We need this for cross-language LTO (bug 1486042) in order to pick up https://github.com/rust-lang/rust/pull/58057.
Initially we'll only care about this on Win64. I have no preference on whether to switch the Rust on all platforms or just Windows.
I believe this will also fix the ARM64 issues in bug 1531986 and related.
This still needs the SIMD fixes from bug 1521249.
I am told that we're OK to use a beta rust on nightly because it will be stable by the time we ship our stable (we'd pick up the stable release on our beta). If this is not correct, please let me know.