Bump MSRV to 1.36

[taiki-e]

Maybe this can fix the CI.

cc #503

[ghost]

Almost there - looks like this just needs a cargo fmt :)

[taiki-e]

CI passed.

[taiki-e]

Ah, wait, we can remove some build scripts and cfgs:

crossbeam/crossbeam-epoch/build.rs

Lines 5 to 7 in c239d8f

	if cfg.probe_rustc_version(1, 31) {
	println!("cargo:rustc-cfg=has_min_const_fn");
	}

[taiki-e]

I added the folloing changes:

• Remove has_min_const_fn cfg and remove a build script of crossbeam-epoch.
• Use std::mem::MaybeUninit directly and remove maybe-uninit dep.

[taiki-e]

Updated MSRV policy based on @jonhoo's suggestion.

Crossbeam supports stable Rust releases going back at least six months, and every time the minimum supported Rust version is increased, a new minor version is released. Currently, the oldest supported version is 1.36.

[jonhoo]

Do we want to give explicit MSRV numbers for the different features too? At least where they differ. Or maybe 1.36 covers all features? Do we want CI to check that MSRV holds for all features?

[jeehoonkang]

I have a very minor comment. Thanks for the PR!

@jonhoo I can't understand your latest comment. Would you please rephrase your question?

[ghost]

@jonhoo I think MSRV policy should cover all features (except for the nightly feature, of course).

[jonhoo]

@jeehoonkang Ah, sorry, what I meant was that we'll want to make sure that the MSRV covers all features, or if it doesn't, document which features have which MSRV.

@stjepang If that's the case, that's awesome! Do we check this on CI at the moment? If not, might be a good thing to add so we don't accidentally break that later on. I think @taiki-e's cargo-hack may be able to help with that.

[taiki-e]

I think @taiki-e's cargo-hack may be able to help with that.

Yup, by using cargo-hack, we can check this with the following one command, and will not miss it if features are added in the future.

cargo +1.36.0 hack check --all --each-feature --no-dev-deps --ignore-private --skip nightly

[jonhoo]

That seems like a fantastic thing to have in CI!