[Diagnostics] Result Builders: Improve diagnostic for uninitialized variable declarations

Resolves: rdar://81532339

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5882,6 +5882,9 @@ NOTE(result_builder_missing_build_array, none,
 NOTE(result_builder_missing_build_limited_availability, none,
      "add 'buildLimitedAvailability(_:)' to the result "
      "builder %0 to erase type information for less-available types", (Type))
+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))
 
 //------------------------------------------------------------------------------
 // MARK: Tuple Shuffle Diagnostics

lib/Sema/CSDiagnostics.cpp

@@ -5872,6 +5872,38 @@ static bool hasMissingElseInChain(IfStmt *ifStmt) {
 
 void SkipUnhandledConstructInResultBuilderFailure::diagnosePrimary(
     bool asNote) {
+
+  if (auto *decl = unhandled.dyn_cast<Decl *>()) {
+    if (auto *PB = dyn_cast<PatternBindingDecl>(decl)) {
+      // Diagnose all of the patterns without explicit initializers.
+      bool diagnosed = false;
+      for (unsigned i : range(PB->getNumPatternEntries())) {
+        if (PB->isExplicitlyInitialized(i))
+          continue;
+
+        auto *pattern = PB->getPattern(i);
+
+        StringRef name;
+
+        if (auto *TP = dyn_cast<TypedPattern>(pattern)) {
+          if (auto *NP = dyn_cast<NamedPattern>(TP->getSubPattern()))
+            name = NP->getNameStr();
+        }
+
+        emitDiagnosticAt(
+            pattern->getLoc(),
+            diag::result_builder_requires_explicit_var_initialization,
+            !name.empty(), name, builder->getName())
+            .fixItInsertAfter(pattern->getEndLoc(), " = <#value#>");
+
+        diagnosed = true;
+      }
+
+      if (diagnosed)
+        return;
+    }
+  }
+
   if (auto stmt = unhandled.dyn_cast<Stmt *>()) {
     emitDiagnostic(asNote ? diag::note_result_builder_control_flow
                           : diag::result_builder_control_flow,

test/Constraints/result_builder_diags.swift

@@ -6,7 +6,7 @@ enum Either<T,U> {
 }
 
 @resultBuilder
-struct TupleBuilder { // expected-note 2 {{struct 'TupleBuilder' declared here}}
+struct TupleBuilder { // expected-note 3 {{struct 'TupleBuilder' declared here}}
   static func buildBlock() -> () { }
 
   static func buildBlock<T1>(_ t1: T1) -> T1 {
@@ -99,10 +99,18 @@ func testDiags() {
   tuplify(true) { _ in
     17
     let x = 17
-    let y: Int // expected-error{{closure containing a declaration cannot be used with result builder 'TupleBuilder'}}
+    let y: Int // expected-error{{local variable 'y' requires explicit initializer to be used with result builder 'TupleBuilder'}} {{15-15= = <#value#>}}
     x + 25
   }
 
+  tuplify(true) { _ in
+    17
+    let y: Int, z: String
+    // expected-error@-1 {{local variable 'y' requires explicit initializer to be used with result builder 'TupleBuilder'}} {{15-15= = <#value#>}}
+    // expected-error@-2 {{local variable 'z' requires explicit initializer to be used with result builder 'TupleBuilder'}} {{26-26= = <#value#>}}
+    y + 25
+  }
+
   // Statements unsupported by the particular builder.
   tuplifyWithoutIf(true) {
     if $0 {    // expected-error{{closure containing control flow statement cannot be used with result builder 'TupleBuilderWithoutIf'}}