BuilderTransform.cpp

//===--- BuilderTransform.cpp - Function-builder transformation -----------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file implements routines associated with the function-builder
// transformation.
//
//===----------------------------------------------------------------------===//

#include "ConstraintSystem.h"
#include "MiscDiagnostics.h"
#include "SolutionResult.h"
#include "TypeChecker.h"
#include "TypeCheckAvailability.h"
#include "swift/Sema/IDETypeChecking.h"
#include "swift/AST/ASTPrinter.h"
#include "swift/AST/ASTVisitor.h"
#include "swift/AST/ASTWalker.h"
#include "swift/AST/NameLookup.h"
#include "swift/AST/NameLookupRequests.h"
#include "swift/AST/ParameterList.h"
#include "swift/AST/TypeCheckRequests.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include <iterator>
#include <map>
#include <memory>
#include <utility>
#include <tuple>

using namespace swift;
using namespace constraints;

namespace {

/// Find the first #available condition within the statement condition,
/// or return NULL if there isn't one.
const StmtConditionElement *findAvailabilityCondition(StmtCondition stmtCond) {
  for (const auto &cond : stmtCond) {
    switch (cond.getKind()) {
    case StmtConditionElement::CK_Boolean:
    case StmtConditionElement::CK_PatternBinding:
      continue;

    case StmtConditionElement::CK_Availability:
      return &cond;
      break;
    }
  }

  return nullptr;
}

/// Visitor to classify the contents of the given closure.
class BuilderClosureVisitor
    : private StmtVisitor<BuilderClosureVisitor, VarDecl *> {

  friend StmtVisitor<BuilderClosureVisitor, VarDecl *>;

  ConstraintSystem *cs;
  DeclContext *dc;
  ASTContext &ctx;
  Type builderType;
  NominalTypeDecl *builder = nullptr;
  Identifier buildOptionalId;
  llvm::SmallDenseMap<Identifier, bool> supportedOps;

  SkipUnhandledConstructInFunctionBuilder::UnhandledNode unhandledNode;

  /// Whether an error occurred during application of the builder closure,
  /// e.g., during constraint generation.
  bool hadError = false;

  /// Counter used to give unique names to the variables that are
  /// created implicitly.
  unsigned varCounter = 0;

  /// The record of what happened when we applied the builder transform.
  AppliedBuilderTransform applied;

  /// Produce a builder call to the given named function with the given
  /// arguments.
  Expr *buildCallIfWanted(SourceLoc loc,
                          Identifier fnName, ArrayRef<Expr *> args,
                          ArrayRef<Identifier> argLabels) {
    if (!cs)
      return nullptr;

    // FIXME: Setting a base on this expression is necessary in order
    // to get diagnostics if something about this builder call fails,
    // e.g. if there isn't a matching overload for `buildBlock`.
    TypeExpr *typeExpr;
    auto simplifiedTy = cs->simplifyType(builderType);
    if (!simplifiedTy->hasTypeVariable()) {
      typeExpr = TypeExpr::createImplicitHack(loc, simplifiedTy, ctx);
    } else if (auto *decl = simplifiedTy->getAnyGeneric()) {
      // HACK: If there's not enough information to completely resolve the
      // builder type, but we have the base available to us, form an *explicit*
      // TypeExpr pointing at it. We cannot form an implicit base without
      // a fully-resolved concrete type. Really, whatever we put here has no
      // bearing on the generated solution because we're going to use this node
      // to stash the builder type and hand it back to the ambient
      // constraint system.
      typeExpr = TypeExpr::createForDecl(DeclNameLoc(loc), decl, dc);
    } else {
      // HACK: If there's not enough information in the constraint system,
      // create a garbage base type to force it to diagnose
      // this as an ambiguous expression.
      // FIXME: We can also construct an UnresolvedMemberExpr here instead of
      // an UnresolvedDotExpr and get a slightly better diagnostic.
      typeExpr = TypeExpr::createImplicitHack(loc, ErrorType::get(ctx), ctx);
    }
    cs->setType(typeExpr, MetatypeType::get(builderType));

    SmallVector<SourceLoc, 4> argLabelLocs;
    for (auto i : indices(argLabels)) {
      argLabelLocs.push_back(args[i]->getStartLoc());
    }

    auto memberRef = new (ctx) UnresolvedDotExpr(
        typeExpr, loc, DeclNameRef(fnName), DeclNameLoc(loc),
        /*implicit=*/true);
    memberRef->setFunctionRefKind(FunctionRefKind::SingleApply);
    SourceLoc openLoc = args.empty() ? loc : args.front()->getStartLoc();
    SourceLoc closeLoc = args.empty() ? loc : args.back()->getEndLoc();
    Expr *result = CallExpr::create(ctx, memberRef, openLoc, args,
                                    argLabels, argLabelLocs, closeLoc,
                                    /*trailing closures*/{},
                                    /*implicit*/true);

    return result;
  }

  /// Check whether the builder supports the given operation.
  bool builderSupports(Identifier fnName,
                       ArrayRef<Identifier> argLabels = {}) {
    auto known = supportedOps.find(fnName);
    if (known != supportedOps.end()) {
      return known->second;
    }

    return supportedOps[fnName] = TypeChecker::typeSupportsBuilderOp(
               builderType, dc, fnName, argLabels);
  }

  /// Build an implicit variable in this context.
  VarDecl *buildVar(SourceLoc loc) {
    // Create the implicit variable.
    Identifier name = ctx.getIdentifier(
        ("$__builder" + Twine(varCounter++)).str());
    auto var = new (ctx) VarDecl(/*isStatic=*/false, VarDecl::Introducer::Var,
                                 loc, name, dc);
    var->setImplicit();
    return var;
  }

  /// Capture the given expression into an implicitly-generated variable.
  VarDecl *captureExpr(Expr *expr, bool oneWay,
                       llvm::PointerUnion<Stmt *, Expr *> forEntity = nullptr) {
    if (!cs)
      return nullptr;

    Expr *origExpr = expr;

    if (oneWay) {
      // Form a one-way constraint to prevent backward propagation.
      expr = new (ctx) OneWayExpr(expr);
    }

    // Generate constraints for this expression.
    expr = cs->generateConstraints(expr, dc);
    if (!expr) {
      hadError = true;
      return nullptr;
    }

    // Create the implicit variable.
    auto var = buildVar(expr->getStartLoc());

    // Record the new variable and its corresponding expression & statement.
    if (auto forStmt = forEntity.dyn_cast<Stmt *>()) {
      applied.capturedStmts.insert({forStmt, { var, { expr } }});
    } else {
      if (auto forExpr = forEntity.dyn_cast<Expr *>())
        origExpr = forExpr;

      applied.capturedExprs.insert({origExpr, {var, expr}});
    }

    cs->setType(var, cs->getType(expr));
    return var;
  }

  /// Build an implicit reference to the given variable.
  DeclRefExpr *buildVarRef(VarDecl *var, SourceLoc loc) {
    return new (ctx) DeclRefExpr(var, DeclNameLoc(loc), /*Implicit=*/true);
  }

public:
  BuilderClosureVisitor(ASTContext &ctx, ConstraintSystem *cs,
                        DeclContext *dc, Type builderType,
                        Type bodyResultType)
      : cs(cs), dc(dc), ctx(ctx), builderType(builderType) {
    builder = builderType->getAnyNominal();
    applied.builderType = builderType;
    applied.bodyResultType = bodyResultType;

    // Use buildOptional(_:) if available, otherwise fall back to buildIf
    // when available.
    if (builderSupports(ctx.Id_buildOptional) ||
        !builderSupports(ctx.Id_buildIf))
      buildOptionalId = ctx.Id_buildOptional;
    else
      buildOptionalId = ctx.Id_buildIf;
  }

  /// Apply the builder transform to the given statement.
  Optional<AppliedBuilderTransform> apply(Stmt *stmt) {
    VarDecl *bodyVar = visit(stmt);
    if (!bodyVar)
      return None;

    applied.returnExpr = buildVarRef(bodyVar, stmt->getEndLoc());

    // If there is a buildFinalResult(_:), call it.
    ASTContext &ctx = cs->getASTContext();
    if (builderSupports(ctx.Id_buildFinalResult, { Identifier() })) {
      applied.returnExpr = buildCallIfWanted(
          applied.returnExpr->getLoc(), ctx.Id_buildFinalResult,
          { applied.returnExpr }, { Identifier() });
    }

    applied.returnExpr = cs->buildTypeErasedExpr(applied.returnExpr,
                                                 dc, applied.bodyResultType,
                                                 CTP_ReturnStmt);

    applied.returnExpr = cs->generateConstraints(applied.returnExpr, dc);
    if (!applied.returnExpr) {
      hadError = true;
      return None;
    }

    return std::move(applied);
  }

  /// Check whether the function builder can be applied to this statement.
  /// \returns the node that cannot be handled by this builder on failure.
  SkipUnhandledConstructInFunctionBuilder::UnhandledNode check(Stmt *stmt) {
    (void)visit(stmt);
    return unhandledNode;
  }

protected:
#define CONTROL_FLOW_STMT(StmtClass)                       \
  VarDecl *visit##StmtClass##Stmt(StmtClass##Stmt *stmt) { \
    if (!unhandledNode)                                    \
      unhandledNode = stmt;                                \
                                                           \
    return nullptr;                                        \
  }

  void visitPatternBindingDecl(PatternBindingDecl *patternBinding) {
    // If any of the entries lacks an initializer, don't handle this node.
    if (!llvm::all_of(range(patternBinding->getNumPatternEntries()),
                      [&](unsigned index) {
            return patternBinding->isExplicitlyInitialized(index);
        })) {
      if (!unhandledNode)
        unhandledNode = patternBinding;
      return;
    }

    // If there is a constraint system, generate constraints for the pattern
    // binding.
    if (cs) {
      SolutionApplicationTarget target(patternBinding);
      if (cs->generateConstraints(target, FreeTypeVariableBinding::Disallow))
        hadError = true;
    }
  }

  VarDecl *visitBraceStmt(BraceStmt *braceStmt) {
    SmallVector<Expr *, 4> expressions;
    auto addChild = [&](VarDecl *childVar) {
      if (!childVar)
        return;

      expressions.push_back(buildVarRef(childVar, childVar->getLoc()));
    };

    for (auto node : braceStmt->getElements()) {
      // Implicit returns in single-expression function bodies are treated
      // as the expression.
      if (auto returnStmt =
              dyn_cast_or_null<ReturnStmt>(node.dyn_cast<Stmt *>())) {
        assert(returnStmt->isImplicit());
        node = returnStmt->getResult();
      }

      if (auto stmt = node.dyn_cast<Stmt *>()) {
        addChild(visit(stmt));
        continue;
      }

      if (auto decl = node.dyn_cast<Decl *>()) {
        // Just ignore #if; the chosen children should appear in the
        // surrounding context.  This isn't good for source tools but it
        // at least works.
        if (isa<IfConfigDecl>(decl))
          continue;

        // Skip #warning/#error; we'll handle them when applying the builder.
        if (isa<PoundDiagnosticDecl>(decl)) {
          continue;
        }

        // Pattern bindings are okay so long as all of the entries are
        // initialized.
        if (auto patternBinding = dyn_cast<PatternBindingDecl>(decl)) {
          visitPatternBindingDecl(patternBinding);
          continue;
        }

        // Ignore variable declarations, because they're always handled within
        // their enclosing pattern bindings.
        if (isa<VarDecl>(decl))
          continue;

        if (!unhandledNode)
          unhandledNode = decl;

        continue;
      }

      auto expr = node.get<Expr *>();
      if (cs && builderSupports(ctx.Id_buildExpression)) {
        expr = buildCallIfWanted(expr->getLoc(), ctx.Id_buildExpression,
                                 { expr }, { Identifier() });
      }

      addChild(captureExpr(expr, /*oneWay=*/true, node.get<Expr *>()));
    }

    if (!cs || hadError)
      return nullptr;

    // Call Builder.buildBlock(... args ...)
    auto call = buildCallIfWanted(braceStmt->getStartLoc(),
                                  ctx.Id_buildBlock, expressions,
                                  /*argLabels=*/{ });
    if (!call)
      return nullptr;

    return captureExpr(call, /*oneWay=*/true, braceStmt);
  }

  VarDecl *visitReturnStmt(ReturnStmt *stmt) {
    if (!unhandledNode)
      unhandledNode = stmt;
    return nullptr;
  }

  VarDecl *visitDoStmt(DoStmt *doStmt) {
    auto childVar = visitBraceStmt(doStmt->getBody());
    if (!childVar)
      return nullptr;

    auto childRef = buildVarRef(childVar, doStmt->getEndLoc());

    return captureExpr(childRef, /*oneWay=*/true, doStmt);
  }

  CONTROL_FLOW_STMT(Yield)
  CONTROL_FLOW_STMT(Defer)

  static bool isBuildableIfChainRecursive(IfStmt *ifStmt,
                                          unsigned &numPayloads,
                                          bool &isOptional) {
    // The 'then' clause contributes a payload.
    ++numPayloads;

    // If there's an 'else' clause, it contributes payloads:
    if (auto elseStmt = ifStmt->getElseStmt()) {
      // If it's 'else if', it contributes payloads recursively.
      if (auto elseIfStmt = dyn_cast<IfStmt>(elseStmt)) {
        return isBuildableIfChainRecursive(elseIfStmt, numPayloads,
                                           isOptional);
      // Otherwise it's just the one.
      } else {
        ++numPayloads;
      }

    // If not, the chain result is at least optional.
    } else {
      isOptional = true;
    }

    return true;
  }

  bool isBuildableIfChain(IfStmt *ifStmt, unsigned &numPayloads,
                          bool &isOptional) {
    if (!isBuildableIfChainRecursive(ifStmt, numPayloads, isOptional))
      return false;

    // If there's a missing 'else', we need 'buildOptional' to exist.
    if (isOptional && !builderSupports(buildOptionalId))
      return false;

    // If there are multiple clauses, we need 'buildEither(first:)' and
    // 'buildEither(second:)' to both exist.
    if (numPayloads > 1) {
      if (!builderSupports(ctx.Id_buildEither, {ctx.Id_first}) ||
          !builderSupports(ctx.Id_buildEither, {ctx.Id_second}))
        return false;
    }

    return true;
  }

  VarDecl *visitIfStmt(IfStmt *ifStmt) {
    // Check whether the chain is buildable and whether it terminates
    // without an `else`.
    bool isOptional = false;
    unsigned numPayloads = 0;
    if (!isBuildableIfChain(ifStmt, numPayloads, isOptional)) {
      if (!unhandledNode)
        unhandledNode = ifStmt;
      return nullptr;
    }

    // Attempt to build the chain, propagating short-circuits, which
    // might arise either do to error or not wanting an expression.
    return buildIfChainRecursive(ifStmt, 0, numPayloads, isOptional,
                                 /*isTopLevel=*/true);
  }

  /// Recursively build an if-chain: build an expression which will have
  /// a value of the chain result type before any call to `buildIf`.
  /// The expression will perform any necessary calls to `buildEither`,
  /// and the result will have optional type if `isOptional` is true.
  VarDecl *buildIfChainRecursive(IfStmt *ifStmt, unsigned payloadIndex,
                                 unsigned numPayloads, bool isOptional,
                                 bool isTopLevel = false) {
    assert(payloadIndex < numPayloads);

    // First generate constraints for the conditions. This can introduce
    // variable bindings that will be used within the "then" branch.
    if (cs && cs->generateConstraints(ifStmt->getCond(), dc)) {
      hadError = true;
      return nullptr;
    }

    // Make sure we recursively visit both sides even if we're not
    // building expressions.

    // Build the then clause.  This will have the corresponding payload
    // type (i.e. not wrapped in any way).
    VarDecl *thenVar = visit(ifStmt->getThenStmt());

    // Build the else clause, if present.  If this is from an else-if,
    // this will be fully wrapped; otherwise it will have the corresponding
    // payload type (at index `payloadIndex + 1`).
    assert(ifStmt->getElseStmt() || isOptional);
    bool isElseIf = false;
    Optional<VarDecl *> elseChainVar;
    if (auto elseStmt = ifStmt->getElseStmt()) {
      if (auto elseIfStmt = dyn_cast<IfStmt>(elseStmt)) {
        isElseIf = true;
        elseChainVar = buildIfChainRecursive(elseIfStmt, payloadIndex + 1,
                                             numPayloads, isOptional);
      } else {
        elseChainVar = visit(elseStmt);
      }
    }

    // Short-circuit if appropriate.
    if (!cs || !thenVar || (elseChainVar && !*elseChainVar))
      return nullptr;

    // If there is a #available in the condition, the 'then' will need to
    // be wrapped in a call to buildLimitedAvailability(_:), if available.
    Expr *thenVarRefExpr = buildVarRef(
        thenVar, ifStmt->getThenStmt()->getEndLoc());
    if (findAvailabilityCondition(ifStmt->getCond()) &&
        builderSupports(ctx.Id_buildLimitedAvailability)) {
      thenVarRefExpr = buildCallIfWanted(
          ifStmt->getThenStmt()->getEndLoc(), ctx.Id_buildLimitedAvailability,
          { thenVarRefExpr }, { Identifier() });
    }

    // Prepare the `then` operand by wrapping it to produce a chain result.
    Expr *thenExpr = buildWrappedChainPayload(
        thenVarRefExpr, payloadIndex, numPayloads, isOptional);

    // Prepare the `else operand:
    Expr *elseExpr;
    SourceLoc elseLoc;

    // - If there's no `else` clause, use `Optional.none`.
    if (!elseChainVar) {
      assert(isOptional);
      elseLoc = ifStmt->getEndLoc();
      elseExpr = buildNoneExpr(elseLoc);

    // - If there's an `else if`, the chain expression from that
    //   should already be producing a chain result.
    } else if (isElseIf) {
      elseExpr = buildVarRef(*elseChainVar, ifStmt->getEndLoc());
      elseLoc = ifStmt->getElseLoc();

    // - Otherwise, wrap it to produce a chain result.
    } else {
      elseLoc = ifStmt->getElseLoc();
      elseExpr = buildWrappedChainPayload(
          buildVarRef(*elseChainVar, ifStmt->getEndLoc()),
          payloadIndex + 1, numPayloads, isOptional);
    }

    // The operand should have optional type if we had optional results,
    // so we just need to call `buildIf` now, since we're at the top level.
    if (isOptional && isTopLevel) {
      thenExpr = buildCallIfWanted(ifStmt->getEndLoc(), buildOptionalId,
                                   thenExpr,  /*argLabels=*/{ });
      elseExpr = buildCallIfWanted(ifStmt->getEndLoc(), buildOptionalId,
                                   elseExpr,  /*argLabels=*/{ });
    }

    thenExpr = cs->generateConstraints(thenExpr, dc);
    if (!thenExpr) {
      hadError = true;
      return nullptr;
    }

    elseExpr = cs->generateConstraints(elseExpr, dc);
    if (!elseExpr) {
      hadError = true;
      return nullptr;
    }

    Type resultType = cs->addJoinConstraint(cs->getConstraintLocator(ifStmt),
        {
          { cs->getType(thenExpr), cs->getConstraintLocator(thenExpr) },
          { cs->getType(elseExpr), cs->getConstraintLocator(elseExpr) }
        });
    if (!resultType) {
      hadError = true;
      return nullptr;
    }

    // Create a variable to capture the result of this expression.
    auto ifVar = buildVar(ifStmt->getStartLoc());
    cs->setType(ifVar, resultType);
    applied.capturedStmts.insert({ifStmt, { ifVar, { thenExpr, elseExpr }}});
    return ifVar;
  }

  /// Wrap a payload value in an expression which will produce a chain
  /// result (without `buildIf`).
  Expr *buildWrappedChainPayload(Expr *operand, unsigned payloadIndex,
                                 unsigned numPayloads, bool isOptional) {
    assert(payloadIndex < numPayloads);

    // Inject into the appropriate chain position.
    //
    // We produce a (left-biased) balanced binary tree of Eithers in order
    // to prevent requiring a linear number of injections in the worst case.
    // That is, if we have 13 clauses, we want to produce:
    //
    //                      /------------------Either------------\
    //           /-------Either-------\                     /--Either--\
    //     /--Either--\          /--Either--\          /--Either--\     \
    //   /-E-\      /-E-\      /-E-\      /-E-\      /-E-\      /-E-\    \
    // 0000 0001  0010 0011  0100 0101  0110 0111  1000 1001  1010 1011 1100
    //
    // Note that a prefix of length D of the payload index acts as a path
    // through the tree to the node at depth D.  On the rightmost path
    // through the tree (when this prefix is equal to the corresponding
    // prefix of the maximum payload index), the bits of the index mark
    // where Eithers are required.
    //
    // Since we naturally want to build from the innermost Either out, and
    // therefore work with progressively shorter prefixes, we can do it all
    // with right-shifts.
    for (auto path = payloadIndex, maxPath = numPayloads - 1;
         maxPath != 0; path >>= 1, maxPath >>= 1) {
      // Skip making Eithers on the rightmost path where they aren't required.
      // This isn't just an optimization: adding spurious Eithers could
      // leave us with unresolvable type variables if `buildEither` has
      // a signature like:
      //    static func buildEither<T,U>(first value: T) -> Either<T,U>
      // which relies on unification to work.
      if (path == maxPath && !(maxPath & 1)) continue;

      bool isSecond = (path & 1);
      operand = buildCallIfWanted(operand->getStartLoc(),
                                  ctx.Id_buildEither, operand,
                                  {isSecond ? ctx.Id_second : ctx.Id_first});
    }

    // Inject into Optional if required.  We'll be adding the call to
    // `buildIf` after all the recursive calls are complete.
    if (isOptional) {
      operand = buildSomeExpr(operand);
    }

    return operand;
  }

  Expr *buildSomeExpr(Expr *arg) {
    auto optionalDecl = ctx.getOptionalDecl();
    auto optionalType = optionalDecl->getDeclaredType();

    auto loc = arg->getStartLoc();
    auto optionalTypeExpr =
      TypeExpr::createImplicitHack(loc, optionalType, ctx);
    auto someRef = new (ctx) UnresolvedDotExpr(
        optionalTypeExpr, loc, DeclNameRef(ctx.getIdentifier("some")),
        DeclNameLoc(loc), /*implicit=*/true);
    return CallExpr::createImplicit(ctx, someRef, arg, { });
  }

  Expr *buildNoneExpr(SourceLoc endLoc) {
    auto optionalDecl = ctx.getOptionalDecl();
    auto optionalType = optionalDecl->getDeclaredType();

    auto optionalTypeExpr =
      TypeExpr::createImplicitHack(endLoc, optionalType, ctx);
    return new (ctx) UnresolvedDotExpr(
        optionalTypeExpr, endLoc, DeclNameRef(ctx.getIdentifier("none")),
        DeclNameLoc(endLoc), /*implicit=*/true);
  }

  VarDecl *visitSwitchStmt(SwitchStmt *switchStmt) {
    // Generate constraints for the subject expression, and capture its
    // type for use in matching the various patterns.
    Expr *subjectExpr = switchStmt->getSubjectExpr();
    if (cs) {
      // Form a one-way constraint to prevent backward propagation.
      subjectExpr = new (ctx) OneWayExpr(subjectExpr);

      // FIXME: Add contextual type purpose for switch subjects?
      SolutionApplicationTarget target(subjectExpr, dc, CTP_Unused, Type(),
                                       /*isDiscarded=*/false);
      if (cs->generateConstraints(target, FreeTypeVariableBinding::Disallow)) {
        hadError = true;
        return nullptr;
      }

      cs->setSolutionApplicationTarget(switchStmt, target);
      subjectExpr = target.getAsExpr();
      assert(subjectExpr && "Must have a subject expression here");
    }

    // Generate constraints and capture variables for all of the cases.
    SmallVector<std::pair<CaseStmt *, VarDecl *>, 4> capturedCaseVars;
    for (auto *caseStmt : switchStmt->getCases()) {
      if (auto capturedCaseVar = visitCaseStmt(caseStmt, subjectExpr)) {
        capturedCaseVars.push_back({caseStmt, capturedCaseVar});
      }
    }

    if (!cs)
      return nullptr;

    // If there are no 'case' statements in the body let's try
    // to diagnose this situation via limited exhaustiveness check
    // before failing a builder transform, otherwise type-checker
    // might end up without any diagnostics which leads to crashes
    // in SILGen.
    if (capturedCaseVars.empty()) {
      TypeChecker::checkSwitchExhaustiveness(switchStmt, dc,
                                             /*limitChecking=*/true);
      hadError = true;
      return nullptr;
    }

    // Form the expressions that inject the result of each case into the
    // appropriate
    llvm::TinyPtrVector<Expr *> injectedCaseExprs;
    SmallVector<std::pair<Type, ConstraintLocator *>, 4> injectedCaseTerms;
    for (unsigned idx : indices(capturedCaseVars)) {
      auto caseStmt = capturedCaseVars[idx].first;
      auto caseVar = capturedCaseVars[idx].second;

      // Build the expression that injects the case variable into appropriate
      // buildEither(first:)/buildEither(second:) chain.
      Expr *caseVarRef = buildVarRef(caseVar, caseStmt->getEndLoc());
      Expr *injectedCaseExpr = buildWrappedChainPayload(
          caseVarRef, idx, capturedCaseVars.size(), /*isOptional=*/false);

      // Generate constraints for this injected case result.
      injectedCaseExpr = cs->generateConstraints(injectedCaseExpr, dc);
      if (!injectedCaseExpr) {
        hadError = true;
        return nullptr;
      }

      // Record this injected case expression.
      injectedCaseExprs.push_back(injectedCaseExpr);

      // Record the type and locator for this injected case expression, to be
      // used in the "join" constraint later.
      injectedCaseTerms.push_back(
        { cs->getType(injectedCaseExpr)->getRValueType(),
          cs->getConstraintLocator(injectedCaseExpr) });
    }

    // Form the type of the switch itself.
    Type resultType = cs->addJoinConstraint(
        cs->getConstraintLocator(switchStmt), injectedCaseTerms);
    if (!resultType) {
      hadError = true;
      return nullptr;
    }

    // Create a variable to capture the result of evaluating the switch.
    auto switchVar = buildVar(switchStmt->getStartLoc());
    cs->setType(switchVar, resultType);
    applied.capturedStmts.insert(
        {switchStmt, { switchVar, std::move(injectedCaseExprs) } });
    return switchVar;
  }

  VarDecl *visitCaseStmt(CaseStmt *caseStmt, Expr *subjectExpr) {
    auto *body = caseStmt->getBody();

    // Explicitly disallow `case` statements with empty bodies
    // since that helps to diagnose other issues with switch
    // statements by excluding invalid cases.
    if (auto *BS = dyn_cast<BraceStmt>(body)) {
      if (BS->getNumElements() == 0) {
        hadError = true;
        return nullptr;
      }
    }

    // If needed, generate constraints for everything in the case statement.
    if (cs) {
      auto locator = cs->getConstraintLocator(
          subjectExpr, LocatorPathElt::ContextualType());
      Type subjectType = cs->getType(subjectExpr);

      if (cs->generateConstraints(caseStmt, dc, subjectType, locator)) {
        hadError = true;
        return nullptr;
      }
    }

    // Translate the body.
    return visit(caseStmt->getBody());
  }

  VarDecl *visitForEachStmt(ForEachStmt *forEachStmt) {
    // for...in statements are handled via buildArray(_:); bail out if the
    // builder does not support it.
    if (!builderSupports(ctx.Id_buildArray)) {
      if (!unhandledNode)
        unhandledNode = forEachStmt;
      return nullptr;
    }

    // For-each statements require the Sequence protocol. If we don't have
    // it (which generally means the standard library isn't loaded), fall
    // out of the function-builder path entirely to let normal type checking
    // take care of this.
    auto sequenceProto = TypeChecker::getProtocol(
        dc->getASTContext(), forEachStmt->getForLoc(),
        KnownProtocolKind::Sequence);
    if (!sequenceProto) {
      if (!unhandledNode)
        unhandledNode = forEachStmt;
      return nullptr;
    }

    // Generate constraints for the loop header. This also wires up the
    // types for the patterns.
    auto target = SolutionApplicationTarget::forForEachStmt(
        forEachStmt, sequenceProto, dc, /*bindPatternVarsOneWay=*/true);
    if (cs) {
      if (cs->generateConstraints(target, FreeTypeVariableBinding::Disallow)) {
        hadError = true;
        return nullptr;
      }

      cs->setSolutionApplicationTarget(forEachStmt, target);
    }

    // Visit the loop body itself.
    VarDecl *bodyVar = visit(forEachStmt->getBody());
    if (!bodyVar)
      return nullptr;

    // If there's no constraint system, there is nothing left to visit.
    if (!cs)
      return nullptr;

    // Form a variable of array type that will capture the result of each
    // iteration of the loop. We need a fresh type variable to remove the
    // lvalue-ness of the array variable.
    SourceLoc loc = forEachStmt->getForLoc();
    VarDecl *arrayVar = buildVar(loc);
    Type arrayElementType = cs->createTypeVariable(
        cs->getConstraintLocator(forEachStmt), 0);
    cs->addConstraint(
        ConstraintKind::Equal, cs->getType(bodyVar), arrayElementType,
        cs->getConstraintLocator(
          forEachStmt, ConstraintLocator::RValueAdjustment));
    Type arrayType = ArraySliceType::get(arrayElementType);
    cs->setType(arrayVar, arrayType);

    // Form an initialization of the array to an empty array literal.
    Expr *arrayInitExpr = ArrayExpr::create(ctx, loc, { }, { }, loc);
    cs->setContextualType(
        arrayInitExpr, TypeLoc::withoutLoc(arrayType), CTP_CannotFail);
    arrayInitExpr = cs->generateConstraints(arrayInitExpr, dc);
    if (!arrayInitExpr) {
      hadError = true;
      return nullptr;
    }
    cs->addConstraint(
        ConstraintKind::Equal, cs->getType(arrayInitExpr), arrayType,
        cs->getConstraintLocator(
          arrayInitExpr, LocatorPathElt::ContextualType()));

    // Form a call to Array.append(_:) to add the result of executing each
    // iteration of the loop body to the array formed above.
    SourceLoc endLoc = forEachStmt->getEndLoc();
    auto arrayVarRef = buildVarRef(arrayVar, endLoc);
    auto arrayAppendRef = new (ctx) UnresolvedDotExpr(
        arrayVarRef, endLoc, DeclNameRef(ctx.getIdentifier("append")),
        DeclNameLoc(endLoc), /*implicit=*/true);
    arrayAppendRef->setFunctionRefKind(FunctionRefKind::SingleApply);
    auto bodyVarRef = buildVarRef(bodyVar, endLoc);
    Expr *arrayAppendCall = CallExpr::create(
        ctx, arrayAppendRef, endLoc, { bodyVarRef } , { Identifier() },
        { endLoc }, endLoc, /*trailingClosures=*/{}, /*implicit=*/true);
    arrayAppendCall = cs->generateConstraints(arrayAppendCall, dc);
    if (!arrayAppendCall) {
      hadError = true;
      return nullptr;
    }

    // Form the final call to buildArray(arrayVar) to allow the function
    // builder to reshape the array into whatever it wants as the result of
    // the for-each loop.
    auto finalArrayVarRef = buildVarRef(arrayVar, endLoc);
    auto buildArrayCall = buildCallIfWanted(
        endLoc, ctx.Id_buildArray, { finalArrayVarRef }, { Identifier() });
    assert(buildArrayCall);
    buildArrayCall = cs->generateConstraints(buildArrayCall, dc);
    if (!buildArrayCall) {
      hadError = true;
      return nullptr;
    }

    // Form a final variable for the for-each expression itself, which will
    // be initialized with the call to the function builder's buildArray(_:).
    auto finalForEachVar = buildVar(loc);
    cs->setType(finalForEachVar, cs->getType(buildArrayCall));
    applied.capturedStmts.insert(
      {forEachStmt, {
          finalForEachVar,
          { arrayVarRef, arrayInitExpr, arrayAppendCall, buildArrayCall }}});

    return finalForEachVar;
  }

  /// Visit a throw statement, which never produces a result.
  VarDecl *visitThrowStmt(ThrowStmt *throwStmt) {
    Type exnType = ctx.getErrorDecl()->getDeclaredInterfaceType();
    if (!exnType) {
      hadError = true;
    }

    if (cs) {
     SolutionApplicationTarget target(
         throwStmt->getSubExpr(), dc, CTP_ThrowStmt, exnType,
         /*isDiscarded=*/false);
     if (cs->generateConstraints(target, FreeTypeVariableBinding::Disallow))
       hadError = true;

     cs->setSolutionApplicationTarget(throwStmt, target);
   }

    return nullptr;
  }

  CONTROL_FLOW_STMT(Guard)
  CONTROL_FLOW_STMT(While)
  CONTROL_FLOW_STMT(DoCatch)
  CONTROL_FLOW_STMT(RepeatWhile)
  CONTROL_FLOW_STMT(Case)
  CONTROL_FLOW_STMT(Break)
  CONTROL_FLOW_STMT(Continue)
  CONTROL_FLOW_STMT(Fallthrough)
  CONTROL_FLOW_STMT(Fail)
  CONTROL_FLOW_STMT(PoundAssert)

#undef CONTROL_FLOW_STMT
};

/// Describes the target into which the result of a particular statement in
/// a closure involving a function builder should be written.
struct FunctionBuilderTarget {
  enum Kind {
    /// The resulting value is returned from the closure.
    ReturnValue,
    /// The temporary variable into which the result should be assigned.
    TemporaryVar,
    /// An expression to evaluate at the end of the block, allowing the update
    /// of some state from an outer scope.
    Expression,
  } kind;

  /// Captured variable information.
  std::pair<VarDecl *, llvm::TinyPtrVector<Expr *>> captured;

  static FunctionBuilderTarget forReturn(Expr *expr) {
    return FunctionBuilderTarget{ReturnValue, {nullptr, {expr}}};
  }

  static FunctionBuilderTarget forAssign(VarDecl *temporaryVar,
                                         llvm::TinyPtrVector<Expr *> exprs) {
    return FunctionBuilderTarget{TemporaryVar, {temporaryVar, exprs}};
  }

  static FunctionBuilderTarget forExpression(Expr *expr) {
    return FunctionBuilderTarget{Expression, { nullptr, { expr }}};
  }
};

/// Handles the rewrite of the body of a closure to which a function builder
/// has been applied.
class BuilderClosureRewriter
    : public StmtVisitor<BuilderClosureRewriter, Stmt *, FunctionBuilderTarget> {
  ASTContext &ctx;
  const Solution &solution;
  DeclContext *dc;
  AppliedBuilderTransform builderTransform;
  std::function<
      Optional<SolutionApplicationTarget> (SolutionApplicationTarget)>
        rewriteTarget;

  /// Retrieve the temporary variable that will be used to capture the
  /// value of the given expression.
  AppliedBuilderTransform::RecordedExpr takeCapturedExpr(Expr *expr) {
    auto found = builderTransform.capturedExprs.find(expr);
    assert(found != builderTransform.capturedExprs.end());

    // Set the type of the temporary variable.
    auto recorded = found->second;
    if (auto temporaryVar = recorded.temporaryVar) {
      Type type = solution.simplifyType(solution.getType(temporaryVar));
      temporaryVar->setInterfaceType(type->mapTypeOutOfContext());
    }

    // Erase the captured expression, so we're sure we never do this twice.
    builderTransform.capturedExprs.erase(found);
    return recorded;
  }

  /// Rewrite an expression without any particularly special context.
  Expr *rewriteExpr(Expr *expr) {
    auto result = rewriteTarget(
      SolutionApplicationTarget(expr, dc, CTP_Unused, Type(),
                                /*isDiscarded=*/false));
    if (result)
      return result->getAsExpr();

    return nullptr;
  }

public:
  /// Retrieve information about a captured statement.
  std::pair<VarDecl *, llvm::TinyPtrVector<Expr *>>
  takeCapturedStmt(Stmt *stmt) {
    auto found = builderTransform.capturedStmts.find(stmt);
    assert(found != builderTransform.capturedStmts.end());

    // Set the type of the temporary variable.
    auto temporaryVar = found->second.first;
    Type type = solution.simplifyType(solution.getType(temporaryVar));
    temporaryVar->setInterfaceType(type->mapTypeOutOfContext());

    // Take the expressions.
    auto exprs = std::move(found->second.second);

    // Erase the statement, so we're sure we never do this twice.
    builderTransform.capturedStmts.erase(found);
    return std::make_pair(temporaryVar, std::move(exprs));
  }

private:
  /// Build the statement or expression to initialize the target.
  ASTNode initializeTarget(FunctionBuilderTarget target) {
    assert(target.captured.second.size() == 1);
    auto capturedExpr = target.captured.second.front();
    SourceLoc implicitLoc = capturedExpr->getEndLoc();
    switch (target.kind) {
    case FunctionBuilderTarget::ReturnValue: {
      // Return the expression.
      Type bodyResultType =
          solution.simplifyType(builderTransform.bodyResultType);

      SolutionApplicationTarget returnTarget(
          capturedExpr, dc, CTP_ReturnStmt, bodyResultType,
          /*isDiscarded=*/false);
      Expr *resultExpr = nullptr;
      if (auto resultTarget = rewriteTarget(returnTarget))
        resultExpr = resultTarget->getAsExpr();

      return new (ctx) ReturnStmt(implicitLoc, resultExpr);
    }

    case FunctionBuilderTarget::TemporaryVar: {
      // Assign the expression into a variable.
      auto temporaryVar = target.captured.first;
      auto declRef = new (ctx) DeclRefExpr(
          temporaryVar, DeclNameLoc(implicitLoc), /*implicit=*/true);
      declRef->setType(LValueType::get(temporaryVar->getType()));

      // Load the right-hand side if needed.
      auto finalCapturedExpr = rewriteExpr(capturedExpr);
      if (finalCapturedExpr->getType()->hasLValueType()) {
        finalCapturedExpr =
            TypeChecker::addImplicitLoadExpr(ctx, finalCapturedExpr);
      }

      auto assign = new (ctx) AssignExpr(
          declRef, implicitLoc, finalCapturedExpr, /*implicit=*/true);
      assign->setType(TupleType::getEmpty(ctx));
      return assign;
    }

    case FunctionBuilderTarget::Expression:
      // Execute the expression.
      return rewriteExpr(capturedExpr);
    }
    llvm_unreachable("invalid function builder target");
  }

  /// Declare the given temporary variable, adding the appropriate
  /// entries to the elements of a brace stmt.
  void declareTemporaryVariable(VarDecl *temporaryVar,
                                std::vector<ASTNode> &elements,
                                Expr *initExpr = nullptr) {
    if (!temporaryVar)
      return;

    // Form a new pattern binding to bind the temporary variable to the
    // transformed expression.
    auto pattern = NamedPattern::createImplicit(ctx, temporaryVar);
    pattern->setType(temporaryVar->getType());

    auto pbd = PatternBindingDecl::create(
        ctx, SourceLoc(), StaticSpellingKind::None, temporaryVar->getLoc(),
        pattern, SourceLoc(), initExpr, dc);
    if (temporaryVar->isImplicit())
      pbd->setImplicit();
    elements.push_back(temporaryVar);
    elements.push_back(pbd);
  }

  /// Produce a final type-checked pattern binding.
  void finishPatternBindingDecl(PatternBindingDecl *patternBinding) {
    for (unsigned index : range(patternBinding->getNumPatternEntries())) {
      // Find the solution application target for this.
      auto knownTarget =
          *solution.getConstraintSystem().getSolutionApplicationTarget(
            {patternBinding, index});

      // Rewrite the target.
      auto resultTarget = rewriteTarget(knownTarget);
      if (!resultTarget)
        continue;

      patternBinding->setPattern(
          index, resultTarget->getInitializationPattern(),
          resultTarget->getDeclContext());
      patternBinding->setInit(index, resultTarget->getAsExpr());
    }
  }

public:
  BuilderClosureRewriter(
      const Solution &solution,
      DeclContext *dc,
      const AppliedBuilderTransform &builderTransform,
      std::function<
          Optional<SolutionApplicationTarget> (SolutionApplicationTarget)>
            rewriteTarget
    ) : ctx(solution.getConstraintSystem().getASTContext()),
        solution(solution), dc(dc), builderTransform(builderTransform),
        rewriteTarget(rewriteTarget) { }

  Stmt *visitBraceStmt(BraceStmt *braceStmt, FunctionBuilderTarget target,
                       Optional<FunctionBuilderTarget> innerTarget = None) {
    std::vector<ASTNode> newElements;

    // If there is an "inner" target corresponding to this brace, declare
    // it's temporary variable if needed.
    if (innerTarget) {
      declareTemporaryVariable(innerTarget->captured.first, newElements);
    }

    for (auto node : braceStmt->getElements()) {
      // Implicit returns in single-expression function bodies are treated
      // as the expression.
      if (auto returnStmt =
              dyn_cast_or_null<ReturnStmt>(node.dyn_cast<Stmt *>())) {
        assert(returnStmt->isImplicit());
        node = returnStmt->getResult();
      }

      if (auto expr = node.dyn_cast<Expr *>()) {
        // Skip error expressions.
        if (isa<ErrorExpr>(expr))
          continue;

        // Each expression turns into a 'let' that captures the value of
        // the expression.
        auto recorded = takeCapturedExpr(expr);

        // Rewrite the expression
        Expr *finalExpr = rewriteExpr(recorded.generatedExpr);

        // Form a new pattern binding to bind the temporary variable to the
        // transformed expression.
        declareTemporaryVariable(recorded.temporaryVar, newElements, finalExpr);
        continue;
      }

      if (auto stmt = node.dyn_cast<Stmt *>()) {
        // "throw" statements produce no value. Transform them directly.
        if (auto throwStmt = dyn_cast<ThrowStmt>(stmt)) {
          if (auto newStmt = visitThrowStmt(throwStmt)) {
            newElements.push_back(stmt);
          }
          continue;
        }

        // Each statement turns into a (potential) temporary variable
        // binding followed by the statement itself.
        auto captured = takeCapturedStmt(stmt);

        declareTemporaryVariable(captured.first, newElements);

        Stmt *finalStmt = visit(
            stmt,
            FunctionBuilderTarget{FunctionBuilderTarget::TemporaryVar,
                                  std::move(captured)});
        newElements.push_back(finalStmt);
        continue;
      }

      auto decl = node.get<Decl *>();

      // Skip #if declarations.
      if (isa<IfConfigDecl>(decl)) {
        newElements.push_back(decl);
        continue;
      }

      // Diagnose #warning / #error during application.
      if (auto poundDiag = dyn_cast<PoundDiagnosticDecl>(decl)) {
        TypeChecker::typeCheckDecl(poundDiag);
        newElements.push_back(decl);
        continue;
      }

      // Skip variable declarations; they're always part of a pattern
      // binding.
      if (isa<VarDecl>(decl)) {
        newElements.push_back(decl);
        continue;
      }

      // Handle pattern bindings.
      if (auto patternBinding = dyn_cast<PatternBindingDecl>(decl)) {
        finishPatternBindingDecl(patternBinding);
        newElements.push_back(decl);
        continue;
      }

      llvm_unreachable("Cannot yet handle declarations");
    }

    // If there is an "inner" target corresponding to this brace, initialize
    // it.
    if (innerTarget) {
      newElements.push_back(initializeTarget(*innerTarget));
    }

    // Capture the result of the buildBlock() call in the manner requested
    // by the caller.
    newElements.push_back(initializeTarget(target));

    return BraceStmt::create(ctx, braceStmt->getLBraceLoc(), newElements,
                             braceStmt->getRBraceLoc());
  }

  Stmt *visitIfStmt(IfStmt *ifStmt, FunctionBuilderTarget target) {
    // Rewrite the condition.
    if (auto condition = rewriteTarget(
            SolutionApplicationTarget(ifStmt->getCond(), dc)))
      ifStmt->setCond(*condition->getAsStmtCondition());

    assert(target.kind == FunctionBuilderTarget::TemporaryVar);
    auto temporaryVar = target.captured.first;

    // Translate the "then" branch.
    auto capturedThen = takeCapturedStmt(ifStmt->getThenStmt());
    auto newThen = visitBraceStmt(cast<BraceStmt>(ifStmt->getThenStmt()),
          FunctionBuilderTarget::forAssign(
            temporaryVar, {target.captured.second[0]}),
          FunctionBuilderTarget::forAssign(
            capturedThen.first, {capturedThen.second.front()}));
    ifStmt->setThenStmt(newThen);

    // Look for a #available condition. If there is one, we need to check
    // that the resulting type of the "then" doesn't refer to any types that
    // are unavailable in the enclosing context.
    //
    // Note that this is for staging in support for buildLimitedAvailability();
    // the diagnostic is currently a warning, so that existing code that
    // compiles today will continue to compile. Once function builder types
    // have had the chance to adopt buildLimitedAvailability(), we'll upgrade
    // this warning to an error.
    if (auto availabilityCond = findAvailabilityCondition(ifStmt->getCond())) {
      SourceLoc loc = availabilityCond->getStartLoc();
      Type thenBodyType = solution.simplifyType(
          solution.getType(target.captured.second[0]));
      thenBodyType.findIf([&](Type type) {
        auto nominal = type->getAnyNominal();
        if (!nominal)
          return false;

        if (auto reason = TypeChecker::checkDeclarationAvailability(
                              nominal, loc, dc)) {
          ctx.Diags.diagnose(
              loc, diag::function_builder_missing_limited_availability,
              builderTransform.builderType);

          // Add a note to the function builder with a stub for
          // buildLimitedAvailability().
          if (auto builder = builderTransform.builderType->getAnyNominal()) {
            SourceLoc buildInsertionLoc;
            std::string stubIndent;
            Type componentType;
            std::tie(buildInsertionLoc, stubIndent, componentType) =
                determineFunctionBuilderBuildFixItInfo(builder);
            if (buildInsertionLoc.isValid()) {
              std::string fixItString;
              {
                llvm::raw_string_ostream out(fixItString);
                printFunctionBuilderBuildFunction(
                    builder, componentType,
                    FunctionBuilderBuildFunction::BuildLimitedAvailability,
                    stubIndent, out);

                builder->diagnose(
                    diag::function_builder_missing_build_limited_availability,
                    builderTransform.builderType)
                  .fixItInsert(buildInsertionLoc, fixItString);
              }
            }
          }

          return true;
        }

        return false;
      });
    }

    if (auto elseBraceStmt =
            dyn_cast_or_null<BraceStmt>(ifStmt->getElseStmt())) {
      // Translate the "else" branch when it's a stmt-brace.
      auto capturedElse = takeCapturedStmt(elseBraceStmt);
      Stmt *newElse = visitBraceStmt(
          elseBraceStmt,
          FunctionBuilderTarget::forAssign(
            temporaryVar, {target.captured.second[1]}),
          FunctionBuilderTarget::forAssign(
            capturedElse.first, {capturedElse.second.front()}));
      ifStmt->setElseStmt(newElse);
    } else if (auto elseIfStmt = cast_or_null<IfStmt>(ifStmt->getElseStmt())){
      // Translate the "else" branch when it's an else-if.
      auto capturedElse = takeCapturedStmt(elseIfStmt);
      std::vector<ASTNode> newElseElements;
      declareTemporaryVariable(capturedElse.first, newElseElements);
      newElseElements.push_back(
          visitIfStmt(
            elseIfStmt,
            FunctionBuilderTarget::forAssign(
              capturedElse.first, capturedElse.second)));
      newElseElements.push_back(
          initializeTarget(
            FunctionBuilderTarget::forAssign(
              temporaryVar, {target.captured.second[1]})));

      Stmt *newElse = BraceStmt::create(
          ctx, elseIfStmt->getStartLoc(), newElseElements,
          elseIfStmt->getEndLoc());
      ifStmt->setElseStmt(newElse);
    } else {
      // Form an "else" brace containing an assignment to the temporary
      // variable.
      auto init = initializeTarget(
          FunctionBuilderTarget::forAssign(
            temporaryVar, {target.captured.second[1]}));
      auto newElse = BraceStmt::create(
          ctx, ifStmt->getEndLoc(), { init }, ifStmt->getEndLoc());
      ifStmt->setElseStmt(newElse);
    }

    return ifStmt;
  }

  Stmt *visitDoStmt(DoStmt *doStmt, FunctionBuilderTarget target) {
    // Each statement turns into a (potential) temporary variable
    // binding followed by the statement itself.
    auto body = cast<BraceStmt>(doStmt->getBody());
    auto captured = takeCapturedStmt(body);

    auto newInnerBody = cast<BraceStmt>(
        visitBraceStmt(
          body,
          target,
          FunctionBuilderTarget::forAssign(
            captured.first, {captured.second.front()})));
    doStmt->setBody(newInnerBody);
    return doStmt;
  }

  Stmt *visitSwitchStmt(SwitchStmt *switchStmt, FunctionBuilderTarget target) {
    // Translate the subject expression.
    ConstraintSystem &cs = solution.getConstraintSystem();
    auto subjectTarget =
        rewriteTarget(*cs.getSolutionApplicationTarget(switchStmt));
    if (!subjectTarget)
      return nullptr;

    switchStmt->setSubjectExpr(subjectTarget->getAsExpr());

    // Handle any declaration nodes within the case list first; we'll
    // handle the cases in a second pass.
    for (auto child : switchStmt->getRawCases()) {
      if (auto decl = child.dyn_cast<Decl *>()) {
        TypeChecker::typeCheckDecl(decl);
      }
    }

    // Translate all of the cases.
    bool limitExhaustivityChecks = false;
    assert(target.kind == FunctionBuilderTarget::TemporaryVar);
    auto temporaryVar = target.captured.first;
    unsigned caseIndex = 0;
    for (auto caseStmt : switchStmt->getCases()) {
      if (!visitCaseStmt(
            caseStmt,
            FunctionBuilderTarget::forAssign(
              temporaryVar, {target.captured.second[caseIndex]})))
        return nullptr;

      // Check restrictions on '@unknown'.
      if (caseStmt->hasUnknownAttr()) {
        checkUnknownAttrRestrictions(
            cs.getASTContext(), caseStmt, limitExhaustivityChecks);
      }

      ++caseIndex;
    }

    TypeChecker::checkSwitchExhaustiveness(
        switchStmt, dc, limitExhaustivityChecks);

    return switchStmt;
  }

  Stmt *visitCaseStmt(CaseStmt *caseStmt, FunctionBuilderTarget target) {
    // Translate the patterns and guard expressions for each case label item.
    for (auto &caseLabelItem : caseStmt->getMutableCaseLabelItems()) {
      SolutionApplicationTarget caseLabelTarget(&caseLabelItem, dc);
      if (!rewriteTarget(caseLabelTarget))
        return nullptr;
    }

    // Transform the body of the case.
    auto body = cast<BraceStmt>(caseStmt->getBody());
    auto captured = takeCapturedStmt(body);
    auto newInnerBody = cast<BraceStmt>(
        visitBraceStmt(
          body,
          target,
          FunctionBuilderTarget::forAssign(
            captured.first, {captured.second.front()})));
    caseStmt->setBody(newInnerBody);

    return caseStmt;
  }

  Stmt *visitForEachStmt(
      ForEachStmt *forEachStmt, FunctionBuilderTarget target) {
    // Translate the for-each loop header.
    ConstraintSystem &cs = solution.getConstraintSystem();
    auto forEachTarget =
        rewriteTarget(*cs.getSolutionApplicationTarget(forEachStmt));
    if (!forEachTarget)
      return nullptr;

    const auto &captured = target.captured;
    auto finalForEachVar = captured.first;
    auto arrayVarRef = captured.second[0];
    auto arrayVar = cast<VarDecl>(cast<DeclRefExpr>(arrayVarRef)->getDecl());
    auto arrayInitExpr = captured.second[1];
    auto arrayAppendCall = captured.second[2];
    auto buildArrayCall = captured.second[3];

    // Collect the three steps to initialize the array variable to an
    // empty array, execute the loop to collect the results of each iteration,
    // then form the buildArray() call to the write the result.
    std::vector<ASTNode> outerBodySteps;

    // Step 1: Declare and initialize the array variable.
    arrayVar->setInterfaceType(solution.simplifyType(cs.getType(arrayVar)));
    arrayInitExpr = rewriteExpr(arrayInitExpr);
    declareTemporaryVariable(arrayVar, outerBodySteps, arrayInitExpr);

    // Step 2. Transform the body of the for-each statement. Each iteration
    // will append the result of executing the loop body to the array.
    auto body = forEachStmt->getBody();
    auto capturedBody = takeCapturedStmt(body);
    auto newBody = cast<BraceStmt>(
        visitBraceStmt(
          body,
          FunctionBuilderTarget::forExpression(arrayAppendCall),
          FunctionBuilderTarget::forAssign(
            capturedBody.first, {capturedBody.second.front()})));
    forEachStmt->setBody(newBody);
    outerBodySteps.push_back(forEachStmt);

    // Step 3. Perform the buildArray() call to turn the array of results
    // collected from the iterations into a single value under the control of
    // the function builder.
    outerBodySteps.push_back(
        initializeTarget(
          FunctionBuilderTarget::forAssign(finalForEachVar, {buildArrayCall})));

    // Form a brace statement to put together the three main steps for the
    // for-each loop translation outlined above.
    return BraceStmt::create(
        ctx, forEachStmt->getStartLoc(), outerBodySteps, newBody->getEndLoc());
  }

  Stmt *visitThrowStmt(ThrowStmt *throwStmt) {
    // Rewrite the error.
    auto target = *solution.getConstraintSystem()
        .getSolutionApplicationTarget(throwStmt);
    if (auto result = rewriteTarget(target))
      throwStmt->setSubExpr(result->getAsExpr());
    else
      return nullptr;

    return throwStmt;
  }

  Stmt *visitThrowStmt(ThrowStmt *throwStmt, FunctionBuilderTarget target) {
    llvm_unreachable("Throw statements produce no value");
  }

#define UNHANDLED_FUNCTION_BUILDER_STMT(STMT) \
  Stmt *visit##STMT##Stmt(STMT##Stmt *stmt, FunctionBuilderTarget target) { \
    llvm_unreachable("Function builders do not allow statement of kind " \
                     #STMT); \
  }

  UNHANDLED_FUNCTION_BUILDER_STMT(Return)
  UNHANDLED_FUNCTION_BUILDER_STMT(Yield)
  UNHANDLED_FUNCTION_BUILDER_STMT(Guard)
  UNHANDLED_FUNCTION_BUILDER_STMT(While)
  UNHANDLED_FUNCTION_BUILDER_STMT(Defer)
  UNHANDLED_FUNCTION_BUILDER_STMT(DoCatch)
  UNHANDLED_FUNCTION_BUILDER_STMT(RepeatWhile)
  UNHANDLED_FUNCTION_BUILDER_STMT(Break)
  UNHANDLED_FUNCTION_BUILDER_STMT(Continue)
  UNHANDLED_FUNCTION_BUILDER_STMT(Fallthrough)
  UNHANDLED_FUNCTION_BUILDER_STMT(Fail)
  UNHANDLED_FUNCTION_BUILDER_STMT(PoundAssert)
#undef UNHANDLED_FUNCTION_BUILDER_STMT
};

} // end anonymous namespace

BraceStmt *swift::applyFunctionBuilderTransform(
    const Solution &solution,
    AppliedBuilderTransform applied,
    BraceStmt *body,
    DeclContext *dc,
    std::function<
        Optional<SolutionApplicationTarget> (SolutionApplicationTarget)>
          rewriteTarget) {
  BuilderClosureRewriter rewriter(solution, dc, applied, rewriteTarget);
  auto captured = rewriter.takeCapturedStmt(body);
  return cast<BraceStmt>(
    rewriter.visitBraceStmt(
      body,
      FunctionBuilderTarget::forReturn(applied.returnExpr),
      FunctionBuilderTarget::forAssign(
        captured.first, captured.second)));
}

Optional<BraceStmt *> TypeChecker::applyFunctionBuilderBodyTransform(
    FuncDecl *func, Type builderType) {
  // Pre-check the body: pre-check any expressions in it and look
  // for return statements.
  //
  // If we encountered an error or there was an explicit result type,
  // bail out and report that to the caller.
  auto &ctx = func->getASTContext();
  auto request =
      PreCheckFunctionBuilderRequest{{AnyFunctionRef(func),
                                      /*SuppressDiagnostics=*/false}};
  switch (evaluateOrDefault(ctx.evaluator, request,
                            FunctionBuilderBodyPreCheck::Error)) {
  case FunctionBuilderBodyPreCheck::Okay:
    // If the pre-check was okay, apply the function-builder transform.
    break;

  case FunctionBuilderBodyPreCheck::Error:
    return nullptr;

  case FunctionBuilderBodyPreCheck::HasReturnStmt: {
    // One or more explicit 'return' statements were encountered, which
    // disables the function builder transform. Warn when we do this.
    auto returnStmts = findReturnStatements(func);
    assert(!returnStmts.empty());

    ctx.Diags.diagnose(
        returnStmts.front()->getReturnLoc(),
        diag::function_builder_disabled_by_return, builderType);

    // Note that one can remove the function builder attribute.
    auto attr = func->getAttachedFunctionBuilder();
    if (!attr) {
      if (auto accessor = dyn_cast<AccessorDecl>(func)) {
        attr = accessor->getStorage()->getAttachedFunctionBuilder();
      }
    }

    if (attr) {
      ctx.Diags.diagnose(
          attr->getLocation(), diag::function_builder_remove_attr)
        .fixItRemove(attr->getRangeWithAt());
      attr->setInvalid();
    }

    // Note that one can remove all of the return statements.
    {
      auto diag = ctx.Diags.diagnose(
          returnStmts.front()->getReturnLoc(),
          diag::function_builder_remove_returns);
      for (auto returnStmt : returnStmts) {
        diag.fixItRemove(returnStmt->getReturnLoc());
      }
    }

    return None;
  }
  }

  ConstraintSystemOptions options = ConstraintSystemFlags::AllowFixes;
  auto resultInterfaceTy = func->getResultInterfaceType();
  auto resultContextType = func->mapTypeIntoContext(resultInterfaceTy);

  // Determine whether we're inferring the underlying type for the opaque
  // result type of this function.
  ConstraintKind resultConstraintKind = ConstraintKind::Conversion;
  if (auto opaque = resultContextType->getAs<OpaqueTypeArchetypeType>()) {
    if (opaque->getDecl()->isOpaqueReturnTypeOfFunction(func)) {
      resultConstraintKind = ConstraintKind::OpaqueUnderlyingType;
    }
  }

  // Build a constraint system in which we can check the body of the function.
  ConstraintSystem cs(func, options);

  if (auto result = cs.matchFunctionBuilder(
          func, builderType, resultContextType, resultConstraintKind,
          cs.getConstraintLocator(func->getBody()))) {
    if (result->isFailure())
      return nullptr;
  }

  // Solve the constraint system.
  SmallVector<Solution, 4> solutions;
  if (cs.solve(solutions) || solutions.size() != 1) {
    // Try to fix the system or provide a decent diagnostic.
    auto salvagedResult = cs.salvage();
    switch (salvagedResult.getKind()) {
    case SolutionResult::Kind::Success:
      solutions.clear();
      solutions.push_back(std::move(salvagedResult).takeSolution());
      break;

    case SolutionResult::Kind::Error:
    case SolutionResult::Kind::Ambiguous:
      return nullptr;

    case SolutionResult::Kind::UndiagnosedError:
      cs.diagnoseFailureFor(SolutionApplicationTarget(func));
      salvagedResult.markAsDiagnosed();
      return nullptr;

    case SolutionResult::Kind::TooComplex:
      func->diagnose(diag::expression_too_complex)
        .highlight(func->getBodySourceRange());
      salvagedResult.markAsDiagnosed();
      return nullptr;
    }

    // The system was salvaged; continue on as if nothing happened.
  }

  if (cs.isDebugMode()) {
    auto &log = llvm::errs();
    log << "--- Applying Solution ---\n";
    solutions.front().dump(log);
    log << '\n';
  }

  // FIXME: Shouldn't need to do this.
  cs.applySolution(solutions.front());

  // Apply the solution to the function body.
  if (auto result = cs.applySolution(
          solutions.front(),
          SolutionApplicationTarget(func))) {
    performSyntacticDiagnosticsForTarget(*result, /*isExprStmt*/ false);
    return result->getFunctionBody();
  }

  return nullptr;
}

Optional<ConstraintSystem::TypeMatchResult>
ConstraintSystem::matchFunctionBuilder(
    AnyFunctionRef fn, Type builderType, Type bodyResultType,
    ConstraintKind bodyResultConstraintKind,
    ConstraintLocatorBuilder locator) {
  auto builder = builderType->getAnyNominal();
  assert(builder && "Bad function builder type");
  assert(builder->getAttrs().hasAttribute<FunctionBuilderAttr>());

  if (InvalidFunctionBuilderBodies.count(fn)) {
    (void)recordFix(
        IgnoreInvalidFunctionBuilderBody::duringConstraintGeneration(
            *this, getConstraintLocator(fn.getBody())));
    return getTypeMatchSuccess();
  }

  // Pre-check the body: pre-check any expressions in it and look
  // for return statements.
  auto request =
      PreCheckFunctionBuilderRequest{{fn, /*SuppressDiagnostics=*/true}};
  switch (evaluateOrDefault(getASTContext().evaluator, request,
                            FunctionBuilderBodyPreCheck::Error)) {
  case FunctionBuilderBodyPreCheck::Okay:
    // If the pre-check was okay, apply the function-builder transform.
    break;

  case FunctionBuilderBodyPreCheck::Error: {
    if (!shouldAttemptFixes())
      return getTypeMatchFailure(locator);

    if (recordFix(IgnoreInvalidFunctionBuilderBody::duringPreCheck(
            *this, getConstraintLocator(fn.getBody()))))
      return getTypeMatchFailure(locator);

    return getTypeMatchSuccess();
  }

  case FunctionBuilderBodyPreCheck::HasReturnStmt:
    // If the body has a return statement, suppress the transform but
    // continue solving the constraint system.
    return None;
  }

  // Check the form of this body to see if we can apply the
  // function-builder translation at all.
  auto dc = fn.getAsDeclContext();
  {
    // Check whether we can apply this specific function builder.
    BuilderClosureVisitor visitor(getASTContext(), nullptr, dc, builderType,
                                  bodyResultType);

    // If we saw a control-flow statement or declaration that the builder
    // cannot handle, we don't have a well-formed function builder application.
    if (auto unhandledNode = visitor.check(fn.getBody())) {
      // If we aren't supposed to attempt fixes, fail.
      if (!shouldAttemptFixes()) {
        return getTypeMatchFailure(locator);
      }

      // Record the first unhandled construct as a fix.
      if (recordFix(
              SkipUnhandledConstructInFunctionBuilder::create(
                *this, unhandledNode, builder,
                getConstraintLocator(locator)))) {
        return getTypeMatchFailure(locator);
      }
    }
  }

  BuilderClosureVisitor visitor(getASTContext(), this, dc, builderType,
                                bodyResultType);

  Optional<AppliedBuilderTransform> applied = None;
  {
    DiagnosticTransaction transaction(dc->getASTContext().Diags);

    applied = visitor.apply(fn.getBody());
    if (!applied)
      return getTypeMatchFailure(locator);

    if (transaction.hasErrors()) {
      InvalidFunctionBuilderBodies.insert(fn);

      if (recordFix(
              IgnoreInvalidFunctionBuilderBody::duringConstraintGeneration(
                  *this, getConstraintLocator(fn.getBody()))))
        return getTypeMatchFailure(locator);

      return getTypeMatchSuccess();
    }
  }

  Type transformedType = getType(applied->returnExpr);
  assert(transformedType && "Missing type");

  // Record the transformation.
  assert(std::find_if(
      functionBuilderTransformed.begin(),
      functionBuilderTransformed.end(),
      [&](const std::pair<AnyFunctionRef, AppliedBuilderTransform> &elt) {
        return elt.first == fn;
      }) == functionBuilderTransformed.end() &&
         "already transformed this body along this path!?!");
  functionBuilderTransformed.push_back(
      std::make_pair(fn, std::move(*applied)));

  // If builder is applied to the closure expression then
  // `closure body` to `closure result` matching should
  // use special locator.
  if (auto *closure = fn.getAbstractClosureExpr()) {
    locator = getConstraintLocator(closure, ConstraintLocator::ClosureResult);
  } else {
    locator = getConstraintLocator(fn.getAbstractFunctionDecl(),
                                   ConstraintLocator::FunctionBuilderBodyResult);
  }

  // Bind the body result type to the type of the transformed expression.
  addConstraint(bodyResultConstraintKind, transformedType, bodyResultType,
                locator);
  return getTypeMatchSuccess();
}

namespace {

/// Pre-check all the expressions in the body.
class PreCheckFunctionBuilderApplication : public ASTWalker {
  AnyFunctionRef Fn;
  bool SkipPrecheck = false;
  bool SuppressDiagnostics = false;
  std::vector<ReturnStmt *> ReturnStmts;
  bool HasError = false;

  bool hasReturnStmt() const { return !ReturnStmts.empty(); }

public:
  PreCheckFunctionBuilderApplication(AnyFunctionRef fn, bool skipPrecheck,
                                     bool suppressDiagnostics)
      : Fn(fn), SkipPrecheck(skipPrecheck),
        SuppressDiagnostics(suppressDiagnostics) {}

  const std::vector<ReturnStmt *> getReturnStmts() const { return ReturnStmts; }

  FunctionBuilderBodyPreCheck run() {
    Stmt *oldBody = Fn.getBody();

    Stmt *newBody = oldBody->walk(*this);

    // If the walk was aborted, it was because we had a problem of some kind.
    assert((newBody == nullptr) == HasError &&
           "unexpected short-circuit while walking body");
    if (HasError)
      return FunctionBuilderBodyPreCheck::Error;

    assert(oldBody == newBody && "pre-check walk wasn't in-place?");

    if (hasReturnStmt())
      return FunctionBuilderBodyPreCheck::HasReturnStmt;

    return FunctionBuilderBodyPreCheck::Okay;
  }

  std::pair<bool, Expr *> walkToExprPre(Expr *E) override {
    if (SkipPrecheck)
      return std::make_pair(false, E);

    // Pre-check the expression.  If this fails, abort the walk immediately.
    // Otherwise, replace the expression with the result of pre-checking.
    // In either case, don't recurse into the expression.
    {
      auto *DC = Fn.getAsDeclContext();
      auto &diagEngine = DC->getASTContext().Diags;

      // Suppress any diangostics which could be produced by this expression.
      DiagnosticTransaction transaction(diagEngine);

      HasError |= ConstraintSystem::preCheckExpression(
          E, DC, /*replaceInvalidRefsWithErrors=*/false);
      HasError |= transaction.hasErrors();

      if (SuppressDiagnostics)
        transaction.abort();

      return std::make_pair(false, HasError ? nullptr : E);
    }
  }

  std::pair<bool, Stmt *> walkToStmtPre(Stmt *S) override {
    // If we see a return statement, note it..
    if (auto returnStmt = dyn_cast<ReturnStmt>(S)) {
      if (!returnStmt->isImplicit()) {
        ReturnStmts.push_back(returnStmt);
        return std::make_pair(false, S);
      }
    }

    // Otherwise, recurse into the statement normally.
    return std::make_pair(true, S);
  }

  /// Ignore patterns.
  std::pair<bool, Pattern*> walkToPatternPre(Pattern *pat) override {
    return { false, pat };
  }
};

}

FunctionBuilderBodyPreCheck PreCheckFunctionBuilderRequest::evaluate(
    Evaluator &evaluator, PreCheckFunctionBuilderDescriptor owner) const {
  // We don't want to do the precheck if it will already have happened in
  // the enclosing expression.
  bool skipPrecheck = false;
  if (auto closure = dyn_cast_or_null<ClosureExpr>(
          owner.Fn.getAbstractClosureExpr()))
    skipPrecheck = shouldTypeCheckInEnclosingExpression(closure);

  return PreCheckFunctionBuilderApplication(
             owner.Fn, /*skipPrecheck=*/false,
             /*suppressDiagnostics=*/owner.SuppressDiagnostics)
      .run();
}

std::vector<ReturnStmt *> TypeChecker::findReturnStatements(AnyFunctionRef fn) {
  PreCheckFunctionBuilderApplication precheck(fn, /*skipPreCheck=*/true,
                                              /*SuppressDiagnostics=*/true);
  (void)precheck.run();
  return precheck.getReturnStmts();
}

bool TypeChecker::typeSupportsBuilderOp(
    Type builderType, DeclContext *dc, Identifier fnName,
    ArrayRef<Identifier> argLabels, SmallVectorImpl<ValueDecl *> *allResults) {
  bool foundMatch = false;
  SmallVector<ValueDecl *, 4> foundDecls;
  dc->lookupQualified(
      builderType, DeclNameRef(fnName),
      NL_QualifiedDefault | NL_ProtocolMembers, foundDecls);
  for (auto decl : foundDecls) {
    if (auto func = dyn_cast<FuncDecl>(decl)) {
      // Function must be static.
      if (!func->isStatic())
        continue;

      // Function must have the right argument labels, if provided.
      if (!argLabels.empty()) {
        auto funcLabels = func->getName().getArgumentNames();
        if (argLabels.size() > funcLabels.size() ||
            funcLabels.slice(0, argLabels.size()) != argLabels)
          continue;
      }

      foundMatch = true;
      break;
    }
  }

  if (allResults)
    allResults->append(foundDecls.begin(), foundDecls.end());

  return foundMatch;
}

Type swift::inferFunctionBuilderComponentType(NominalTypeDecl *builder) {
  Type componentType;

  SmallVector<ValueDecl *, 4> potentialMatches;
  ASTContext &ctx = builder->getASTContext();
  bool supportsBuildBlock = TypeChecker::typeSupportsBuilderOp(
      builder->getDeclaredInterfaceType(), builder, ctx.Id_buildBlock,
      /*argLabels=*/{}, &potentialMatches);
  if (supportsBuildBlock) {
    for (auto decl : potentialMatches) {
      auto func = dyn_cast<FuncDecl>(decl);
      if (!func || !func->isStatic())
        continue;

      // If we haven't seen a component type before, gather it.
      if (!componentType) {
        componentType = func->getResultInterfaceType();
        continue;
      }

      // If there are inconsistent component types, bail out.
      if (!componentType->isEqual(func->getResultInterfaceType())) {
        componentType = Type();
        break;
      }
    }
  }

  return componentType;
}

std::tuple<SourceLoc, std::string, Type>
swift::determineFunctionBuilderBuildFixItInfo(NominalTypeDecl *builder) {
  SourceLoc buildInsertionLoc = builder->getBraces().Start;
  std::string stubIndent;
  Type componentType;

  if (buildInsertionLoc.isInvalid())
    return std::make_tuple(buildInsertionLoc, stubIndent, componentType);

  ASTContext &ctx = builder->getASTContext();
  buildInsertionLoc = Lexer::getLocForEndOfToken(
      ctx.SourceMgr, buildInsertionLoc);

  StringRef extraIndent;
  StringRef currentIndent = Lexer::getIndentationForLine(
      ctx.SourceMgr, buildInsertionLoc, &extraIndent);
  stubIndent = (currentIndent + extraIndent).str();

  componentType = inferFunctionBuilderComponentType(builder);
  return std::make_tuple(buildInsertionLoc, stubIndent, componentType);
}

void swift::printFunctionBuilderBuildFunction(
      NominalTypeDecl *builder, Type componentType,
      FunctionBuilderBuildFunction function,
      Optional<std::string> stubIndent, llvm::raw_ostream &out) {
  // Render the component type into a string.
  std::string componentTypeString;
  if (componentType)
    componentTypeString = componentType.getString();
  else
    componentTypeString = "<#Component#>";

  // Render the code.
  ExtraIndentStreamPrinter printer(out, stubIndent.getValueOr(std::string()));

  // If we're supposed to provide a full stub, add a newline and the introducer
  // keywords.
  if (stubIndent) {
    printer.printNewline();

    if (builder->getFormalAccess() >= AccessLevel::Public)
      printer << "public ";

    printer << "static func ";
  }

  bool printedResult = false;
  switch (function) {
  case FunctionBuilderBuildFunction::BuildBlock:
    printer << "buildBlock(_ components: " << componentTypeString << "...)";
    break;

  case FunctionBuilderBuildFunction::BuildExpression:
    printer << "buildExpression(_ expression: <#Expression#>)";
    break;

  case FunctionBuilderBuildFunction::BuildOptional:
    printer << "buildOptional(_ component: " << componentTypeString << "?)";
    break;

  case FunctionBuilderBuildFunction::BuildEitherFirst:
    printer << "buildEither(first component: " << componentTypeString << ")";
    break;

  case FunctionBuilderBuildFunction::BuildEitherSecond:
    printer << "buildEither(second component: " << componentTypeString << ")";
    break;

  case FunctionBuilderBuildFunction::BuildArray:
    printer << "buildArray(_ components: [" << componentTypeString << "])";
    break;

  case FunctionBuilderBuildFunction::BuildLimitedAvailability:
    printer << "buildLimitedAvailability(_ component: " << componentTypeString
            << ")";
    break;

  case FunctionBuilderBuildFunction::BuildFinalResult:
    printer << "buildFinalResult(_ component: " << componentTypeString
            << ") -> <#Result#>";
    printedResult = true;
    break;
  }

  if (!printedResult)
    printer << " -> " << componentTypeString;

  if (stubIndent) {
    printer << " {";
    printer.printNewline();
    printer << "  <#code#>";
    printer.printNewline();
    printer << "}";
  }
}