[wip] most of the way there, just need diagnostic I think

Author: kavon

include/swift/Sema/ConstraintSystem.h

@@ -333,6 +333,9 @@ enum TypeVariableOptions {
 
   /// Whether the type variable can be bound to a pack type or not.
   TVO_CanBindToPack = 0x20,
+
+  /// Whether the type variable can be bound to a move-only (noncopyable) type.
+  TVO_CanBindToMoveOnly = 0x40,
 };
 
 /// The implementation object for a type variable used within the
@@ -405,6 +408,9 @@ class TypeVariableType::Implementation {
   /// Whether this type variable can bind to a PackType.
   bool canBindToPack() const { return getRawOptions() & TVO_CanBindToPack; }
 
+  /// Whether the type variable can be bound to a move-only (noncopyable) type.
+  bool canBindToMoveOnly() const { return getRawOptions() & TVO_CanBindToMoveOnly; }
+
   /// Whether this type variable prefers a subtype binding over a supertype
   /// binding.
   bool prefersSubtypeBinding() const {
@@ -565,6 +571,13 @@ class TypeVariableType::Implementation {
         recordBinding(*record);
       getTypeVariable()->Bits.TypeVariableType.Options &= ~TVO_CanBindToNoEscape;
     }
+
+    if (canBindToMoveOnly() && !otherRep->getImpl().canBindToMoveOnly()) {
+      assert(false && "yay, not dead code!");
+      if (record)
+        recordBinding(*record);
+      getTypeVariable()->Bits.TypeVariableType.Options &= ~TVO_CanBindToMoveOnly;
+    }
   }
 
   /// Retrieve the fixed type that corresponds to this type variable,
@@ -645,6 +658,7 @@ class TypeVariableType::Implementation {
     ENTRY(TVO_CanBindToHole, "hole");
     ENTRY(TVO_PrefersSubtypeBinding, "");
     ENTRY(TVO_CanBindToPack, "pack");
+    ENTRY(TVO_CanBindToMoveOnly, "moveonly");
     }
   #undef ENTRY
   }

lib/Sema/CSSimplify.cpp

@@ -4201,6 +4201,12 @@ ConstraintSystem::matchTypesBindTypeVar(
     }
   }
 
+  // If the type variable cannot bind to a move-only type, fail.
+  if (!typeVar->getImpl().canBindToMoveOnly() && type->isPureMoveOnly()) {
+    // TODO: record a fix that just emits an error to reject the substitution.
+    return getTypeMatchFailure(locator);
+  }
+
   // We do not allow keypaths to go through AnyObject. Let's create a fix
   // so this can be diagnosed later.
   if (auto loc = typeVar->getImpl().getLocator()) {

lib/Sema/TypeCheckConstraints.cpp

@@ -67,6 +67,8 @@ void TypeVariableType::Implementation::print(llvm::raw_ostream &OS) {
     bindingOptions.push_back(TypeVariableOptions::TVO_CanBindToNoEscape);
   if (canBindToHole())
     bindingOptions.push_back(TypeVariableOptions::TVO_CanBindToHole);
+  if (canBindToMoveOnly())
+    bindingOptions.push_back(TypeVariableOptions::TVO_CanBindToMoveOnly);
   if (!bindingOptions.empty()) {
     OS << " [allows bindings to: ";
     interleave(bindingOptions, OS,