[Diagnostics] Switch to use contextual purpose associated with locator

`ContextualFailure` is the main beneficiary of additional information
associated with `ContextualType` element because it no longer has to
query solution for "purpose" of the contextual information.

Resolves: rdar://68795727

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -2447,8 +2447,9 @@ bool ContextualFailure::diagnoseConversionToNil() const {
 
   Optional<ContextualTypePurpose> CTP;
   // Easy case were failure has been identified as contextual already.
-  if (locator->isLastElement<LocatorPathElt::ContextualType>()) {
-    CTP = getContextualTypePurpose();
+  if (auto contextualTy =
+          locator->getLastElementAs<LocatorPathElt::ContextualType>()) {
+    CTP = contextualTy->getPurpose();
   } else {
     // Here we need to figure out where where `nil` is located.
     // It could be e.g. an argument to a subscript/call, assignment

lib/Sema/CSDiagnostics.h

@@ -594,8 +594,11 @@ class ContextualFailure : public FailureDiagnostic {
                     ConstraintLocator *locator)
       : ContextualFailure(
             solution,
-            solution.getConstraintSystem().getContextualTypePurpose(
-                locator->getAnchor()),
+            locator->isForContextualType()
+                ? locator->castLastElementTo<LocatorPathElt::ContextualType>()
+                      .getPurpose()
+                : solution.getConstraintSystem().getContextualTypePurpose(
+                      locator->getAnchor()),
             lhs, rhs, locator) {}
 
   ContextualFailure(const Solution &solution, ContextualTypePurpose purpose,

lib/Sema/CSFix.cpp

@@ -391,15 +391,16 @@ AllowWrappedValueMismatch *AllowWrappedValueMismatch::create(ConstraintSystem &c
 /// and the contextual type.
 static Optional<std::tuple<ContextualTypePurpose, Type, Type>>
 getStructuralTypeContext(const Solution &solution, ConstraintLocator *locator) {
-  if (locator->findLast<LocatorPathElt::ContextualType>()) {
+  if (auto contextualTypeElt =
+          locator->findLast<LocatorPathElt::ContextualType>()) {
     assert(locator->isLastElement<LocatorPathElt::ContextualType>() ||
            locator->isLastElement<LocatorPathElt::FunctionArgument>());
 
     auto &cs = solution.getConstraintSystem();
     auto anchor = locator->getAnchor();
     auto contextualType = cs.getContextualType(anchor);
     auto exprType = cs.getType(anchor);
-    return std::make_tuple(cs.getContextualTypePurpose(anchor), exprType,
+    return std::make_tuple(contextualTypeElt->getPurpose(), exprType,
                            contextualType);
   } else if (auto argApplyInfo = solution.getFunctionArgApplyInfo(locator)) {
     return std::make_tuple(CTP_CallArgument,

lib/Sema/CSSimplify.cpp

@@ -5754,12 +5754,8 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       }
 
       // Single expression function with implicit `return`.
-      if (elt->is<LocatorPathElt::ContextualType>()) {
-        auto anchor = locator.getAnchor();
-        auto contextualInfo = getContextualTypeInfo(anchor);
-        assert(contextualInfo &&
-               "Found contextual type locator without additional information");
-        if (contextualInfo->purpose == CTP_ReturnSingleExpr) {
+      if (auto contextualType = elt->getAs<LocatorPathElt::ContextualType>()) {
+        if (contextualType->isFor(CTP_ReturnSingleExpr)) {
           increaseScore(SK_FunctionConversion);
           return getTypeMatchSuccess();
         }

validation-test/Sema/SwiftUI/rdar68795727.swift

@@ -0,0 +1,28 @@
+// RUN: %target-typecheck-verify-swift -target %target-cpu-apple-macosx10.15 -swift-version 5
+// REQUIRES: objc_interop
+// REQUIRES: OS=macosx
+
+import SwiftUI
+
+struct UnitVolumeView: View {
+  var measurement: Measurement<Unit>
+
+  var body: some View {
+    Text("")
+  }
+}
+struct UnitView_Previews: PreviewProvider {
+  static var previews: some View {
+    let volume: Measurement<Unit> = Measurement<UnitVolume>(value: 200, unit: UnitVolume.milliliters)
+    // expected-error@-1 {{cannot assign value of type 'Measurement<UnitVolume>' to type 'Measurement<Unit>'}}
+    // expected-note@-2 {{arguments to generic parameter 'UnitType' ('UnitVolume' and 'Unit') are expected to be equal}}
+
+    Group {
+      ForEach(["en", "de", "fr"], id: \.self) { id in
+        UnitVolumeView(measurement: volume)
+          .previewLayout(PreviewLayout.sizeThatFits)
+          .environment(\.locale, .init(identifier: id))
+      }
+    }
+  }
+}