Unknown annotations in salsa::tracked structs do not lead to an error

[sharkdp]

Tracked struct fields that are annotated with unknown attributes are just silently ignored. For example, when changing ParsedFile in examples/lazy-input/main.rs to this …

#[salsa::tracked]
struct ParsedFile<'db> {
    value: u32,
    #[returns_ref]  // oops, should be 'return_ref'!
    links: Vec<ParsedFile<'db>>,
}

… everything still compiles without any errors, but parsed_file.links(db) returns a clone of the vector now.

It looks to me like the developer experience could possibly be improved here.

[MichaReiser]

I think what we have to do here is to preserve any unknown attributes and carry them along so that Rust can complain if it is indeed an unknown attribute. Unless we don't want to support custom attributes altogether (e.g. allow?) in which case we can just error early