Speed improvements for cargo based linting

[aomader]

Instead of using cargo build to generate the lint messages, it might make sense to use cargo rustc in combination with -Z no-trans, as it is already done with rustc based linting. The downside of this approach is, that one has to specify which target to build, .i.e., --lib, --bin NAME, etc, which might require multiple runs to really get all messages.

[HybridEidolon]

I'll add that because cargo build is used, the entire crate and all of its dependencies is built upon saving a file. This causes a number of problems:

1. if you tab into a terminal to run cargo build, undefined behavior occurs due to two instances of cargo building the same thing. More often than not this causes errors and fails to output rlibs and the executables. This gets much worse when dependencies aren't fully built.
2. if you save two or more files in quick succession, multiple cargo builds are initiated. Same issue as Spawning rustc process on every save #16, this can result in total saturation of the computer's resources.

The main issue can be solved if cargo had an option to only build the crate specified by the -p argument or the current crate, and simply return a non-zero exit code otherwise. The linter could then identify that error code and respond with a warning telling the user to first run a full cargo build of the dependencies. An additional option to build only dependencies would be helpful here, since the current crate code is expected not to build if linting is necessary anyway.

The side effects (and issue #16) can be solved by rate-limiting linting such that only one lint can be running at any time. This is probably an issue with the linter package itself, only it becomes such a problem for rust and not other languages due to it being a compiled language using the compiler directly for linting. I think the linter API should have an option to rate-limit calls to lint per linter, since this same problem can affect other compiled languages.

Cargo issue for build only: rust-lang/cargo#1891

[wackywendell]

Another way around this is cargo check, which is part of cargo-extras. An option to use that instead of build or instead of rustc -Z no-trans would be nice. cargo check is just a shortcut for rustc -Z no-trans, but I believe it has the added benefit of checking the whole crate instead of just the current file.

[colin-kiegel]

PR #43 adds an option for cargo check to linter-rust. :-)

[wackywendell]

Hurray!

[White-Oak]

This one should probably be closed.