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I'm pretty sure all uses of RUSTC_BOOTSTRAP are to get around limitations in Rust's SIMD support. The main users are encoding_rs which depends on packed_simd a

I'm pretty sure all uses of RUSTC_BOOTSTRAP are to get around limitations in Rust's SIMD support. The main users are encoding_rs which depends on packed_simd and qcms which needs NEON support. So it's not like we're going around using all sorts of unstable features. It's very targeted and at least in qcms's case unlikely to change before stabilization.

deleted 5y ago
[deleted]
hsivonen 5y ago
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).