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``

2 files changed, 25 insertions, 1 deletions

diff --git a/compiler/rustc_builtin_macros/src/autodiff.rs b/compiler/rustc_builtin_macros/src/autodiff.rs
index 7b5c4a159b0..cd7d79862c6 100644
--- a/compiler/rustc_builtin_macros/src/autodiff.rs
+++ b/compiler/rustc_builtin_macros/src/autodiff.rs
@@ -8,7 +8,8 @@ mod llvm_enzyme {
     use std::string::String;
 
     use rustc_ast::expand::autodiff_attrs::{
-        AutoDiffAttrs, DiffActivity, DiffMode, valid_input_activity, valid_ty_for_activity,
+        AutoDiffAttrs, DiffActivity, DiffMode, valid_input_activity, valid_ret_activity,
+        valid_ty_for_activity,
     };
     use rustc_ast::ptr::P;
     use rustc_ast::token::{Token, TokenKind};
@@ -576,6 +577,8 @@ mod llvm_enzyme {
     //
     // Error handling: If the user provides an invalid configuration (incorrect numbers, types, or
     // both), we emit an error and return the original signature. This allows us to continue parsing.
+    // FIXME(Sa4dUs): make individual activities' span available so errors
+    // can point to only the activity instead of the entire attribute
     fn gen_enzyme_decl(
         ecx: &ExtCtxt<'_>,
         sig: &ast::FnSig,
@@ -623,10 +626,22 @@ mod llvm_enzyme {
                 errors = true;
             }
         }
+
+        if has_ret && !valid_ret_activity(x.mode, x.ret_activity) {
+            dcx.emit_err(errors::AutoDiffInvalidRetAct {
+                span,
+                mode: x.mode.to_string(),
+                act: x.ret_activity.to_string(),
+            });
+            // We don't set `errors = true` to avoid annoying type errors relative
+            // to the expanded macro type signature
+        }
+
         if errors {
             // This is not the right signature, but we can continue parsing.
             return (sig.clone(), new_inputs, idents, true);
         }
+
         let unsafe_activities = x
             .input_activity
             .iter()
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]