addressing final feedback

Author: ZuseZ4

Diff for: compiler/rustc_builtin_macros/src/autodiff.rs

@@ -591,6 +591,7 @@ mod llvm_enzyme {
         if x.mode.is_fwd() {
             // Fwd mode is easy. If the return activity is Const, we support arbitrary types.
             // Otherwise, we only support a scalar, a pair of scalars, or an array of scalars.
+            // We checked that (on a best-effort base) in the preceding gen_enzyme_decl function.
             // In all three cases, we can return `std::hint::black_box(<T>::default())`.
             if x.ret_activity == DiffActivity::Const {
                 // Here we call the primal function, since our dummy function has the same return
@@ -625,15 +626,16 @@ mod llvm_enzyme {
                         exprs = ecx.expr_tuple(new_decl_span, exprs2);
                     }
                     _ => {
+                        // Interestingly, even the `-> ArbitraryType` case
+                        // ends up getting matched and handled correctly above,
+                        // so we don't have to handle any other case for now.
                         panic!("Unsupported return type: {:?}", d_ret_ty);
                     }
                 }
             }
             exprs = ecx.expr_call(new_decl_span, bb_call_expr, thin_vec![exprs]);
         } else {
-            // The return type of our dummy function is the same as the return type of our primal.
-            // In that case, we can just use the primal call as last instruction.
-            panic!("Unsupported mode: {:?}", x.mode);
+            unreachable!("Unsupported mode: {:?}", x.mode);
         }
 
         body.stmts.push(ecx.stmt_expr(exprs));

Diff for: tests/codegen/autodiffv.rs

@@ -1,6 +1,13 @@
 //@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
 //@ no-prefer-dynamic
 //@ needs-enzyme
+//
+// In Enzyme, we test against a large range of LLVM versions (5+) and don't have overly many
+// breakages. One benefit is that we match the IR generated by Enzyme only after running it
+// through LLVM's O3 pipeline, which will remove most of the noise.
+// However, our integration test could also be affected by changes in how rustc lowers MIR into
+// LLVM-IR, which could cause additional noise and thus breakages. If that's the case, we should
+// reduce this test to only match the first lines and the ret instructions.
 
 #![feature(autodiff)]
 

Diff for: tests/ui/autodiff/autodiff_illegal.rs

@@ -177,4 +177,11 @@ fn f21(x: f32) -> f32 {
     unimplemented!()
 }
 
+struct DoesNotImplDefault;
+#[autodiff(df22, Forward, Dual)]
+pub fn f22() -> DoesNotImplDefault {
+    //~^^ ERROR the function or associated item `default` exists for tuple `(DoesNotImplDefault, DoesNotImplDefault)`, but its trait bounds were not satisfied
+    unimplemented!()
+}
+
 fn main() {}

Diff for: tests/ui/autodiff/autodiff_illegal.stderr

@@ -151,7 +151,24 @@ error[E0433]: failed to resolve: use of undeclared type `F64Trans`
 LL | #[autodiff(df18, Reverse, Active, Active)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ use of undeclared type `F64Trans`
 
-error: aborting due to 22 previous errors
+error[E0599]: the function or associated item `default` exists for tuple `(DoesNotImplDefault, DoesNotImplDefault)`, but its trait bounds were not satisfied
+  --> $DIR/autodiff_illegal.rs:181:1
+   |
+LL | struct DoesNotImplDefault;
+   | ------------------------- doesn't satisfy `DoesNotImplDefault: Default`
+LL | #[autodiff(df22, Forward, Dual)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ function or associated item cannot be called on `(DoesNotImplDefault, DoesNotImplDefault)` due to unsatisfied trait bounds
+   |
+   = note: the following trait bounds were not satisfied:
+           `DoesNotImplDefault: Default`
+           which is required by `(DoesNotImplDefault, DoesNotImplDefault): Default`
+help: consider annotating `DoesNotImplDefault` with `#[derive(Default)]`
+   |
+LL + #[derive(Default)]
+LL | struct DoesNotImplDefault;
+   |
+
+error: aborting due to 23 previous errors
 
-Some errors have detailed explanations: E0428, E0433, E0658.
+Some errors have detailed explanations: E0428, E0433, E0599, E0658.
 For more information about an error, try `rustc --explain E0428`.