[AST] Drop makeIterator reference from ForEachStmt.

Rather than having the type checker form the ConcreteDeclRef for
makeIterator, have SILGen do it, because it's fairly trivial.
Eliminates some redundant state from the AST.

Author: DougGregor

include/swift/AST/Stmt.h

@@ -808,7 +808,6 @@ class ForEachStmt : public LabeledStmt {
 
   // Set by Sema:
   ProtocolConformanceRef sequenceConformance = ProtocolConformanceRef();
-  ConcreteDeclRef makeIterator;
   ConcreteDeclRef iteratorNext;
   VarDecl *iteratorVar = nullptr;
   Expr *iteratorVarRef = nullptr;
@@ -838,9 +837,6 @@ class ForEachStmt : public LabeledStmt {
   void setConvertElementExpr(Expr *expr) { convertElementExpr = expr; }
   Expr *getConvertElementExpr() const { return convertElementExpr; }
 
-  void setMakeIterator(ConcreteDeclRef declRef) { makeIterator = declRef; }
-  ConcreteDeclRef getMakeIterator() const { return makeIterator; }
-
   void setIteratorNext(ConcreteDeclRef declRef) { iteratorNext = declRef; }
   ConcreteDeclRef getIteratorNext() const { return iteratorNext; }
 

lib/AST/ASTDumper.cpp

@@ -1593,9 +1593,6 @@ class PrintStmt : public StmtVisitor<PrintStmt> {
   }
   void visitForEachStmt(ForEachStmt *S) {
     printCommon(S, "for_each_stmt");
-    PrintWithColorRAII(OS, LiteralValueColor) << " make_generator=";
-    S->getMakeIterator().dump(
-        PrintWithColorRAII(OS, LiteralValueColor).getOS());
     PrintWithColorRAII(OS, LiteralValueColor) << " next=";
     S->getIteratorNext().dump(
         PrintWithColorRAII(OS, LiteralValueColor).getOS());

lib/SILGen/SILGenStmt.cpp

@@ -909,8 +909,20 @@ void StmtEmitter::visitForEachStmt(ForEachStmt *S) {
     auto initialization =
         SGF.emitInitializationForVarDecl(S->getIteratorVar(), false);
     SILLocation loc = SILLocation(S->getSequence());
+
+    // Compute the reference to the Sequence's makeIterator().
+    FuncDecl *makeIteratorReq = SGF.getASTContext().getSequenceMakeIterator();
+    auto sequenceProto =
+        SGF.getASTContext().getProtocol(KnownProtocolKind::Sequence);
+    auto sequenceConformance = S->getSequenceConformance();
+    Type sequenceType = S->getSequence()->getType();
+    auto sequenceSubs = SubstitutionMap::getProtocolSubstitutions(
+        sequenceProto, sequenceType, sequenceConformance);
+    ConcreteDeclRef makeIteratorRef(makeIteratorReq, sequenceSubs);
+
+    // Call makeIterator().
     RValue result = SGF.emitApplyMethod(
-        loc, S->getMakeIterator(), ArgumentSource(S->getSequence()),
+        loc, makeIteratorRef, ArgumentSource(S->getSequence()),
         PreparedArguments(ArrayRef<AnyFunctionType::Param>({})),
         SGFContext(initialization.get()));
     if (!result.isInContext()) {

lib/Sema/TypeCheckConstraints.cpp

@@ -3013,17 +3013,8 @@ auto TypeChecker::typeCheckForEachBinding(
 
       // Perform any necessary conversions of the sequence (e.g. [T]! -> [T]).
       expr = solution.coerceToType(expr, SequenceType, Locator);
-      
       if (!expr) return nullptr;
 
-      // Convert the sequence as appropriate for the makeIterator() call.
-      auto makeIteratorOverload = solution.getOverloadChoice(ContextualLocator);
-      auto makeIteratorSelfType = solution.simplifyType(
-          makeIteratorOverload.openedFullType
-        )->castTo<AnyFunctionType>()->getParams()[0].getPlainType();
-      expr = solution.coerceToType(expr, makeIteratorSelfType,
-                                   ContextualLocator);
-
       cs.cacheExprTypes(expr);
       Stmt->setSequence(expr);
 
@@ -3054,14 +3045,6 @@ auto TypeChecker::typeCheckForEachBinding(
           ConformanceCheckFlags::InExpression,
           expr->getLoc());
 
-      // Record the makeIterator declaration we used.
-      auto makeIteratorDecl = makeIteratorOverload.choice.getDecl();
-      auto makeIteratorSubs = SequenceType->getMemberSubstitutionMap(
-          cs.DC->getParentModule(), makeIteratorDecl);
-      auto makeIteratorDeclRef =
-          ConcreteDeclRef(makeIteratorDecl, makeIteratorSubs);
-      Stmt->setMakeIterator(makeIteratorDeclRef);
-
       solution.setExprTypes(expr);
       return expr;
     }