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.

This transitively depends on bug 1521734 as discussed with dmajor on IRC, but mentioning it explicitly for other readers. (That is, bug 1521734 is the reason bug 1521249 hasn't landed despite having reviewed patches.)

This is needed for cross-language LTO (bug 1512723). We don't want to block on waiting for 1.34's release, so we'll get a head start now, but we'll update to the final 1.34 release when available.

https://hg.mozilla.org/mozilla-central/rev/e3809f2c3cb2