Rollup merge of #138231 - Sa4dUs:autodiff-ice, r=ZuseZ4

Prevent ICE in autodiff validation by emitting user-friendly errors

This PR moves `valid_ret_activity` and `valid_input_activity` checks to the macro expansion phase in compiler/rustc_builtin_macros/src/autodiff.rs, replacing the following internal compiler error (ICE):
```
error: internal compiler error:
compiler/rustc_codegen_ssa/src/codegen_attrs.rs:935:13:
Invalid input activity Dual for Reverse mode
```
with a more user-friendly message.

The issue specifically affected the test file `tests/ui/autodiff/autodiff_illegal.rs`, impacting the functions `f5` and `f6`.

The ICE can be reproduced by following [Enzyme's Rustbook](https://enzymead.github.io/rustbook/installation.html) installation guide.

Additionally, this PR adds tests for invalid return activity in `autodiff_illegal.rs`, which previously triggered an unnoticed ICE before these fixes.

r? ``@oli-obk``

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index ab1e0d8ee89..30597944124 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -186,6 +186,15 @@ mod autodiff {
     }
 
     #[derive(Diagnostic)]
+    #[diag(builtin_macros_autodiff_ret_activity)]
+    pub(crate) struct AutoDiffInvalidRetAct {
+        #[primary_span]
+        pub(crate) span: Span,
+        pub(crate) mode: String,
+        pub(crate) act: String,
+    }
+
+    #[derive(Diagnostic)]
     #[diag(builtin_macros_autodiff_mode)]
     pub(crate) struct AutoDiffInvalidMode {
         #[primary_span]