Rustc integration setup

[matprec]

The current rustc integration setup is strictly speaking not conforming with the cargo rule, that features are additive. LSP implementations like rust-analyzer assume that crates strictly adhere to that rule and compile/check with --all-features by default, which breaks because of the rustc-dep-of-std feature. This causes issues like rust-lang/rust-analyzer#4702.

I can't think of an elegant solution for this problem, but maybe something along the lines of explicitly failing with compile_error! and a helpful message would make this more discoverable.

[philipc]

@alexcrichton Do you have any ideas about what to do for this?

[philipc]

#233 is the best I could come up with.

[alexcrichton]

Yeah unfortunately I don't know of a great way to solve this. The root of the issue is the weirdness of getting built as part of libstd. I'd love to fix that but I don't really know of another way myself.