[Sema] Stop visiting existential exprs an extra time checking for uses (swiftlang#27150)

This was causing an exponential amount of time traversing the AST with
deeply chained protocol extension methods, such as in the
TestCodableRouter.swift test in Kitura.

If the OpaqueValueExpr is referenced more than once within the
OpenExistentialExpr it'll still get visited more than once, but that
doesn't seem to happen in practice. If it turns out to be a problem,
we can weaken the assertion I'm adding here.

https://bugs.swift.org/browse/SR-11012

Author: jrose-apple

lib/Sema/MiscDiagnostics.cpp

@@ -22,13 +22,16 @@
 #include "swift/AST/Pattern.h"
 #include "swift/Basic/Defer.h"
 #include "swift/Basic/SourceManager.h"
+#include "swift/Basic/Statistic.h"
 #include "swift/Basic/StringExtras.h"
 #include "swift/Parse/Lexer.h"
 #include "swift/Parse/Parser.h"
 #include "swift/Sema/IDETypeChecking.h"
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/SaveAndRestore.h"
+
+#define DEBUG_TYPE "Sema"
 using namespace swift;
 
 /// Return true if this expression is an implicit promotion from T to T?.
@@ -1992,6 +1995,10 @@ class VarDeclUsageChecker : public ASTWalker {
   /// This is a mapping from VarDecls to the if/while/guard statement that they
   /// occur in, when they are in a pattern in a StmtCondition.
   llvm::SmallDenseMap<VarDecl*, LabeledConditionalStmt*> StmtConditionForVD;
+
+#ifndef NDEBUG
+  llvm::SmallPtrSet<Expr*, 32> AllExprsSeen;
+#endif
 
   bool sawError = false;
 
@@ -2732,13 +2739,19 @@ void VarDeclUsageChecker::markStoredOrInOutExpr(Expr *E, unsigned Flags) {
 
 /// The heavy lifting happens when visiting expressions.
 std::pair<bool, Expr *> VarDeclUsageChecker::walkToExprPre(Expr *E) {
+  STATISTIC(VarDeclUsageCheckerExprVisits,
+            "# of times VarDeclUsageChecker::walkToExprPre is called");
+  ++VarDeclUsageCheckerExprVisits;
+
   // Sema leaves some subexpressions null, which seems really unfortunate.  It
   // should replace them with ErrorExpr.
   if (E == nullptr || !E->getType() || E->getType()->hasError()) {
     sawError = true;
     return { false, E };
   }
 
+  assert(AllExprsSeen.insert(E).second && "duplicate traversal");
+
   // If this is a DeclRefExpr found in a random place, it is a load of the
   // vardecl.
   if (auto *DRE = dyn_cast<DeclRefExpr>(E)) {
@@ -2783,9 +2796,13 @@ std::pair<bool, Expr *> VarDeclUsageChecker::walkToExprPre(Expr *E) {
     return { false, E };
   }
 
-  // If we see an OpenExistentialExpr, remember the mapping for its OpaqueValue.
-  if (auto *oee = dyn_cast<OpenExistentialExpr>(E))
+  // If we see an OpenExistentialExpr, remember the mapping for its OpaqueValue
+  // and only walk the subexpr.
+  if (auto *oee = dyn_cast<OpenExistentialExpr>(E)) {
     OpaqueValueMap[oee->getOpaqueValue()] = oee->getExistentialValue();
+    oee->getSubExpr()->walk(*this);
+    return { false, E };
+  }
 
   // Visit bindings.
   if (auto ove = dyn_cast<OpaqueValueExpr>(E)) {

validation-test/compiler_scale/var_decl_usage_checker.swift.gyb

@@ -0,0 +1,17 @@
+// RUN: %scale-test --begin 1 --end 5 --step 1 --select VarDeclUsageCheckerExprVisits %s
+// REQUIRES: OS=macosx
+// REQUIRES: asserts
+
+protocol Proto {}
+extension Proto {
+  var selfish: Proto { return self }
+}
+
+struct X: Proto {}
+
+func test(_ x: X) -> Proto {
+  return x
+%for i in range(0, N*5):
+    .selfish
+%end
+}