Rollup merge of #139465 - EnzymeAD:autodiff-sret, r=oli-obk

add sret handling for scalar autodiff

r? `@oli-obk`

Fixing one of the todo's which I left in my previous batching PR.
This one handles sret for scalar autodiff.  `sret` mostly shows up when we try to return a lot of scalar floats.
People often start testing autodiff which toy functions which just use a few scalars as inputs and outputs, and those were the most likely to be affected by this issue. So this fix should make learning/teaching hopefully a bit easier.

Tracking:

- https://github.com/rust-lang/rust/issues/124509

compiler/rustc_ast/src/expand/autodiff_attrs.rs

@@ -92,6 +92,12 @@ pub struct AutoDiffAttrs {
     pub input_activity: Vec<DiffActivity>,
 }
 
+impl AutoDiffAttrs {
+    pub fn has_primal_ret(&self) -> bool {
+        matches!(self.ret_activity, DiffActivity::Active | DiffActivity::Dual)
+    }
+}
+
 impl DiffMode {
     pub fn is_rev(&self) -> bool {
         matches!(self, DiffMode::Reverse)