Poor error messages with cargo fix with broken code

[ehuss]

There are two related issues with error messages generated with cargo fix when the code starts in a broken state.

Example 1

If the code starts with both an error, and suggestion diagnostics, cargo fix --broken-code prints a large message that tells you there is a bug and to use --broken-code. That is not a good suggestion since the user is already using --broken-code, and there isn't a bug.

// WARNING: unused_imports suggestion will remove this line
use std::mem;

fn main() {
    // ERROR: mismatched types
    let x: i32 = "foo";
}

results in:

    Checking foo v0.1.0 (/Users/eric/Temp/foo)
warning: failed to automatically apply fixes suggested by rustc to crate `foo`

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/rust/issues
quoting the full output of this command we'd be very appreciative!
Note that you may be able to make some more progress in the near-term
fixing code with the `--broken-code` flag

The following errors were reported:
error[E0308]: mismatched types
 --> src/main.rs:6:18
  |
6 |     let x: i32 = "foo";
  |            ---   ^^^^^ expected `i32`, found `&str`
  |            |
  |            expected due to this

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
Original diagnostics will follow.

error[E0308]: mismatched types
 --> src/main.rs:6:18
  |
6 |     let x: i32 = "foo";
  |            ---   ^^^^^ expected `i32`, found `&str`
  |            |
  |            expected due to this

For more information about this error, try `rustc --explain E0308`.
error: could not compile `foo` due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `foo` due to previous error

If the first pass running rustc fails, then I don't think PLEASE_REPORT_THIS_BUG should be displayed. Instead, a different error message should be used to explain that the code is starting in a broken state and cargo fix cannot handle this. This message may need to be customized whether or not --broken-code is used.

Example 2

Similar to above, if the code does not have any suggestions, and is a broken state, then it displays a relatively terse output.

fn main() {
    this_does_not_compile(); // ERROR
}

cargo fix displays (with or without --broken-code):

    Checking foo v0.1.0 (/Users/eric/Temp/foo)
error[E0425]: cannot find function `this_does_not_compile` in this scope
 --> src/main.rs:2:5
  |
2 |     this_does_not_compile(); // ERROR
  |     ^^^^^^^^^^^^^^^^^^^^^ not found in this scope

For more information about this error, try `rustc --explain E0425`.
error: could not compile `foo` due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `foo` due to previous error

I think in this situation, it might be good to display some kind of message explaining that the code is starting in a broken state and that cargo fix needs to start with passing code. This message may need to be customized based on whether --broken-code is used.

Notes

These two examples may have the same solution.

A closely related issue is #8806 where Cargo does not tell you that non-machine-applicable suggestions aren't applied. This can cause confusion as to why suggestions aren't being applied, even with --broken-code.

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cargo 1.64.0-nightly (85b500cca 2022-07-24)
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commit-date: 2022-07-24
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