[Type checker] Experimental support for one-way parameter constraints.

Introduce an experimental mode (behind the flag
`experimental-one-way-closure-params`) that places one-way
constraints between closure parameter types and references to those
parameters within the body of the closure. The intent here is to
break up constraint systems further, potentially improving type
checking performance and making way for larger closure bodies to be
supported.

This is a source-breaking change when the body of a single-expression
closure is used to determine the parameter types. One obvious example
is when there is no contextual type, e.g.,

    let _ = { $0 + 1 }

this type-checks today because `1` becomes `Int`, which matches the
`+` overload with the type `(Int, Int) -> Int`, determining the
parameter type `Int` for the closure. Such code would not type-check
with one-way constraints.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -549,7 +549,10 @@ namespace swift {
     /// Enable constraint solver support for experimental
     ///        operator protocol designator feature.
     bool SolverEnableOperatorDesignatedTypes = false;
-    
+
+    /// Enable experimental support for one-way constraints for the
+    /// parameters of closures.
+    bool EnableOneWayClosureParameters = false;
   };
 } // end namespace swift
 

include/swift/Option/FrontendOptions.td

@@ -642,6 +642,10 @@ def experimental_print_full_convention :
  HelpText<"When emitting a module interface, emit additional @convention "
           "arguments, regardless of whether they were written in the source">;
 
+def experimental_one_way_closure_params :
+  Flag<["-"], "experimental-one-way-closure-params">,
+  HelpText<"Enable experimental support for one-way closure parameters">;
+
 def prebuilt_module_cache_path :
   Separate<["-"], "prebuilt-module-cache-path">,
   HelpText<"Directory of prebuilt modules for loading module interfaces">;

lib/Frontend/CompilerInvocation.cpp

@@ -724,6 +724,8 @@ static bool ParseTypeCheckerArgs(TypeCheckerOptions &Opts, ArgList &Args,
 
   Opts.SolverEnableOperatorDesignatedTypes |=
       Args.hasArg(OPT_solver_enable_operator_designated_types);
+  Opts.EnableOneWayClosureParameters |=
+      Args.hasArg(OPT_experimental_one_way_closure_params);
 
   Opts.DebugConstraintSolver |= Args.hasArg(OPT_debug_constraints);
   Opts.DebugGenericSignatures |= Args.hasArg(OPT_debug_generic_signatures);

lib/Sema/CSBindings.cpp

@@ -710,7 +710,8 @@ ConstraintSystem::getPotentialBindings(TypeVariableType *typeVar) const {
       }
       break;
 
-    case ConstraintKind::OneWayEqual: {
+    case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam: {
       // Don't produce any bindings if this type variable is on the left-hand
       // side of a one-way binding.
       auto firstType = constraint->getFirstType();

lib/Sema/CSSimplify.cpp

@@ -1322,6 +1322,7 @@ ConstraintSystem::matchTupleTypes(TupleType *tuple1, TupleType *tuple2,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Not a conversion");
   }
@@ -1387,6 +1388,7 @@ static bool matchFunctionRepresentations(FunctionTypeRepresentation rep1,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     return false;
   }
@@ -1699,6 +1701,7 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
   case ConstraintKind::FunctionInput:
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Not a relational constraint");
   }
@@ -4406,6 +4409,7 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     case ConstraintKind::FunctionInput:
     case ConstraintKind::FunctionResult:
     case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
       llvm_unreachable("Not a relational constraint");
     }
@@ -7121,9 +7125,16 @@ ConstraintSystem::simplifyOneWayConstraint(
     return SolutionKind::Solved;
   }
 
-  // Translate this constraint into a one-way binding constraint.
-  return matchTypes(first, secondSimplified, ConstraintKind::Equal, flags,
-                    locator);
+  // Translate this constraint into an equality or bind-parameter constraint,
+  // as appropriate.
+  if (kind == ConstraintKind::OneWayEqual) {
+    return matchTypes(first, secondSimplified, ConstraintKind::Equal, flags,
+                      locator);
+  }
+
+  assert(kind == ConstraintKind::OneWayBindParam);
+  return matchTypes(
+      secondSimplified, first, ConstraintKind::BindParam, flags, locator);
 }
 
 static Type getFunctionBuilderTypeFor(ConstraintSystem &cs, unsigned paramIdx,
@@ -7175,12 +7186,27 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
 
     Type internalType;
 
+    bool oneWayConstraints =
+      getASTContext().TypeCheckerOpts.EnableOneWayClosureParameters;
     if (paramList->get(i)->getTypeRepr()) {
       // Internal type is the type used in the body of the closure,
       // so "external" type translates to it as follows:
       //  - `Int...` -> `[Int]`,
       //  - `inout Int` -> `@lvalue Int`.
       internalType = param.getParameterType();
+
+      // When there are type variables in the type and we have enabled
+      // one-way constraints, create a fresh type variable to handle the
+      // binding.
+      if (oneWayConstraints && internalType->hasTypeVariable()) {
+        auto *paramLoc =
+            getConstraintLocator(closure, LocatorPathElt::TupleElement(i));
+        auto *typeVar = createTypeVariable(paramLoc, TVO_CanBindToLValue |
+                                                         TVO_CanBindToNoEscape);
+        addConstraint(
+            ConstraintKind::OneWayBindParam, typeVar, internalType, paramLoc);
+        internalType = typeVar;
+      }
     } else {
       auto *paramLoc =
           getConstraintLocator(closure, LocatorPathElt::TupleElement(i));
@@ -7194,7 +7220,13 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
           param.isVariadic() ? ArraySliceType::get(typeVar) : Type(typeVar);
 
       auto externalType = param.getOldType();
-      addConstraint(ConstraintKind::BindParam, externalType, typeVar, paramLoc);
+      if (oneWayConstraints) {
+        addConstraint(
+            ConstraintKind::OneWayBindParam, typeVar, externalType, paramLoc);
+      } else {
+        addConstraint(
+            ConstraintKind::BindParam, externalType, typeVar, paramLoc);
+      }
     }
 
     setType(paramList->get(i), internalType);
@@ -9759,6 +9791,7 @@ ConstraintSystem::addConstraintImpl(ConstraintKind kind, Type first,
                                                subflags, locator);
 
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     return simplifyOneWayConstraint(kind, first, second, subflags, locator);
 
   case ConstraintKind::ValueMember:
@@ -10271,6 +10304,7 @@ ConstraintSystem::simplifyConstraint(const Constraint &constraint) {
     return SolutionKind::Unsolved;
 
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     return simplifyOneWayConstraint(constraint.getKind(),
                                     constraint.getFirstType(),
                                     constraint.getSecondType(),

lib/Sema/CSSolver.cpp

@@ -1688,6 +1688,7 @@ void ConstraintSystem::ArgumentInfoCollector::walk(Type argType) {
       case ConstraintKind::ConformsTo:
       case ConstraintKind::Defaultable:
       case ConstraintKind::OneWayEqual:
+      case ConstraintKind::OneWayBindParam:
       case ConstraintKind::DefaultClosureType:
         break;
       }

lib/Sema/Constraint.cpp

@@ -66,6 +66,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
     assert(!First.isNull());
     assert(!Second.isNull());
     break;
@@ -138,6 +139,7 @@ Constraint::Constraint(ConstraintKind Kind, Type First, Type Second, Type Third,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     llvm_unreachable("Wrong constructor");
 
@@ -265,6 +267,7 @@ Constraint *Constraint::clone(ConstraintSystem &cs) const {
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     return create(cs, getKind(), getFirstType(), getSecondType(), getLocator());
 
@@ -348,6 +351,7 @@ void Constraint::print(llvm::raw_ostream &Out, SourceManager *sm) const {
   case ConstraintKind::EscapableFunctionOf: Out << " @escaping type of "; break;
   case ConstraintKind::OpenedExistentialOf: Out << " opened archetype of "; break;
   case ConstraintKind::OneWayEqual: Out << " one-way bind to "; break;
+  case ConstraintKind::OneWayBindParam: Out << " one-way bind param to "; break;
   case ConstraintKind::DefaultClosureType:
     Out << " closure can default to ";
     break;
@@ -564,6 +568,7 @@ gatherReferencedTypeVars(Constraint *constraint,
   case ConstraintKind::FunctionResult:
   case ConstraintKind::OpaqueUnderlyingType:
   case ConstraintKind::OneWayEqual:
+  case ConstraintKind::OneWayBindParam:
   case ConstraintKind::DefaultClosureType:
     constraint->getFirstType()->getTypeVariables(typeVars);
     constraint->getSecondType()->getTypeVariables(typeVars);

lib/Sema/Constraint.h

@@ -161,6 +161,12 @@ enum class ConstraintKind : char {
   /// type). At that point, this constraint will be treated like an `Equal`
   /// constraint.
   OneWayEqual,
+  /// The second type is the type of a function parameter, and the first type
+  /// is the type of a reference to that function parameter within the body.
+  /// Once the second type has been fully determined (and mapped down to a
+  /// concrete type), this constraint will be treated like a 'BindParam'
+  /// constraint.
+  OneWayBindParam,
   /// If there is no contextual info e.g. `_ = { 42 }` default first type
   /// to a second type (inferred closure type). This is effectively a
   /// `Defaultable` constraint which a couple of differences:
@@ -549,6 +555,7 @@ class Constraint final : public llvm::ilist_node<Constraint>,
     case ConstraintKind::OptionalObject:
     case ConstraintKind::OpaqueUnderlyingType:
     case ConstraintKind::OneWayEqual:
+    case ConstraintKind::OneWayBindParam:
     case ConstraintKind::DefaultClosureType:
       return ConstraintClassification::Relational;
 
@@ -669,7 +676,8 @@ class Constraint final : public llvm::ilist_node<Constraint>,
 
   /// Whether this is a one-way constraint.
   bool isOneWayConstraint() const {
-    return Kind == ConstraintKind::OneWayEqual;
+    return Kind == ConstraintKind::OneWayEqual ||
+        Kind == ConstraintKind::OneWayBindParam;
   }
 
   /// Retrieve the overload choice for an overload-binding constraint.

test/Constraints/one_way_closure_params.swift

@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -swift-version 4 -experimental-one-way-closure-params
+
+func testBasic() {
+  let _: (Float) -> Float = { $0 + 1 }
+
+  let _ = { $0 + 1 } // expected-error{{unable to infer type of a closure parameter $0 in the current context}}
+}
+