Support for file checking beyond syntactic correctness

[jonhoo]

Currently, the syntastic integration (which will hopefully soon also be used by neomake) runs rustc with -Zparse-only to check a particular file. While this is fine, it doesn't catch the vast majority of errors that come up during development.

Obviously, just dropping -Zparse-only won't work, as the compiler will complain about missing imports/types, but maybe some kind of integration with cargo could work? Maybe look up the file tree to find Cargo.toml, run cargo build, and parse out any warning/error that pertains to the currently open file? It's not pretty, but would be extremely useful!

[Seeker14491]

Can't we use cargo rustc -Zno-trans like this Atom plugin does? This will catch those errors while being faster than cargo build.

[da-x]

Today we have the 'cargo' Syntastic checker in rust.vim, plus ALE and experimental RLS. Standalone .rs file checking is still supported by rust.vim for the situation where the Rust file is really standalone. If a "errors only for the current file" filter would be useful in Syntastic, ALE, or RLS, then it can be a separate issue?

Also, for limited single-file checking, using rustfmt as a third check #80 may be an interesting option.

Anyway, this can be closed :)