must_use lint only triggered when code is modified on clean build

[lopopolo]

In my project with deny(clippy::pedantic), I get these warnings for code that aleady existed, triggered by writing to the file:

$ cargo clippy -- -D warnings
warning: couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
warning: set the LLVM_CONFIG_PATH environment variable to a valid `llvm-config` executable
    Checking artichoke-backend v0.1.0 (/Users/lopopolo/dev/artichoke/artichoke/artichoke-backend)
error: this method could have a `#[must_use]` attribute
   --> artichoke-backend/src/sys/mod.rs:154:5
    |
154 |     fn info(&self) -> String {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] fn info(&self) -> String`
    |
note: lint level defined here
   --> artichoke-backend/src/lib.rs:2:9
    |
2   | #![deny(clippy::pedantic)]
    |         ^^^^^^^^^^^^^^^^
    = note: `#[deny(clippy::must_use_candidate)]` implied by `#[deny(clippy::pedantic)]`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

error: this method could have a `#[must_use]` attribute
   --> artichoke-backend/src/sys/mod.rs:158:5
    |
158 |     fn debug(&self) -> String {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] fn debug(&self) -> String`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

error: this method could have a `#[must_use]` attribute
   --> artichoke-backend/src/sys/mod.rs:162:5
    |
162 |     fn version(&self) -> String {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] fn version(&self) -> String`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

error: this method could have a `#[must_use]` attribute
   --> artichoke-backend/src/sys/mod.rs:168:5
    |
168 |     fn info(&self) -> String {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] fn info(&self) -> String`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

error: this method could have a `#[must_use]` attribute
   --> artichoke-backend/src/sys/mod.rs:172:5
    |
172 |     fn debug(&self) -> String {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^ help: add the attribute: `#[must_use] fn debug(&self) -> String`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate

error: aborting due to 5 previous errors

The following warnings were emitted during compilation:

warning: couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
warning: set the LLVM_CONFIG_PATH environment variable to a valid `llvm-config` executable

error: could not compile `artichoke-backend`.

To learn more, run the command again with --verbose.

On any write to artichoke-backend/src/sys/mod.rs in vim, subsequent runs of clippy produce no errors.

I added #[must_use] to every function Clippy asked me to when I upgraded to Rust 1.40.0, but this file generated no complaints, even in CI, until I modified it to add some additional impls of the trait these methods are in when writing a recent commit.

Any idea what is going on here?

[flip1995]

Can you try if the same thing happens with cargo clippy-preview -Zunstable-options -- -Wclippy::pedantic? If that's the case it will be fixed by #3837.

[lopopolo]

with cargo +nightly clippy-preview -Zunstable-options -- -Wclippy::pedantic on cargo 1.42.0-nightly (f6449ba23 2020-01-21) the lint doesn't trigger at all, even after writing the file like before.

[flip1995]

When I checkout artichoke/artichoke@1c7de40 and run cargo +stable clippy -- -D warnings, I get the error messages of the must_use lint. At least on a clean build.

[flip1995]

With the nightly compiler/clippy, this lint won't be emitted on these functions. It seems that the lint has changed somehow since 1.40.0.

[lopopolo]

It seems that this is not scoped to must_use_candidate. Clippy does not run on incremental builds sometimes.

I got this build failure in CI after a clean incremental run on my local box.

https://circleci.com/gh/artichoke/artichoke/5748?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

$ cargo clippy -- --version
clippy 0.0.212 (c8e3cfbd 2019-10-28)
$ cargo --version
cargo 1.40.0 (bc8e4c8be 2019-11-22)
$ rustc --version
rustc 1.40.0 (73528e339 2019-12-16)

[flip1995]

Clippy does not run on incremental builds sometimes.

Oh yeah, we know this. This will be fixed with #3837. If you want clippy to run reliably on incremental builds you can use cargo +nightly clippy-preview -Zunstable-options, until #3837 is finished.

[flip1995]

See also #2604