Emit compiler errors for unknown field attributes

[sharkdp]

Emit an error if an unknown field attribute is used on a salsa::tracked struct. For example, when changing the attribute in ParsedFile inside the examples/lazy-input/main.rs example from return_ref to returns_ref, the compiler emits:

error: unrecognized field attribute
   --> examples/lazy-input/main.rs:164:5
    |
164 |     #[returns_ref]
    |     ^^^^^^^^^^^^^^

I'm opening this as a draft for two reasons:

• It's unclear to me if this is actually desired behavior. Could non-salsa attributes be mixed with salsa attributes?
• The error message above seems fine, but there are actually a lot more downstream compiler errors if that change is made to the example. Maybe there is a way to get rid of those downstream errors?

closes #728

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[MichaReiser]

I'm not sure if possible but it would be nice if we could carry along unknown attributes. I would have found that useful in the past, e.g. to suppress a clippy warning (allow a reference to some deprecated code).

It would also be great to have a test for this. You can add a compilation test by adding a new file in tests/compile-fail.