Build error with extern crates after upgrading to 2018 edition

[elliottslaughter]

I took my project here and converted it to the 2018 edition: https://github.com/elliottslaughter/integrity-checker

The flow I used was:

git clone https://github.com/elliottslaughter/integrity-checker
cd integrity-checker
cargo +beta fix --edition # no warnings on first try, one change
git commit -a -m 'Prep for edition.'
# go to Cargo.toml and set edition keyword
cargo +beta build

But even though cargo fix completes without warnings, the build is giving me:

   Compiling integrity-checker v0.1.0 (/Users/elliott/Programming/Rust/integrity-checker)                                                   
error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)                      
 --> src/error.rs:2:5                                                                                                                       
  |                                                                                                                                         
2 | use ignore;                                                                                                                             
  |     ^^^^^^ not an extern crate passed with `--extern`                                                                                   
  |                                                                                                                                         
note: this import refers to the built-in attribute imported here                                                                            
 --> src/error.rs:2:5                                                                                                                       
  |                                                                                                                                         
2 | use ignore;                                                                                                                             
  |     ^^^^^^                                                                                                                              
                                                                                                                                            
error: aborting due to previous error                                                                                                       
                                                                                                                                            
For more information about this error, try `rustc --explain E0658`.                                                                         
error: Could not compile `integrity-checker`.                                                                                               

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

For posterity I've saved the result of having applied cargo fix here:

https://github.com/elliottslaughter/integrity-checker/tree/rust-2018

$ rustc +beta --version
rustc 1.31.0-beta.20 (8940af3ce 2018-11-30)
$ cargo +beta --version
cargo 1.31.0-beta (339d9f9c8 2018-11-16)

[petrochenkov]

Duplicate of #56326

[ehuss]

@petrochenkov This issue seems a little different because it does not involve edition idioms or migration lints. A smaller example, assume you are writing new code in 2018 from scratch:

cargo +beta new foo --lib
cd foo
cargo +beta new ignore --lib
echo 'ignore = {path = "ignore"}' >> Cargo.toml
echo 'use ignore;' >> src/lib.rs
cargo +beta check

The error message (as shown above) on current beta is somewhat confusing. If you're not aware that use ignore is attempting to import both an extern crate and an attribute from the standard prelude, as a user I would be thoroughly flummoxed. It doesn't help that ignore is passed in with --extern and the word "only" may not be clear enough.

Perhaps the fix for this and #56326 is the same, but that issue is currently marked as an idiom lint, but this affects "normal" code.

[petrochenkov]

Ultimately, the fix to both issues is to stabilize uniform paths (#56417), so that the feature gate error is no longer reported.