cache-messages will cache incorrect messages when switching between cargo check and cargo clippy

[Nemo157]

> cargo new --lib foo
     Created library `foo` package
> cd foo
> echo 'fn foo(_: &u8) {}' > src/lib.rs

> cargo check
    Checking foo v0.1.0 (/tmp/tmp.aMWSnXGUN2/foo)
warning: function is never used: `foo`
 --> src/lib.rs:1:4
  |
1 | fn foo(_: &u8) {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.13s

> cargo clippy
warning: function is never used: `foo`
 --> src/lib.rs:1:4
  |
1 | fn foo(_: &u8) {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.06s

> cargo clean

> cargo clippy
    Checking foo v0.1.0 (/tmp/tmp.aMWSnXGUN2/foo)
warning: function is never used: `foo`
 --> src/lib.rs:1:4
  |
1 | fn foo(_: &u8) {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> src/lib.rs:1:11
  |
1 | fn foo(_: &u8) {}
  |           ^^^ help: consider passing by value instead: `u8`
  |
  = note: `#[warn(clippy::trivially_copy_pass_by_ref)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

    Finished dev [unoptimized + debuginfo] target(s) in 0.22s

> cargo --version
cargo 1.41.0-nightly (8280633db 2019-11-11)

also the inverse applies, running `cargo check` now will return clippy's results

> cargo clippy
warning: function is never used: `foo`
 --> src/lib.rs:1:4
  |
1 | fn foo(_: &u8) {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> src/lib.rs:1:11
  |
1 | fn foo(_: &u8) {}
  |           ^^^ help: consider passing by value instead: `u8`
  |
  = note: `#[warn(clippy::trivially_copy_pass_by_ref)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
> cargo check
warning: function is never used: `foo`
 --> src/lib.rs:1:4
  |
1 | fn foo(_: &u8) {}
  |    ^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: this argument (1 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
 --> src/lib.rs:1:11
  |
1 | fn foo(_: &u8) {}
  |           ^^^ help: consider passing by value instead: `u8`
  |
  = note: `#[warn(clippy::trivially_copy_pass_by_ref)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref

    Finished dev [unoptimized + debuginfo] target(s) in 0.05s

[ehuss]

Thanks for the report. We're trying to fix this in #7533, so I'm going to close since we're already working on it.