Auto merge of #142544 - Sa4dUs:prevent-abi-changes, r=ZuseZ4

Prevent ABI changes affect EnzymeAD

This PR handles ABI changes for autodiff input arguments to improve Enzyme compatibility. Fundamentally this adjusts activities when a function argument is lowered as an `ScalarPair`, so there's no mismatch between diff activities and args. Also removes activities corresponding to ZSTs.

fixes: https://github.com/rust-lang/rust/issues/144025

r? `@ZuseZ4`

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/ui/autodiff/zst.rs b/tests/ui/autodiff/zst.rs
new file mode 100644
index 00000000000..7b9b5f5f20b
--- /dev/null
+++ b/tests/ui/autodiff/zst.rs
@@ -0,0 +1,17 @@
+//@ compile-flags: -Zautodiff=Enable -C opt-level=3 -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+//@ build-pass
+
+// Check that differentiating functions with ZST args does not break
+
+#![feature(autodiff)]
+
+#[core::autodiff::autodiff_forward(fd_inner, Const, Dual)]
+fn f(_zst: (), _x: &mut f64) {}
+
+fn fd(x: &mut f64, xd: &mut f64) {
+    fd_inner((), x, xd);
+}
+
+fn main() {}