Emit debug info for catch let errorvar error variables.

This implementation adds the debug info emission to
SILGenFunction::emitTemporaryAllocation() which may not be the optimal place to
do this. It may be better to change SILGen to unconditionally emit an
alloc_stack instead of relying on a temporary alloca to be requested.

rdar://75499821

lib/SILGen/SILGenExpr.cpp

@@ -30,6 +30,7 @@
 #include "swift/AST/CanTypeVisitor.h"
 #include "swift/AST/Decl.h"
 #include "swift/AST/DiagnosticsCommon.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Expr.h"
 #include "swift/AST/ForeignErrorConvention.h"
 #include "swift/AST/GenericEnvironment.h"
@@ -1031,6 +1032,16 @@ SILValue SILGenFunction::emitTemporaryAllocation(SILLocation loc, SILType ty,
   Optional<SILDebugVariable> DbgVar;
   if (auto *VD = loc.getAsASTNode<VarDecl>())
     DbgVar = SILDebugVariable(VD->isLet(), 0);
+  // Recognize "catch let errorvar" bindings.
+  if (auto *DRE = loc.getAsASTNode<DeclRefExpr>())
+    if (auto *VD = dyn_cast<VarDecl>(DRE->getDecl()))
+      if (!isa<ParamDecl>(VD) && VD->isImplicit() &&
+          (VD->getType()->is<ProtocolType>() ||
+           VD->getType()->is<ProtocolCompositionType>()) &&
+          VD->getType()->getExistentialLayout().isErrorExistential()) {
+        DbgVar = SILDebugVariable(VD->isLet(), 0);
+        loc = SILLocation(VD);
+      }
   auto alloc = B.createAllocStack(loc, ty, DbgVar, hasDynamicLifetime);
   enterDeallocStackCleanup(alloc);
   return alloc;

test/DebugInfo/catch_let.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend %s -emit-ir -g -o - | %FileCheck %s
+enum MyError : Error {
+  case Yikes
+}
+
+func throwing() throws -> () {
+  throw MyError.Yikes
+}
+
+func use<T>(_ t: T) {}
+
+public func foo() {
+  do {
+    try throwing()
+  }
+  catch let error {
+    // CHECK: call void @llvm.dbg.declare(metadata %swift.error** %{{.*}}, metadata ![[ERROR:[0-9]+]],
+    // CHECK: ![[ERROR]] = !DILocalVariable(name: "error"
+    use(error)
+  }
+}
+foo();