Minimum supported version of Rust (MSVR) config and support

Many lints suggest using new features in the std lib like clippy::manual_strip. I think these are great because they help people upgrade their code to take advantage of these features. However, there are many projects that support old versions of Rust that can't use these. These lints only make more work for those projects because they have to allows to fix their CI. See this recent commit in serde-yaml for example.

I'd like to add a key to clippy.toml to specify the minimum version of rust (and the stdlib) that a project supports. We can conditionally register these lints that target features not supported on that version.

I understand that eventually proper support for this will be added to Cargo and rustc to support this but at that stage we can just deprecate the setting and use the new stuff.

[flip1995]

I think this makes sense. This should be possible by just adding a "lint configuration" and passing that to the relevant lint passes.

[suyashb95]

@flip1995 I'd like to pick this up!

[djc]

This would be awesome to have. I know we have a bunch of these allows in the url crate already at least.

[flip1995]

@djc Can you post a list of them here, so we can apply this configuration option to those lints? That would be really helpful!

[djc]

In url we have these:

#[allow(clippy::option_as_ref_deref)] // introduced in 1.40, MSRV is 1.36
#[allow(clippy::manual_non_exhaustive)] // introduced in 1.40, MSRV is 1.36

The matches!() macro is also an important one that I ran into before.

[alex]

clippy::match-like-matches-macro requires matches!() which is new in 1.42

[suyashb95]

@flip1995 do we need to add an msvr attribute to all the lints to denote the targeted Rust version?

[flip1995]

No, just the lints mentioned here. And then, we can iteratively add it to lints in the future, if a issue for them comes up.

[llogiq]

Just a heads up: manual_range_contains should have a msrv of 1.35.0.

[Kixunil]

THIS! Just got bitten by matches!() being used in postgres-protocol motivated by clippy.

Can we perhaps also add a lint that will suggest setting the msrv key for projects that don't have it? Explicit latest would turn it off. This would motivate people to at least think about supporting older versions.

Also, oddly enough, Debian stable contains rustc 1.41.1, but oddly enough cargo 1.42.1, so please make sure the version is compared against rustc not against cargo.