Comment by hsivonen

The crate linked to from the comment you replied to uses RUSTC_BOOTSTRAP for a non-SIMD reason: To allow Firefox to be built with somewhat old rustc.

Firefox no longer updates its MSRV in sync with Rust release trains. I’d be happy to remove this particular RUSTC_BOOTSTRAP if we decided to increase Firefox’s MSRV (currently stuck at 1.47.0).