Warn when .rs files in examples/tests/benches are unused

[jonas-schievink]

By default, Cargo will compile all files inside examples/tests/benches. However, when at least one example, test or benchmark is specified in the Cargo.toml, all other files in the respective directory are ignored. It would be nice to emit a warning since such a directory layout is very unlikely to be intentional (multi-file targets are supposed to get their own subdirectory).

Related: #4013

(this could also be done for files in src/bin/*, but that configuration is used intentionally sometimes, for example by Cargo itself)

[MathieuDuponchelle]

Alternatively, other files could also not be ignored, I have a use case where I want to specify required-features for one target, but end up being forced to define all the other targets, which makes me sad, curious what the reason is for that behaviour?

[dwijnand]

Resolved with #5330, I'd say.

[jonas-schievink]

Yep, looks like it!