bug#50355: [core-updates-frozen] Building mozjs with rust@1.54 fails

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello Guix!

I'm nearly done updating the Rust bootstrap for core-updates-frozen; one
of the changes is that only the current rust (1.54) gets exposed and
used everywhere.  This seems to be OK so far, except for mozjs, which
fails to build like so:

--8<---------------cut here---------------start------------->8---
error: aborting due to 8 previous errors

Some errors have detailed explanations: E0432, E0557.
For more information about an error, try `rustc --explain E0432`.
The following warnings were emitted during compilation:

warning: Cannot set `RUSTC_BOOTSTRAP=1` from build script of `packed_simd v0.3.3 (https://github.com/hsivonen/packed_simd?rev=3541e3818fdc7c2a24f87e3459151a4ce955a67a#3541e381)`.
note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.

error: could not compile `packed_simd`

Caused by:
  process didn't exit successfully: `CARGO=/gnu/store/sgvw6h637pgw3mrn9wva29b8645z37m0-rust-1.54.0-cargo/bin/cargo CARGO_CRATE_NAME=packed_simd CARGO_MANIFEST_DIR=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/third_party/rust/packed_simd CARGO_PKG_AUTHORS='Gonzalo Brito Gadeschi <gonzalobg88@gmail.com>' CARGO_PKG_DESCRIPTION='Portable Packed SIMD vectors' CARGO_PKG_HOMEPAGE='https://github.com/rust-lang-nursery/packed_simd' CARGO_PKG_LICENSE=MIT/Apache-2.0 CARGO_PKG_LICENSE_FILE='' CARGO_PKG_NAME=packed_simd CARGO_PKG_REPOSITORY='https://github.com/rust-lang-nursery/packed_simd' CARGO_PKG_VERSION=0.3.3 CARGO_PKG_VERSION_MAJOR=0 CARGO_PKG_VERSION_MINOR=3 CARGO_PKG_VERSION_PATCH=3 CARGO_PKG_VERSION_PRE='' LD_LIBRARY_PATH='/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/release/deps:/gnu/store/89iv0hicjlmgdxk01sj9lc95xkdkb8f0-rust-1.54.0/lib' OUT_DIR=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/x86_64-unknown-linux-gnu/release/build/packed_simd-36a821efa8c03e42/out /gnu/store/89iv0hicjlmgdxk01sj9lc95xkdkb8f0-rust-1.54.0/bin/rustc --crate-name packed_simd --edition=2018 /tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/third_party/rust/packed_simd/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C opt-level=2 -C panic=abort -C embed-bitcode=no --cfg 'feature="default"' --cfg 'feature="into_bits"' -C metadata=220aa1d9e77763e7 -C extra-filename=-220aa1d9e77763e7 --out-dir /tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -C linker=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/build/cargo-linker -L dependency=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/x86_64-unknown-linux-gnu/release/deps -L dependency=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/release/deps --extern cfg_if=/tmp/guix-build-mozjs-78.10.1.drv-0/firefox-78.10.1/run-configure-from-here/x86_64-unknown-linux-gnu/release/deps/libcfg_if-646d80c97708097e.rmeta --cap-lints warn -C opt-level=2 --cap-lints warn -Cembed-bitcode=yes -C codegen-units=1` (exit status: 1)
--8<---------------cut here---------------end--------------->8---

If someone has an idea, please shout :-)

Thanks,

Maxim