Document and test Rust 1.59.0 as minimum supported version

[SimonSapin]

1.59.0 is the version that stabilized the asm! macro for inline assembly.

The README previously said that Nightly is required, but that turned to be the case only for cross-compiling to i686-unknown-none.

[lion328]

Thanks for the PR!

Yeah this LGTM, but I noticed that Rust 1.59.0 CI doesn't seem to do anything. Did you planned to add more tests to that later?

[SimonSapin]

Yes. At some point I changed it to build on x86_64-unknown-linux-gnu as a placeholder for "something that has pre-compiled libcore in rustup" but then decided it wasn’t particularly useful. #2 does add CI for RISC-V which works on stable (if alloc is disabled).

By the way, how do you feel about making alloc a non-default feature? I tend to prefer avoiding default features, though the effects are more pronounced for crates that typically have multiple dependents in the same project: all of them need to opt out for a default feature not to be implicitly requested, and there’s no way to opt out of a single feature.

[lion328]

By the way, how do you feel about making alloc a non-default feature? I tend to prefer avoiding default features, though the effects are more pronounced for crates that typically have multiple dependents in the same project: all of them need to opt out for a default feature not to be implicitly requested, and there’s no way to opt out of a single feature.

For my use cases I normally use alloc, that's why it's a default feature. But yes I agree with you. If you want to change it then go ahead. I'm going to merge this one for now.