attribute above), or conflict with other names in the root module.

[petrochenkov]

We should either

• Suggest use ::foo; (easy, correct, but crude).
• Check whether use foo; brings names in value or macro namespaces, and suggest use foo; only if it doesn't, otherwise suggest use ::foo (this may be harder).

[alexcrichton]

Ok thanks for the info @petrochenkov! I've moved this to rust-lang/rust#56326 and will close this in favor of that.