Fix build of std for thumbv7a-pc-windows-msvc

[arlosi]

Attempting to build std for the tier-3 target thumbv7a-pc-windows-msvc fails with the following error:

Building stage1 std artifacts (x86_64-pc-windows-msvc -> thumbv7a-pc-windows-msvc)
..
LLVM ERROR: WinEH not implemented for this target
error: could not compile `panic_unwind`

EH (unwinding) is not supported by LLVM for 32 bit arm msvc targets. This changes panic unwind to use the dummy implementation for thumbv7a-pc-windows-msvc.

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[wesleywiser]

Nominating for beta backport to fix a Tier 3 target build error.

[Amanieu]

Why did this work before? What changed?

If unwinding doesn't work on this target, wouldn't it be preferable to force it to use panic=abort instead?

[ChrisDenton]

It does have the abort panic strategy:

rust/compiler/rustc_target/src/spec/thumbv7a_pc_windows_msvc.rs

Line 24 in 63c748e

panic_strategy: PanicStrategy::Abort,

And the following works:

./x.py build library/alloc --host x86_64-pc-windows-msvc --target thumbv7a-pc-windows-msvc --stage 1

But building library/std gives the error in the OP. So I guess building std implies building panic_unwind?

[arlosi]

Bisection shows that it was caused by the LLVM 15 update - #99464.

Maybe LLVM previously generated bad unwinding code (that was unused due to panic=abort), and now LLVM errors instead?

It looked like other targets that didn't have unwinding support used the "dummy" implementation, so that what I did here.

[Amanieu]

LGTM, it seems this is what we do for all targets that don't support unwinding.

@bors r+

@wesleywiser Is a backport useful? Aren't tier-3 targets only usable with -Z build-std which is unstable?

[Amanieu]

@bors r+

[bors]

📌 Commit 7c60236 has been approved by Amanieu

It is now in the queue for this repository.

[wesleywiser]

@Amanieu I figured since we hit this in 1.65, it might be nice to have this resolved in 1.66. It's a Tier 3 target though, so I completely understand if T-libs doesn't want to backport!