Unable to `-Zbuild-std` because std depends on `compiler-builtins` without the `no-f16-f128` features

[TheBlueMatt]

When trying to build for macOS from Linux for determinism reasons a recent (last week or three?) update to compiler-builtins appears to have started causing issues. Something like RUSTFLAGS="-C target-cpu=apple-a14 -C link-arg=-isysroot/home/matt/Xcode-12.2-12B45b-extracted-SDK-with-libcxx-headers -C link-arg=-mmacosx-version-min=10.9 -C link-arg=--target=aarch64-apple-darwin -C link-arg=-mcpu=apple-a14 -C link-arg=-fuse-ld=lld -C link-arg=-mcpu=apple-a14" cargo build --features=std --offline -v --release --target aarch64-apple-darwin -Zbuild-std=std,panic_abort now fails with

error[E0412]: cannot find type `f16` in this scope
  --> registry/src/github.spider-man.dpdns.org-1ecc6299db9ec823/compiler_builtins-0.1.112/src/float/extend.rs:91:40
   |
91 |     pub extern "C" fn __extendhfsf2(a: f16) -> f32 {
   |                                        ^^^ help: a builtin type with a similar name exists: `i16`
.....
error[E0635]: unknown feature `f16`
  --> registry/src/github.spider-man.dpdns.org-1ecc6299db9ec823/compiler_builtins-0.1.112/src/lib.rs:16:51
   |
16 | #![cfg_attr(not(feature = "no-f16-f128"), feature(f16))]

[bjorn3]

Which nightly version are you using?

[TheBlueMatt]

This is not on nightly, but rather using RUSTC_BOOTSTRAP=1

[bjorn3]

compiler-builtins is only expected to work with the latest nightly version. -Zbuild-std should probably be respecting the lockfile for the standard library, but that is non-trivial to implement as the Cargo.lock in the rust-src component is a direct copy of the Cargo.lock in the root of the rust repo, which also includes all crates of the compiler and thus is outdated as far as cargo is concerned.

[TheBlueMatt]

Fair enough, I have a hack right now that forces cargo to fetch an old compiler_builtins (and cc), deletes the new ones out of the cargo repository, then builds with --offline that works, but boy its a hack :)