Display errors when cargo fix fails.

[ehuss]

It can be difficult to figure out what's wrong when a user reports that cargo fix fails. It can be hard to figure out which suggestion caused a compile error, especially if the error is in another file/location.

[rust-highfive]

r? @alexcrichton

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[ehuss]

cc @killercup

• I'm not sure if it was intentional to not show the errors, but I think they would be useful.
• An update to rustfix will be needed before this can be merged. I needed pub for the rendered diagnostic (ehuss/rustfix@d904271).
• This also updates broken_fixes_backed_out. In Warn users when fixes break code rustfix#93 the check was removed for whether or not it actually backed out the changes. I'm not sure why. I've updated it so that it actually checks whether or not it was backed out.
• I'm a bit surprised that cargo fix exits 0 with a warning when it fails. Is there some reason it's not a error?

Here's an example of what the full output looks like:

    Checking badfix v0.1.0 (.../badfix)
warning: failed to automatically apply fixes suggested by rustc to crate `badfix`

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!

The following errors were reported:
error[E0408]: variable `_x` is not bound in all patterns
 --> src/main.rs:7:20
  |
7 |         E::A(_x) | E::B(x) => {}
  |              --    ^^^^^^^ pattern doesn't bind `_x`
  |              |
  |              variable not in all patterns

error[E0408]: variable `x` is not bound in all patterns
 --> src/main.rs:7:9
  |
7 |         E::A(_x) | E::B(x) => {}
  |         ^^^^^^^^        - variable not in all patterns
  |         |
  |         pattern doesn't bind `x`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0408`.
Original diagnostics will follow.

warning: unused variable: `x`
 --> src/main.rs:7:14
  |
7 |         E::A(x) | E::B(x) => {}
  |              ^ help: consider using `_x` instead
  |
  = note: #[warn(unused_variables)] on by default

warning: variant is never constructed: `B`
 --> src/main.rs:4:9
  |
4 |         B(i32,),
  |         ^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

warning: unused variable: `x`
 --> src/main.rs:7:14
  |
7 |         E::A(x) | E::B(x) => {}
  |              ^ help: consider using `_x` instead
  |
  = note: #[warn(unused_variables)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 1.02s

[alexcrichton]

I'm not sure if it was intentional to not show the errors, but I think they would be useful.

Definitely looks useful to me, these look like much better diagnostics!

I've updated it so that it actually checks whether or not it was backed out.

Makes sense!

I'm a bit surprised that cargo check exits 0 with a warning when it fails. Is there some reason it's not a error?

I think this is because cargo check actually succeeds (the underlying implementation) because the broken code was backed out. Seems plausible to me to change though

[ehuss]

Updated with new rustfix.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 502ab65 has been approved by alexcrichton

[bors]

⌛ Testing commit 502ab65 with merge a3a3c25...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing a3a3c25 to master...