[CS] Add custom diagnostic for missing result builder element

If we have a single missing argument for an empty
`buildBlock` call, emit a custom diagnostic.

Author: hamishknight

include/swift/AST/DiagnosticsSema.def

@@ -1605,6 +1605,11 @@ NOTE(candidate_with_extraneous_args,none,
      "candidate %0 has %1 parameter%s1, but context %2 has %3",
      (Type, unsigned, Type, unsigned))
 
+ERROR(result_builder_missing_element,none,
+      "expected expression in result builder %0", (DeclName))
+ERROR(result_builder_missing_element_of_type,none,
+      "expected expression of type %0 in result builder %1", (Type, DeclName))
+
 ERROR(no_accessible_initializers,none,
       "%0 cannot be constructed because it has no accessible initializers",
       (Type))

lib/Sema/CSDiagnostics.cpp

@@ -177,6 +177,21 @@ FailureDiagnostic::getArgumentListFor(ConstraintLocator *locator) const {
   return S.getArgumentList(locator);
 }
 
+StringRef FailureDiagnostic::getEditorPlaceholder(
+    StringRef description, Type ty,
+    llvm::SmallVectorImpl<char> &scratch) const {
+  llvm::raw_svector_ostream OS(scratch);
+  OS << "<#";
+  if (!ty || ty->is<UnresolvedType>()) {
+    OS << description;
+  } else {
+    OS << "T##";
+    ty.print(OS);
+  }
+  OS << "#>";
+  return StringRef(scratch.data(), scratch.size());
+}
+
 Expr *FailureDiagnostic::getBaseExprFor(const Expr *anchor) const {
   if (!anchor)
     return nullptr;
@@ -5178,6 +5193,9 @@ bool MissingArgumentsFailure::diagnoseAsError() {
     return true;
   }
 
+  if (diagnoseMissingResultBuilderElement())
+    return true;
+
   if (diagnoseInvalidTupleDestructuring())
     return true;
 
@@ -5515,6 +5533,55 @@ bool MissingArgumentsFailure::diagnoseClosure(const ClosureExpr *closure) {
   return true;
 }
 
+bool MissingArgumentsFailure::diagnoseMissingResultBuilderElement() const {
+  auto &ctx = getASTContext();
+
+  // Only handle a single missing argument in an empty builder for now. This
+  // should be the most common case though since most builders support N >= 1
+  // elements.
+  if (SynthesizedArgs.size() != 1)
+    return false;
+
+  auto *call = getAsExpr<CallExpr>(getRawAnchor());
+  if (!call || !call->isImplicit() || !call->getArgs()->empty())
+    return false;
+
+  auto *UDE = dyn_cast<UnresolvedDotExpr>(call->getFn());
+  if (!UDE || !UDE->isImplicit())
+    return false;
+
+  auto overload = getCalleeOverloadChoiceIfAvailable(getLocator());
+  if (!overload)
+    return false;
+
+  auto *decl = overload->choice.getDeclOrNull();
+  if (!decl || decl->getBaseName() != ctx.Id_buildBlock)
+    return false;
+
+  auto resultBuilder =
+      getType(UDE->getBase())->getMetatypeInstanceType()->getAnyNominal();
+  if (!resultBuilder)
+    return false;
+
+  auto paramType = resolveType(SynthesizedArgs.front().param.getPlainType());
+
+  SmallString<64> scratch;
+  auto fixIt = getEditorPlaceholder("result", paramType, scratch);
+  auto fixItLoc = call->getStartLoc();
+
+  if (paramType->is<UnresolvedType>()) {
+    emitDiagnostic(diag::result_builder_missing_element,
+                   resultBuilder->getName())
+        .fixItInsertAfter(fixItLoc, fixIt);
+  } else {
+    emitDiagnostic(diag::result_builder_missing_element_of_type, paramType,
+                   resultBuilder->getName())
+        .fixItInsertAfter(fixItLoc, fixIt);
+  }
+  emitDiagnosticAt(decl, diag::decl_declared_here, decl);
+  return true;
+}
+
 bool MissingArgumentsFailure::diagnoseInvalidTupleDestructuring() const {
   auto *locator = getLocator();
   if (!locator->isLastElement<LocatorPathElt::ApplyArgument>())

lib/Sema/CSDiagnostics.h

@@ -203,6 +203,11 @@ class FailureDiagnostic {
           [](GenericTypeParamType *, Type) {});
 
   bool conformsToKnownProtocol(Type type, KnownProtocolKind protocol) const;
+
+  /// Retrieve an editor placeholder with a given description, or a given
+  /// type if specified.
+  StringRef getEditorPlaceholder(StringRef description, Type ty,
+                                 llvm::SmallVectorImpl<char> &scratch) const;
 };
 
 /// Base class for all of the diagnostics related to generic requirement
@@ -1505,6 +1510,9 @@ class MissingArgumentsFailure final : public FailureDiagnostic {
   /// let's produce tailored diagnostics.
   bool diagnoseClosure(const ClosureExpr *closure);
 
+  /// Diagnose a single missing argument to a buildBlock call.
+  bool diagnoseMissingResultBuilderElement() const;
+
   /// Diagnose cases when instead of multiple distinct arguments
   /// call got a single tuple argument with expected arity/types.
   bool diagnoseInvalidTupleDestructuring() const;

test/Constraints/result_builder_diags.swift

@@ -1013,3 +1013,19 @@ func test_partially_resolved_closure_params() {
     42
   }
 }
+
+func testMissingElementInEmptyBuilder() {
+  @resultBuilder
+  struct SingleElementBuilder {
+    static func buildBlock<T>(_ x: T) -> T { x }
+    // expected-note@-1 2{{'buildBlock' declared here}}
+  }
+
+  func test1(@SingleElementBuilder fn: () -> Int) {}
+  test1 {}
+  // expected-error@-1 {{expected expression of type 'Int' in result builder 'SingleElementBuilder'}} {{10-10=<#T##Int#>}}
+
+  @SingleElementBuilder
+  func test2() -> Int {}
+  // expected-error@-1 {{expected expression of type 'Int' in result builder 'SingleElementBuilder'}} {{24-24=<#T##Int#>}}
+}

validation-test/Sema/SwiftUI/rdar88256059.swift

@@ -9,7 +9,7 @@ struct MyView: View {
 
   var body: some View {
     Table(self.data) {
-      // expected-error@-1 {{missing argument for parameter #1 in call}}
+      // expected-error@-1 {{expected expression in result builder 'TableColumnBuilder'}} {{23-23=<#result#>}}
       // expected-error@-2 {{cannot infer return type of empty closure}} {{23-23=<#result#>}}
     }
   }