rustc throws an error when compiling with target-feature=-sse

[wumb0]

[build]
rustflags = [
    "-C", "target-feature=-sse",
]

Having proc-macro2 as a dependency in a project with the above .cargo/config throws an error when compiling.

cargo +nightly build --release --verbose
PS C:\Users\wumb0\Documents\nosse> cargo +nightly build --release --verbose
   Compiling winapi v0.3.9
   Compiling proc-macro2 v1.0.23 (C:\Users\wumb0\Documents\nosse\proc-macro2)
   Compiling unicode-xid v0.2.1
     Running `rustc --crate-name build_script_build C:\Users\wumb0\.cargo\registry\src\garden.eu.org-1ecc6299db9ec823\winapi-0.3.9\build.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debug-assertions=off -C metadata=aa02bc61400bc7b8 -C extra-filename=-aa02bc61400bc7b8 --out-dir C:\Users\wumb0\Documents\nosse\target\release\build\winapi-aa02bc61400bc7b8 -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps --cap-lints allow -C target-feature=-sse`
     Running `rustc --crate-name build_script_build --edition=2018 proc-macro2\build.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debug-assertions=off --cfg "feature=\"default\"" --cfg "feature=\"proc-macro\"" -C metadata=9cd313b41f79eba7 -C extra-filename=-9cd313b41f79eba7 --out-dir C:\Users\wumb0\Documents\nosse\target\release\build\proc-macro2-9cd313b41f79eba7 -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps -C target-feature=-sse`
     Running `rustc --crate-name unicode_xid C:\Users\wumb0\.cargo\registry\src\garden.eu.org-1ecc6299db9ec823\unicode-xid-0.2.1\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debug-assertions=off --cfg "feature=\"default\"" -C metadata=491c09ef3a3cb1eb -C extra-filename=-491c09ef3a3cb1eb --out-dir C:\Users\wumb0\Documents\nosse\target\release\deps -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps --cap-lints allow -C target-feature=-sse`
     Running `C:\Users\wumb0\Documents\nosse\target\release\build\winapi-aa02bc61400bc7b8\build-script-build`
     Running `C:\Users\wumb0\Documents\nosse\target\release\build\proc-macro2-9cd313b41f79eba7\build-script-build`
     Running `rustc --crate-name winapi C:\Users\wumb0\.cargo\registry\src\garden.eu.org-1ecc6299db9ec823\winapi-0.3.9\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debug-assertions=off -C metadata=6bc6f9c49ceb85d2 -C extra-filename=-6bc6f9c49ceb85d2 --out-dir C:\Users\wumb0\Documents\nosse\target\release\deps -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps --cap-lints allow -C target-feature=-sse`
     Running `rustc --crate-name proc_macro2 --edition=2018 proc-macro2\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debug-assertions=off --cfg "feature=\"default\"" --cfg "feature=\"proc-macro\"" -C metadata=9e27134fd2d02e77 -C extra-filename=-9e27134fd2d02e77 --out-dir C:\Users\wumb0\Documents\nosse\target\release\deps -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps --extern unicode_xid=C:\Users\wumb0\Documents\nosse\target\release\deps\libunicode_xid-491c09ef3a3cb1eb.rmeta -C target-feature=-sse`
LLVM ERROR: SSE register return with SSE disabled
error: could not compile `proc-macro2`

Caused by:
  process didn't exit successfully: `rustc --crate-name proc_macro2 --edition=2018 proc-macro2\src\lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C 
embed-bitcode=no -C debug-assertions=off --cfg "feature=\"default\"" --cfg "feature=\"proc-macro\"" -C metadata=9e27134fd2d02e77 -C extra-filename=-9e27134fd2d02e77 --out-dir C:\Users\wumb0\Documents\nosse\target\release\deps -L dependency=C:\Users\wumb0\Documents\nosse\target\release\deps --extern unicode_xid=C:\Users\wumb0\Documents\nosse\target\release\deps\libunicode_xid-491c09ef3a3cb1eb.rmeta -C target-feature=-sse` (exit code: 101)

I can get the behavior to go away if I explicitly set -Z allow-features in RUSTFLAGS only to the features I need.
It does not work if I set allow-features in the cargo config, which is weird. Because of this I thought it was something to do with proc_macro_span since feature_allowed in build.rs only checks RUSTFLAGS and is only used to see whether to enable proc_macro_span:

    if version.nightly && feature_allowed("proc_macro_span") {
        println!("cargo:rustc-cfg=proc_macro_span");
    }

I have tried cloning the repo and manually commenting out things in build.rs but it still throws the error, so I am stumped at this point.

With nightly rust versions:

> cargo --version
cargo 1.46.0 (149022b1d 2020-07-17)
> rustc --version
rustc 1.46.0 (04488afe3 2020-08-24)

It compiles fine on stable.

[dtolnay]

I am closing because this doesn't seem like something where a fix in proc-macro2 would be appropriate. Possibly a fix in llvm or rustc; but you would need to provide a minimal repro of your use case to one of those projects.

rust-lang/rust#66644 (comment) seems applicable:

It is possible that LLVM simply does not handle this, since you're building for x86-64, which always has SSE available.

[wumb0]

Here is a minimal repro: https://github.com/wumb0/proc-macro2-nosse
run it with cargo build on toolchain x86_64-pc-windows-msvc and it should fail.
run .\setenv.ps1 to set RUSTFLAGS with a bogus -Z allow-features setting.
run cargo build again. it should build properly.

my bad on the test case, it was overriding RUSTFLAGS in .cargo/config, removing -C target-feature=-sse
I just wonder why it only happens on this package hmm.
I don't disagree that this is probably not a proc-macro2 issue, but I'm trying to find a workaround here regardless.

[wumb0]

the kicker here is I'd love to compile it with SSE. I really don't need to disable it for the proc macro I'm writing, as it runs on the host. but the project I'm using the macro in is a target I'd like to disable SSE on. there just doesn't seem to be a way to compile the proc macro crate with SSE, but the main crate as -SSE.

the use case is fully position independent code. the compiler is generating movaps instructions, which must have 16-byte aligned memory or an exception is thrown. i want it to not generate those instructions!

but again, the proc macro crate runs on the host so should not be subject to the restriction :/

[wumb0]

It seems passing --target makes this issue go away:
rust-lang/cargo#6375 (comment)

Very interesting. Resolved, but good to document.