Sync from rust 2026/09/14 - #977
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Co-authored-by: Jubilee <workingjubilee@gmail.com>
This avoids a delayed bug if compilation is aborted between checking function ABIs and codegening all functions.
Refactor the way cg_ssa handles indirect returns (returns via `sret`) — Take 2 This continues the work that @FractalFir done in rust-lang/rust#144976. **I managed to make this work, so this confirms the original approach works and is ready to merge.** r? workingjubilee (assigning the same reviewer as the original PR) # The problem The current way `cg_ssa` handles `PassMode::Indirect` is fundamentally incompatible with the requirements of the GCC backend. This is currently worked around in a very brittle way, that breaks on ARM. In order to function correctly, `cg_gcc` requires the `sret`(indirect return) pointer to be treated in a special way. # The solution This PR separates the `sret` pointer from all the other arguments, allowing each backend to decide how it wants to handle that pointer. This will allow GCC to do it's own thing, while changing nothing on the LLVM side. Currently, the PR just makes both backends preappend the sret arg, mimicking the previous behaviour. The PR is based on this [zulip suggestion](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Bootstrap.20of.20rustc.20with.20rustc_codegen_gcc/near/526487725). # Assumptions I made. The PR makes certain assumptions about the ABI handling. 1. No LLVM intrinsic returns via sret. [According to my sources](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/Can.20LLVM.20intrinsics.20return.20via.20PassMode.3A.3AIndirect.3F), this is always true. 2. Tail calls don't currently return via `sret`. From [asking about](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/Can.20tail.20calls.20return.20indirectly.28via.20a.20pointer.29.3F/near/532984507), it seems like the tail call handling does not currently support indirect returns. So, I did not add support for that. 3. TLS shims don't perform indirect returns, and asserts and drops also don't perform indirect returns(since they return nothing) - those assumptions seemed reasonable. 4. Inline assembly calls don't use indirect returns. I *assumed* functions like `inline_asm_call` will not need the `sret` handling code. I am not 100% sure about this, tough.
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