Mark generated impl de::Visitor blocks as #[automatically_derived]

[tdittr]

When using cargo llvm-cov on one of my projects I noticed that the #[derive(Deserialize)] annotations on my structs were marked as not covered.

This PR adds #[automatically_derived] also to the impl de::Visitor blocks that are generated. In my tests, they now no longer show up in the coverage as not covered.

rustc automatically excludes impl blocks marked with #[automatically_derived] since rust-lang/rust#120185.
The generated impl Deserialize block is already marked with #[automatically_derived] and thus correctly not counted.
However, the visitors and field visitors which are generated within the deserialize method were counted towards the coverage.

[oli-obk]

Thanks! That makes sense

[dtolnay]

The generated impl Deserialize block is already marked with #[automatically_derived] and thus correctly not counted.

There were some Deserialize and other impls (DeserializeSeed, Serialize, remote deserialize/serialize) that were not marked. I have opened #2868 to add #[automatically_derived] to them all. Please let me know if there were any of those impls intentionally omitted from this PR and should not get #[automatically_derived]. I am not familiar with using llvm-cov in my own projects.

[tdittr]

Oh interesting, I guess I just missed those. Thanks for also adding the annotation there!