Add a file allowing cross-crate configuration of lints

[oli-obk]

Right now, if your project has a certain set of lints it wants to use, it needs to repeat the #![deny(foo)] entries in every single crate of the project.

In the clippy project it has frequently been requested (rust-lang/rust-clippy#1313) that we add a clippy.toml file which would have entries for allowing, warning and denying lints, instead of requiring -A, -W or -D flags to the compiler.

I think this is something that should be solved on the rustc side. As a minimal version I propose to add a --lint-file=foo.toml or --lint-settings=foo.toml setting, which is default-filled by cargo by crawling up the directory tree and looking for a lints.toml or rustc.toml (or a green bikshed).

To reduce repeating the lint modifiers, I propose to use the keys allow, deny, warn and forbid of Vec<String> type:

deny = [
    "foo",
    "bar",
]

Does this require an RFC?

[kennytm]

Cargo can parse the lints.toml and pass the -D/-W/-A flags to rustc directly, without introducing the --lint-file flag. I think this is purely a cargo feature.

[sanmai-NL]

Right now the README says:

Some lints can be configured in a TOML file named with clippy.toml or .clippy.toml. It contains basic variable = value mapping eg.

Which lints can be configured?

[oli-obk]

closing in favour of rust-lang/cargo#5034

[oli-obk]

Which lints can be configured?

That's a clippy topic, have a look at https://github.com/rust-lang-nursery/rust-clippy/#configuration for details