[DebugInfo] When referencing structured bindings use the reference's location, not the binding's declaration's location (#153637)

For structured bindings that use custom `get` specializations, the
resulting LLVM IR ascribes the load of the result of `get` to the
binding's declaration, rather than the place where the binding is
referenced - this caused awkward sequencing in the debug info where,
when stepping through the code you'd step back to the binding
declaration every time there was a reference to the binding.

To fix that - when we cross into IRGening a binding - suppress the debug
info location of that subexpression.

I don't represent this as a great bit of API design - certainly open to
ideas, but putting it out here as a place to start.

It's /possible/ this is an incomplete fix, even - if the binding decl
had other subexpressions, those would still get their location applied &
it'd likely be wrong.

So maybe that's a direction to go with to productionize this - add a new
location scoped device that suppresses any overriding - this might be
more robust. How do people feel about that?

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 844b445b98c1..26fba751e6f9 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -3387,7 +3387,14 @@ LValue CodeGenFunction::EmitDeclRefLValue(const DeclRefExpr *E) {
       auto *FD = LambdaCaptureFields.lookup(BD);
       return EmitCapturedFieldLValue(*this, FD, CXXABIThisValue);
     }
-    return EmitLValue(BD->getBinding());
+    // Suppress debug location updates when visiting the binding, since the
+    // binding may emit instructions that would otherwise be associated with the
+    // binding itself, rather than the expression referencing the binding. (this
+    // leads to jumpy debug stepping behavior where the location/debugger jump
+    // back to the binding declaration, then back to the expression referencing
+    // the binding)
+    DisableDebugLocationUpdates D(*this);
+    return EmitLValue(BD->getBinding(), NotKnownNonNull);
   }
 
   // We can form DeclRefExprs naming GUID declarations when reconstituting