Cargo fix fails to apply --edition to block-utils crate

[cholcombe973]

I tried to use cargo fix --edition to upgrade my block-utils crate to the 2018 edition. It fails to apply and gives output suggesting I file a bug so here i am :)

Steps

1. Checkout master of https://github.com/cholcombe973/block-utils
2. run cargo fix --edition against it.
3. Patch fails to apply

Possible Solution(s)

Notes

cargo fix --edition --allow-dirty
    Checking block-utils v0.6.0 (/home/chris/repos/block-utils)                                                                                                        
warning: failed to automatically apply fixes suggested by rustc to crate `block_utils`                                                                                 

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

  * src/lib.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: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition                                                        
 --> src/nvme.rs:5:5                                                                                                                                                   
  |                                                                                                                                                                    
5 | use {BlockResult, BlockUtilsError};                                                                                                                                
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `crate`: `crate::{BlockResult, BlockUtilsError}`                                                                      
  |                                                                                                                                                                    
  = note: `-W absolute-paths-not-starting-with-crate` implied by `-W 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 #53130 <https://github.com/rust-lang/rust/issues/53130>                                                                      
                                                                                                                                                                       
warning: unused variable: `e`                                                                                                                                          
    --> src/lib.rs:1520:33                                                                                                                                             
     |                                                                                                                                                                 
1520 |             Err(nom::Err::Error(e)) | Err(nom::Err::Failure(e)) => Err(BlockUtilsError::new(                                                                    
     |                                 ^ help: consider using `_e` instead                                                                                             
     |                                                                                                                                                                 
     = note: #[warn(unused_variables)] on by default                                                                                                                   
                                                                                                                                                                       
warning: absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition                                                        
 --> src/nvme.rs:5:5                                                                                                                                                   
  |                                                                                                                                                                    
5 | use {BlockResult, BlockUtilsError};                                                                                                                                
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `crate`: `crate::{BlockResult, BlockUtilsError}`                                                                      
  |                                                                                                                                                                    
  = note: `-W absolute-paths-not-starting-with-crate` implied by `-W 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 #53130 <https://github.com/rust-lang/rust/issues/53130>                                                                      
                                                                                                                                                                       
warning: unused variable: `e`                                                                                                                                          
    --> src/lib.rs:1520:33                                                                                                                                             
     |                                                                                                                                                                 
1520 |             Err(nom::Err::Error(e)) | Err(nom::Err::Failure(e)) => Err(BlockUtilsError::new(                                                                    
     |                                 ^ help: consider using `_e` instead                                                                                             
     |                                                                                                                                                                 
     = note: #[warn(unused_variables)] on by default                                                                                                                   
                                                                                                                                                                       
    Finished dev [unoptimized + debuginfo] target(s) in 5.56s  

Output of cargo version: cargo 1.31.0 (339d9f9 2018-11-16)

rustc --version
rustc 1.31.0 (abe02cefd 2018-12-04)

running on Ubuntu 16.04.4 LTS

[ehuss]

Thanks for the report. Here is a reproduction:

fn main() {
    enum E {
        A(i32,),
        B(i32,),
    }
    match E::A(1) {
        E::A(x) | E::B(x) => {}
    }
}

The unused_variables lint does not give proper suggestions with | patterns in match arms. It only suggests _x for the first variable, but not the second, causing compilation to fail. Surprisingly I can't find any upstream issues for this.

To work around this, just update your match blocks to use _ or _e for the bindings.

[alexcrichton]

Thanks for the report @cholcombe973 and the reduction @ehuss! I've moved the suggestion bug to rust-lang/rust as it's a compiler bug and the otherwise relevant but for edition migration is at #5738, so I'm going to close this.