Extend dead code detection to cover files that are not loaded

[chris-morgan]

If I have a whole tree of *.rs files and one of them is not included in the compilation, that is probably an error on my part. I propose a new lint to this effect.

When compiling without cfg(test), I suggest that files named test.rs or tests.rs and the contents of directories named test or tests be ignored.

To suppress this lint for individual files, we would add an attribute such as #[allow_unloaded_file = "foo.rs"]. That could go on a module in the appropriate place in the filesystem or on the crate itself.

This occurred to me when I found src/librustc/middle/typeck/infer/test.rs with a few syntax errors (#12564).

[SiegeLord]

I'm not sure which files this is meant to trigger for. E.g. in my projects I have a directory examples with several example#.rs files. If I compile one of them, would it tell me that I forgot to compile the rest alongside with it?

It seems like this would have too many false positives and (or false negatives depending on how you handle the above case) in legitimate directory structures.

[chris-morgan]

@SiegeLord I see your point there. I see several primary options (mutually inclusive):

• By default, only complain if at least one other file is used. This will take care of the majority of examples.

• Don't turn this lint on by default.

• Push people back into the structure that was the rustpkg recommendation whereby each directory will be self-contained:

  + src/
    + mycrate/
      - lib.rs
      - test.rs
      - foo.rs
    + examples/
      + example1/
        - main.rs
      + example2/
        - main.rs
  

  (i.e. move your exampleN.rs to exampleN/main.rs.)

[fhahn]

I've started working on a compiler plugin that adds a lint to detect unused files. At the moment, it only analysis a single crate (e.g. src/lib.rs) and all files in subdirectories of the crate root.

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!