Prevent ICE at codegen_attrs.rs in autodiff validation by emitting nice error message

[Sa4dUs]

This PR replaces the following ICE error with a more user-friendly error message.

error: internal compiler error:
compiler/rustc_codegen_ssa/src/codegen_attrs.rs:935:13:
Invalid input activity Dual for Reverse mode

The issue was occurring specifically in the test file tests/ui/autodiff/autodiff_illegal.rs, affecting functions f5 and f6.

[rustbot]

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[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa/src/codegen_attrs.rs

cc @jdonszelmann

[compiler-errors]

This needs a test. Also, there's no issue linked -- does this actually fix a real issue found by someone? You mention f5 and f6 from tests/ui/autodiff/autodiff_illegal.rs, but these are not ICEing on master, so can you explain more about how you encountered an ICE?

[Sa4dUs]

The errors were found by following the build guide from Enzyme's Rust installation page, running:

git clone --depth=1 [email protected]:rust-lang/rust.git
cd rust
./configure --enable-llvm-link-shared --enable-llvm-plugins --enable-llvm-enzyme --release-channel=nightly --enable-llvm-assertions --enable-clang --enable-lld --enable-option-checking --enable-ninja --disable-docs

./x.py build --stage 1 library

rustup toolchain link enzyme build/host/stage1
rustup toolchain install nightly # enables -Z unstable-options

And after that, testing that specific file using:

./x test tests/ui/autodiff/autodiff_illegal.rs

As long as I know, there is no issue related to this problem. I started looking into it because it was mentioned in a Zulip chat, specifically in this conversation. Since I did this, there doesn't seem to be any PR fixing it, so the error should still be there.