unused_variables suggestion has false positives for variables that are only named in unreachable code

[Moises-Herradon-Cueto]

Running cargo clippy --fix or cargo +nightly clippy --fix on this code causes it to break, and tells me to come here and file a bug report

fn example() -> u8 {
    let result = 0_u8;
    return 0;
    let a = result;
}

I think clippy wants to turn result into _result, and then the second result is not found.

Meta

rustc --version --verbose:

rustc 1.67.0 (fc594f156 2023-01-24)
binary: rustc
commit-hash: fc594f15669680fa70d255faec3ca3fb507c3405
commit-date: 2023-01-24
host: x86_64-unknown-linux-gnu
release: 1.67.0
LLVM version: 15.0.6

Full output of clippy

warning: failed to automatically apply fixes suggested by rustc to crate `test_bug`

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/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[E0425]: cannot find value `result` in this scope
 --> src/lib.rs:4:14
  |
4 |     let _a = result;
  |              ^^^^^^ help: a local variable with a similar name exists: `_result`

warning: unreachable statement
 --> src/lib.rs:4:5
  |
3 |     return 0;
  |     -------- any code following this expression is unreachable
4 |     let _a = result;
  |     ^^^^^^^^^^^^^^^^ unreachable statement
  |
  = note: `#[warn(unreachable_code)]` on by default

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0425`.
Original diagnostics will follow.

warning: unreachable statement
 --> src/lib.rs:4:5
  |
3 |     return 0;
  |     -------- any code following this expression is unreachable
4 |     let a = result;
  |     ^^^^^^^^^^^^^^^ unreachable statement
  |
  = note: `#[warn(unreachable_code)]` on by default

warning: unused variable: `result`
 --> src/lib.rs:2:9
  |
2 |     let result = 0_u8;
  |         ^^^^^^ help: if this is intentional, prefix it with an underscore: `_result`
  |
  = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `a`
 --> src/lib.rs:4:9
  |
4 |     let a = result;
  |         ^ help: if this is intentional, prefix it with an underscore: `_a`

warning: function `example` is never used
 --> src/lib.rs:1:10
  |
1 | async fn example() -> u8 {
  |          ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `test_bug` (lib test) generated 4 warnings (run `cargo clippy --fix --lib -p test_bug --tests` to apply 2 suggestions)
warning: `test_bug` (lib) generated 4 warnings (4 duplicates)
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s

[Moises-Herradon-Cueto]

I've just realized it's the same if the function is not async

[clubby789]

This also happens with cargo fix, and is not a clippy-specific issue.

The problem is that clippy fails to recognise that the warnings also existed before the fix was applied
My mistake, this specific case actually is breaking the code by changing the name of a used variable.

[jieyouxu]

I think it's because renaming unused variable names to be prepended with an underscore is a machine-applicable level lint. The lint probably adheres strictly to variable liveness analysis and considers the let result = 0_u8; variable dead because it's never used (since the variable name is referenced only in future unreachable expressions) so it doesn't need to rename the unreachable result. Not sure what the right fix is.

Relevant function:

rust/compiler/rustc_passes/src/liveness.rs

Line 1570 in a12d31d

fn report_unused(