mir: prohibit projection into scalable vec - #160642

davidtwco · 2026-08-06T14:15:30Z

Fixes #160580. Preventing projections into scalable vectors is an oversight from the initial implementation and something we should fix.

I'm surprised it caused a stdarch CI failure as reported by #160580, as nothing in rustc or stdarch seems to have changed that would have caused that to start happening as far as I can tell. This likely won't fix that stdarch CI failure if it keeps happening, because if there is a projection coming from somewhere then that needs to be fixed - nevertheless, preventing them as in this patch is the right thing to do.

I've tested this against the stdarch CI locally.

rustbot · 2026-08-06T14:15:34Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

rustbot · 2026-08-06T14:15:37Z

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davidtwco · 2026-08-06T14:15:40Z

r? @folkertdev

oli-obk · 2026-08-06T15:27:54Z

+//@ compile-flags: --target aarch64-unknown-linux-gnu -Copt-level=0
+//@ check-fail
+//@ needs-llvm-components: aarch64
+//@ only-aarch64


is the --target and needs-llvm-components necessary when it's already restricted to aarch64?

View changes since the review

going in the other direction, I think this can be a minicore test?

is the --target and needs-llvm-components necessary when it's already restricted to aarch64?

are you suggesting to remove both? (if you have a test with a --target, tidy will require needs-llvm-components)

But combining //@ only-aarch64 with compile-flags: --target aarch64-unknown-linux-gnu doesn't really make sense I think?

Anyhow, this should be a minicore test so it run everywhere.

Removed --target and needs-llvm-components, added minicore

Oh, sorry, I meant to keep --target and needs-llvm-components in combination with minicore, so that this test runs even on my x86_64 machine. The logic here isn't os-specific, so defaulting to aarch64-unknown-linux-gnu should be fine. It's the only-aarch64 that should go.

workingjubilee · 2026-08-06T19:08:33Z

cc @scottmcm Since you worked on prohibiting similar projections in the past.

davidtwco · 2026-08-07T09:00:32Z

cc @scottmcm Since you worked on prohibiting similar projections in the past.

I looked at #143833 while doing this - the rustc_mir_transform/src/validate.rs change is the same, not strictly necessary in my patch but gives a more deliberate ICE than the previous one as a fallback, but I didn't think the rustc_codegen_ssa/src/mir/operand.rs change was relevant here. ~~I went further than that patch with the rustc_hir_typeck change to give a nicer error than the MIR validation one.~~

scottmcm · 2026-08-07T09:03:36Z


+#[derive(Diagnostic)]
+#[diag("cannot project into scalable vector type `{$ty}`")]
+pub(crate) struct ScalableVectorFieldAccess<'tcx> {


Are you expecting scalable vectors to be something that people can mess up like this outside of the library? We intentionally just ICE under rust-lang/compiler-team#620 for the misuses of repr(simd).

(The tricky cases were things that didn't have a great place in the code to point at.)

View changes since the review

Ah, that makes sense, I hadn't considered that the visibility errors would prevent field access normally, so this was really only for direct uses of rustc_scalable_vector, so the MIR validator error is sufficient - removed the typeck error.

scottmcm · 2026-08-07T09:04:49Z

                        }

-                        if adt_def.repr().simd() {
+                        if adt_def.repr().simd() || adt_def.repr().scalable() {


Would it worth having a negative test that mir-opt SRoA doesn't try to explode one of these, or similar?

View changes since the review

I've added one now

folkertdev

Thanks, r=me when CI is green.

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folkertdev · 2026-08-07T13:52:13Z

Hmm i've been running into this too. Tidy already lints on tests having --target without an llvm component, maybe it can also lint on whether the target is available with gcc and error when it's not and suggest to ignore gcc.

folkertdev · 2026-08-07T20:42:55Z

@bors r+

rust-bors · 2026-08-07T20:42:59Z

📌 Commit 0e878d6 has been approved by folkertdev

It is now in the queue for this repository.

…folkertdev mir: prohibit projection into scalable vec Fixes rust-lang#160580. Preventing projections into scalable vectors is an oversight from the initial implementation and something we should fix. I'm surprised it caused a stdarch CI failure as reported by rust-lang#160580, as nothing in rustc or stdarch seems to have changed that would have caused that to start happening as far as I can tell. This likely won't fix that stdarch CI failure if it keeps happening, because if there is a projection coming from somewhere then that needs to be fixed - nevertheless, preventing them as in this patch is the right thing to do. I've tested this against the stdarch CI locally.

Rollup of 5 pull requests Successful merges: - #160634 (miri subtree update) - #160642 (mir: prohibit projection into scalable vec) - #160303 (rustc_parse: A few cleanups to expression parsing next to attributes) - #160429 (tidy: Update Python version requirements to 3.10) - #160543 (Default `RawOsError` to `i16` for 16-bit targets)

jhpratt · 2026-08-08T07:09:04Z

+extern crate minicore;
+
+#[rustc_scalable_vector(4)]
+pub struct Sv(f32);


This needs to silence a lint.

@bors r- #160735 (comment)

---- [ui] tests/ui/scalable-vectors/project-into-field-extern.rs stdout ---- error: auxiliary build of /Users/runner/work/rust/rust/tests/ui/scalable-vectors/auxiliary/simple.rs failed to compile: status: exit status: 1 command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/stage2/bin/rustc" "/Users/runner/work/rust/rust/tests/ui/scalable-vectors/auxiliary/simple.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/Users/runner/.cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/Users/runner/work/rust/rust/vendor" "--sysroot" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/stage2" "--target=aarch64-apple-darwin" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "--out-dir" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/Users/runner/work/rust/rust/build/aarch64-apple-darwin/native/rust-test-helpers" "-Cpanic=abort" "-Cforce-unwind-tables=yes" "-Copt-level=0" "--crate-type" "dylib" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary" "--extern" "minicore=/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/libminicore.rlib" stdout: none --- stderr ------------------------------- warning: field `0` is never read ##[warning] --> /Users/runner/work/rust/rust/tests/ui/scalable-vectors/auxiliary/simple.rs:13:15 | LL | pub struct Sv(f32); | -- ^^^ | | | field in this struct | = help: consider removing this field = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default error: linking with `cc` failed: exit status: 1 | = note: "cc" "-Wl,-exported_symbols_list" "-Wl,/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary/rustcnDOBCi/list" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary/rustcnDOBCi/symbols.o" "<1 object files omitted>" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary/rustcnDOBCi/rmeta.o" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/libminicore.rlib" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/native/rust-test-helpers" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary" "-o" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/scalable-vectors/project-into-field-extern/auxiliary/libsimple.dylib" "-Wl,-dead_strip" "-dynamiclib" "-Wl,-install_name" "-Wl,@rpath/libsimple.dylib" "-nodefaultlibs" = note: some arguments are omitted. use `--verbose` to show all linker arguments = note: Undefined symbols for architecture arm64: "dyld_stub_binder", referenced from: <initial-undefines> ld: symbol(s) not found for architecture arm64 clang: error: linker command failed with exit code 1 (use -v to see invocation) error: aborting due to 1 previous error; 1 warning emitted ------------------------------------------

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rust-bors · 2026-08-08T07:09:11Z

This pull request was unapproved.

This PR was contained in a rollup (#160735), which was unapproved.

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rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 6, 2026

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davidtwco mentioned this pull request Aug 6, 2026

Tracking Issue for Scalable Vectors #145052

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oli-obk reviewed Aug 6, 2026

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Comment thread library/stdarch/crates/core_arch/src/aarch64/sve/mod.rs

Comment thread compiler/rustc_hir_typeck/src/expr.rs Outdated

davidtwco force-pushed the sve-field-projection branch from 303387d to 168f22f Compare August 7, 2026 08:55

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davidtwco force-pushed the sve-field-projection branch from 168f22f to c1f9520 Compare August 7, 2026 11:54

davidtwco changed the title ~~hir_typeck: prohibit projection into scalable vec~~ mir: prohibit projection into scalable vec Aug 7, 2026

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mir: prohibit projection into scalable vec

0e878d6

davidtwco force-pushed the sve-field-projection branch from 8574222 to 0e878d6 Compare August 7, 2026 14:59

rust-bors Bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Aug 7, 2026

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Rollup of 5 pull requests #160735

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