Various improvements to the `incompatible_msrv` lint (#14433)

This PR supersedes #14328 following the [2025-03-18 Clippy meeting
discussion](https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/Meeting.202025-03-18/with/506527762).
It uses a simpler approach than what was proposed initially in #14328
and does not add new options.

First, it documents how `cfg_attr` can be used to change the MSRV
considered by Clippy to trigger the lint. This allows the MSRV to be
feature gated, or to be raised in tests.

Also, the lint stops warning about items which have been explicitly
allowed through a rustc feature. This works even if the feature has been
stabilized since. It allows using an older compiler with some features
turned on, as is done in Rust for Linux. This fixes #14425.

Then, if the lint triggers, and it looks like the code is located below
a `cfg` or `cfg_attr` attribute, an additional note is issued, once, to
indicate that the `clippy::msrv` attribute can be controlled by an
attribute.

Finally, the lint is extended to cover any path, not just method and
function calls. For example, enumeration variants, or constants, were
not MSRV checked. This required replacing two `u32::MAX` by
`u32::max_value()` in MSRV-limited tests.

An extra commit adds a TODO for checking the const stability also, as
this is not done right now.

@Centri3 I'll assign this to you because you were assigned #14328 and
you were the one who nominated the issue for discussion (thanks!), but
of course feel free to reroll!

r? @Centri3

changelog: [`incompatible_msrv`]: better documentation, honor the
`features` attribute, and lint non-function entities as well

0 files changed, 0 insertions, 0 deletions