unsafe(naked) functions might not be called via a thunk

[thejpster]

• Using nightly 2026-01-30
• And the minimal reproducer at https://github.com/thejpster/aarch32-hell
• And a target that supports two modes of operation but needs either a thunk to switch modes or a special mode switching branch instruction
• And with a naked function given an instruction_set attribute to compile it in the non-default mode
• calling the naked function does not insert the thunk or the special instruction

Discussion at https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/naked.20functions.20broken.20-.20missing.20thunk/with/571236055

Cc: @folkertdev

[folkertdev]

The real problem is that naked functions are emitted as hidden since #148339 (rather than e.g. dso_local or internal). Hidden symbols are not picked up for the thunk generation.

I'm not sure that makes sense (e.g. perhaps it's a bug that hidden functions are not considered), but that is what I'm observing. We changed the visibility for LTO reasons, but we should probably (try to) mirror normal extern "C" { ... } definitions more closely.

[folkertdev]

I filed llvm/llvm-project#180358, I believe what we're running into is the arm backend not handling hidden functions well.

cc @Jamesbarford this should go on the arm maintainers board I think.