Feature-gate #[no_std]

[kmcallister]

It breaks various things at the syntax layer, e.g. #21827, #16803, #14342. I plan to fix those before the beta but I'm not sure what other issues there are.

[sfackler]

If we feature gate this, it'd be nice to move std injection either into expansion or after it. Right now you can't do #![cfg_attr(feature = "foo", no_std)], which means that crates that want to compile both in normal and no_std contexts have to use no_std. For example, take rust-phf: rust-phf/rust-phf@24555b1.

[pnkfelix]

1.0 beta