[CS] Avoid penalizing holes for placeholder vars for completion

When doing completion in a result builder, we avoid
solving unrelated expressions by replacing them with
unbound placeholder variables. Avoid penalizing
assigning holes to these placeholders, since
otherwise we end up unnecessarily exploring the
entire solution space, and bailing due to too
complex.

rdar://144382123

Author: hamishknight

lib/Sema/CSBindings.cpp

@@ -2895,6 +2895,21 @@ static bool shouldIgnoreHoleForCodeCompletion(ConstraintSystem &cs,
   if (typeVar->getImpl().isCodeCompletionToken())
     return true;
 
+  // When doing completion in a result builder, we avoid solving unrelated
+  // expressions by replacing them with unbound placeholder variables.
+  // As such, we need to avoid penalizing holes for references to
+  // placeholder variables.
+  if (srcLocator->isLastElement<LocatorPathElt::PlaceholderType>()) {
+    if (auto *DRE = getAsExpr<DeclRefExpr>(srcLocator->getAnchor())) {
+      if (auto *VD = dyn_cast_or_null<VarDecl>(DRE->getDecl())) {
+        if (auto *PBD = VD->getParentPatternBinding()) {
+          if (isPlaceholderVar(PBD))
+            return true;
+        }
+      }
+    }
+  }
+  
   // Don't penalize solutions with holes due to missing arguments after the
   // code completion position.
   auto argLoc = srcLocator->findLast<LocatorPathElt::SynthesizedArgument>();

test/IDE/issue-79213.swift

@@ -0,0 +1,38 @@
+// RUN: %batch-code-completion
+
+protocol View {}
+
+struct EmptyView: View {}
+struct Either<T: View, U: View>: View {}
+struct Tuple<T>: View {}
+extension Optional: View where Wrapped: View {}
+
+@resultBuilder
+struct ViewBuilder {
+  static func buildBlock() -> EmptyView { .init() }
+  static func buildBlock<T: View>(_ x: T) -> T { x }
+  @_disfavoredOverload static func buildBlock<each T: View>(
+    _ x: repeat each T
+  ) -> Tuple<(repeat each T)> { .init() }
+  static func buildEither<T, U>(first component: T) -> Either<T, U> { .init() }
+  static func buildEither<T, U>(second component: U) -> Either<T, U> { .init() }
+  static func buildIf<T: View>(_ x: T?) -> T? { x }
+}
+
+struct R {
+  var bar: Bool
+}
+
+func baz<R>(@ViewBuilder _ fn: () -> R) {}
+
+// https://github.com/swiftlang/swift/issues/79213 - Make sure we get a result here.
+func foo(_ x: R, _ b: Bool) {
+  baz {
+    if b {
+      EmptyView()
+    } else if b {
+      if x.#^COMPLETE^# {}
+      // COMPLETE: Decl[InstanceVar]/CurrNominal/TypeRelation[Convertible]: bar[#Bool#]; name=bar
+    }
+  }
+}