Fix ICE for repr simd on non struct

[chenyukang]

Fixes #148634

The ICE happened because

rust/compiler/rustc_middle/src/ty/mod.rs

Line 1531 in 995c118

attr::ReprSimd => ReprFlags::IS_SIMD,

will always set IS_SIMD according to get_all_attrs, and since we already report error attribute should be applied to a struct, it's OK to bypass here.

[rustbot]

r? @lcnr

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[Kivooeo]

r=me when ci is green

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

---- [ui] tests/ui/asm/invalid-repr-simd-on-enum-148634.rs stdout ----
Saved the actual stderr to `/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/asm/invalid-repr-simd-on-enum-148634/invalid-repr-simd-on-enum-148634.stderr`
diff of stderr:

1 error[E0517]: attribute should be applied to a struct
-   --> $DIR/invalid-repr-simd-on-enum.rs:5:8
+   --> $DIR/invalid-repr-simd-on-enum-148634.rs:5:8
---
19 error: cannot use value of type `Es` for inline assembly
-   --> $DIR/invalid-repr-simd-on-enum.rs:13:29
+   --> $DIR/invalid-repr-simd-on-enum-148634.rs:13:29
21    |
22 LL |         asm!("{}", out(reg) x);
23    |                             ^

Note: some mismatched output was normalized before being compared
-   --> /checkout/tests/ui/asm/invalid-repr-simd-on-enum-148634.rs:5:8
-   --> /checkout/tests/ui/asm/invalid-repr-simd-on-enum-148634.rs:5:8
---
To only update this specific test, also pass `--test-args asm/invalid-repr-simd-on-enum-148634.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/asm/invalid-repr-simd-on-enum-148634.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--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" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/asm/invalid-repr-simd-on-enum-148634" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0517]: attribute should be applied to a struct
##[error]  --> /checkout/tests/ui/asm/invalid-repr-simd-on-enum-148634.rs:5:8
   |
---

error: cannot use value of type `Es` for inline assembly
##[error]  --> /checkout/tests/ui/asm/invalid-repr-simd-on-enum-148634.rs:13:29
   |
LL |         asm!("{}", out(reg) x);
   |                             ^
   |
   = note: only integers, floats, SIMD vectors, pointers and function pointers can be used as arguments for inline assembly

error: aborting due to 3 previous errors

[lcnr]

hmm, would it be possible to instead filter the ReprOptions so that we don't end up with unsupported ones? It feels brittle to simply handle incorrect info by ignoring it, e.g. it adds an implicit assumption that repr(simd) is only ever used for structs, and if we were to idk, extend support to unions at some point (even if unlikely) this code would suddenly be incorrect

alternatively, accept all Adts, but then match on the kind and skip + span_delay_bug if it's not a struct

[Kivooeo]

it adds an implicit assumption that repr(simd) is only ever used for structs

according to the rfc it seems (at least for me) that this is only for structs https://github.com/rust-lang/rfcs/blob/master/text/1199-simd-infrastructure.md#types

[lcnr]

Yes, but is this a guarantee that we'll never extend it to unions? My point is not that the code is wrong, it's that this code adds an implicit dependency which may result in bugs in the future.

Most bugs in large projects happen by:

• somebody writes code which has an unchecked assumption which holds right now
• somebody else modifies another part of the project, causing this assumption to no longer hold, resulting in a bug in entirely untouched code

This implies that a good way to avoid bugs in large projects it to avoid unchecked assumptions. It's why exhaustive matches are so powerful. It's also why we do stuff like

rust/compiler/rustc_middle/src/ty/context.rs

Lines 628 to 635 in 0bbef55

	// This causes a compiler error if any new float kinds are added.
	let (ty::FloatTy::F16 | ty::FloatTy::F32 | ty::FloatTy::F64 | ty::FloatTy::F128);
	let possible_floats = [
	ty::SimplifiedType::Float(ty::FloatTy::F16),
	ty::SimplifiedType::Float(ty::FloatTy::F32),
	ty::SimplifiedType::Float(ty::FloatTy::F64),
	ty::SimplifiedType::Float(ty::FloatTy::F128),
	];