Clippy is an official tool for linting your code. It's named after the venerable Clippit Office Assistant from Windows XP. It has like a bajillion lints. It works really well.

Unfortunately, the rust compiler has a weird bespoke build system, so adding things to it is rather difficult. As a result, I have been trying to add a reliable x clippy command to the build system for going on checks notes three and a half years now.

This blog post is about what makes it hard, how I fixed it, and why it took so long.

Say you don't know anything about compilers or bootstrapping. How might you approach this problem? Well, a good first try is probably just to run cargo clippy. It works everywhere else, after all. How bad could it be?

error: expected identifier, found `$`
   --> library/core/src/tuple.rs:172:26
    |
172 |                     $( ${ignore($T)} self.${index()} == other.${index()} )&&+
    |                          ^^^^^^ - help: try removing `$`

... snip ...

error[E0277]: the trait bound `(FromA, FromB): default::Default` is not satisfied
    --> library/core/src/iter/traits/iterator.rs:3442:44
     |
3442 |         let mut unzipped: (FromA, FromB) = Default::default();
     |                                            ^^^^^^^^^^^^^^^^ the trait `default::Default` is not implemented for `(FromA, FromB)`
     |
     = help: the trait `default::Default` is implemented for `()`
error: could not compile `core` (lib) due to 16 previous errors; 716 warnings emitted

Well. That wasn't particularly helpful now, was it. The problem, as explained in more detail in the rustc dev guide, is that standard library uses unstable features that may not match the features available in our default Rust toolchain. As a result, it only supports exactly two versions of the compiler: the previous beta, and the latest commit on the master branch.

Now, we can't use the previous beta directly because that's gated behind a #[cfg(bootstrap)] attribute, but we can jerry-rig that into the build system.