Respect Cargo `[$target-kind.edition]` when formatting a file

[Xanewok]

At the moment we only respect Rustfmt file configuration, however it'd be good to also detect crate kind edition if specified in a Cargo.toml and use that as a default.

Not sure if I'm overthinking this, but it seems it's possible to have two distinct crate targets (e.g. [[bin]]) using a different edition but still including the same file - in this case the edition-specific formatting would be ambiguous. What does cargo fmt do in this case? I believe Cargo should try to detect that and possibly emit a warning since this could result in some subtle bugs or unexpected behaviour, while RLS should fail and with a message.

While we have the file -> package mapping thanks to project model, since the edition can be specified per crate target we'd need to resolve that information in a more fine-grained way. One way would be to parse the Cargo-emitted .d files for bin and libs or use something like build plan with computed file inputs to create a file -> [crate target] mapping.

cc @otavio

[otavio]

The way I implemented it in cargo fmt is splitting the targets per edition settings and calling rustfmt with the respective --edition flag passed as argument.

The cargo_metadata crate already offers a convenient access to this information, what I did on rustfmt was basically make use of it.