Add ability to check rustc version via cfg_attr

[iddm]

I am using diesel and I have to use this thing:

#![allow(proc_macro_derive_resolution_fallback)]

However, it is allowed to use it only in rustc >= 1.29 so it breaks compatibility if I write it so. I'd like to write it as the following:

#![cfg_attr(
    rustc_version_higher_than(1, 28),
    allow(proc_macro_derive_resolution_fallback)
)]

This requires adding such a attribute (rustc_version_higher_than) with corresponding checks. If anyone agreed, I'd do it myself, just point me where I can see for such a code as an example and where I can put it.

P.S. For checking is it less (if anyone needs that someday) it would be good just to use not:

#![cfg_attr(
    not(rustc_version_higher_than(1, 28)),
    allow(proc_macro_derive_resolution_fallback)
)]

[cuviper]

The current state of the art is to check the version in your build.rs, perhaps using crates like version_check or rustc_version, then print your own "cargo:rustc-cfg=whatever" to use in your main sources.

The compiler could add something like you suggest, with enough bikeshedding design work, but then you'd have to wait for that compiler release as the minimum version you could ever check for.

[Centril]

This is proposed in rust-lang/rfcs#2523.

[iddm]

@cuviper I know that there is a more or less hacky-way of doing this but this should be done via the compiler preprocessor things as I have written above because there must be an easy way of doing this, obviously.

[Mark-Simulacrum]

Closing in favor of rust-lang/rfcs#2523.