[Diagnostics] Result Builders: Improve diagnostic for unsupported lazy/wrapped/computed vars

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5885,6 +5885,9 @@ NOTE(result_builder_missing_build_limited_availability, none,
 ERROR(result_builder_requires_explicit_var_initialization,none,
       "local variable%select{| '%1'}0 requires explicit initializer to be used with "
       "result builder %2", (bool, StringRef, DeclName))
+ERROR(cannot_declare_computed_var_in_result_builder,none,
+      "cannot declare local %select{lazy|wrapped|computed}0 variable "
+      "in result builder", (unsigned))
 
 //------------------------------------------------------------------------------
 // MARK: Tuple Shuffle Diagnostics

lib/Sema/CSDiagnostics.cpp

@@ -5875,14 +5875,44 @@ void SkipUnhandledConstructInResultBuilderFailure::diagnosePrimary(
 
   if (auto *decl = unhandled.dyn_cast<Decl *>()) {
     if (auto *PB = dyn_cast<PatternBindingDecl>(decl)) {
+      // Tailored diagnostics for computed properties.
+      if (!PB->hasStorage()) {
+      }
+
+      enum class PropertyKind : unsigned { lazy, wrapped, computed };
+
       // Diagnose all of the patterns without explicit initializers.
       bool diagnosed = false;
       for (unsigned i : range(PB->getNumPatternEntries())) {
+        auto *pattern = PB->getPattern(i);
+
+        // Each variable bound by the pattern must be stored.
+        SmallVector<VarDecl *, 8> variables;
+        pattern->collectVariables(variables);
+
+        for (auto *var : variables) {
+          if (var->getImplInfo().isSimpleStored())
+            continue;
+
+          PropertyKind kind;
+          if (var->getAttrs().hasAttribute<LazyAttr>()) {
+            kind = PropertyKind::lazy;
+          } else if (var->hasAttachedPropertyWrapper()) {
+            kind = PropertyKind::wrapped;
+          } else {
+            kind = PropertyKind::computed;
+          }
+
+          emitDiagnosticAt(PB->getLoc(),
+                           diag::cannot_declare_computed_var_in_result_builder,
+                           static_cast<unsigned>(kind));
+
+          diagnosed = true;
+        }
+
         if (PB->isExplicitlyInitialized(i))
           continue;
 
-        auto *pattern = PB->getPattern(i);
-
         StringRef name;
 
         if (auto *TP = dyn_cast<TypedPattern>(pattern)) {

test/Constraints/result_builder_invalid_vars.swift

@@ -10,12 +10,12 @@ struct DummyBuilder { // expected-note 5 {{struct 'DummyBuilder' declared here}}
 func dummy<T>(@DummyBuilder _: () -> T) {}
 
 dummy {
-  var computedVar: Int { return 123 } // expected-error {{closure containing a declaration cannot be used with result builder 'DummyBuilder'}}
+  var computedVar: Int { return 123 } // expected-error {{cannot declare local computed variable in result builder}}
   ()
 }
 
 dummy {
-  lazy var lazyVar: Int = 123 // expected-error {{closure containing a declaration cannot be used with result builder 'DummyBuilder'}}
+  lazy var lazyVar: Int = 123 // expected-error {{cannot declare local lazy variable in result builder}}
   ()
 }
 
@@ -40,7 +40,7 @@ dummy {
 }
 
 dummy {
-  @Wrapper var wrappedVar: Int = 123 // expected-error {{closure containing a declaration cannot be used with result builder 'DummyBuilder'}}
+  @Wrapper var wrappedVar: Int = 123 // expected-error {{cannot declare local wrapped variable in result builder}}
   ()
 }