swift-mirror/lib/AST/RequirementEnvironment.cpp

//===--- RequirementEnvironment.cpp - Requirement Environments ------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 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 the RequirementEnvironment class, which is used to
// capture how a witness to a protocol requirement maps type parameters.
//
//===----------------------------------------------------------------------===//

#include "swift/AST/RequirementEnvironment.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/ConformanceLookup.h"
#include "swift/AST/Decl.h"
#include "swift/AST/DeclContext.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/AST/GenericSignature.h"
#include "swift/AST/ProtocolConformance.h"
#include "swift/AST/TypeCheckRequests.h"
#include "swift/AST/Types.h"
#include "swift/Basic/Assertions.h"
#include "llvm/ADT/Statistic.h"

#define DEBUG_TYPE "Protocol conformance checking"

using namespace swift;

STATISTIC(NumRequirementEnvironments, "# of requirement environments");

RequirementEnvironment::RequirementEnvironment(
                                           DeclContext *conformanceDC,
                                           GenericSignature reqSig,
                                           ProtocolDecl *proto,
                                           ClassDecl *covariantSelf,
                                           RootProtocolConformance *conformance)
    : reqSig(reqSig) {
  ASTContext &ctx = conformanceDC->getASTContext();

  auto concreteType = conformanceDC->getSelfInterfaceType();
  auto conformanceSig = conformanceDC->getGenericSignatureOfContext();

  auto conformanceToWitnessThunkGenericParamFn = [&](GenericTypeParamType *genericParam)
      -> GenericTypeParamType * {
    return genericParam->withDepth(
        genericParam->getDepth() + (covariantSelf ? 1 : 0));
  };

  // This is a substitution function from the generic parameters of the
  // conforming type to the witness thunk environment.
  //
  // For structs, enums and protocols, this is a 1:1 mapping; for classes,
  // we increase the depth of each generic parameter by 1 so that we can
  // introduce a class-bound 'Self' parameter.
  //
  // This is a raw function rather than a substitution map because we need to
  // keep generic parameters as generic, even if the conformanceSig (the best
  // way to create the substitution map) equates them to concrete types.
  auto conformanceToWitnessThunkTypeFn = [&](SubstitutableType *type) -> Type {
    auto *genericParam = cast<GenericTypeParamType>(type);
    auto t = conformanceToWitnessThunkGenericParamFn(genericParam);
    if (t->isParameterPack())
      return PackType::getSingletonPackExpansion(t);

    return t;
  };

  auto substConcreteType = concreteType.subst(
      conformanceToWitnessThunkTypeFn,
      LookUpConformanceInModule());

  // Calculate the depth at which the requirement's generic parameters
  // appear in the witness thunk signature.
  unsigned depth = conformanceSig.getNextDepth();
  if (covariantSelf)
    ++depth;

  // Build a substitution map to replace the protocol's \c Self and the type
  // parameters of the requirement into a combined context that provides the
  // type parameters of the conformance context and the parameters of the
  // requirement.
  reqToWitnessThunkSigMap = SubstitutionMap::get(reqSig,
    [substConcreteType, depth, covariantSelf, &ctx]
    (SubstitutableType *type) -> Type {
      // If the conforming type is a class, the protocol 'Self' maps to
      // the class-constrained 'Self'. Otherwise, it maps to the concrete
      // type.
      if (type->isEqual(ctx.TheSelfType)) {
        if (covariantSelf)
          return ctx.TheSelfType;
        return substConcreteType;
      }
      // Other requirement generic parameters map 1:1 with their depth
      // increased appropriately.
      auto *genericParam = cast<GenericTypeParamType>(type);
      // In a protocol requirement, the only generic parameter at depth 0
      // should be 'Self', and all others at depth 1. Anything else is
      // invalid code.
      if (genericParam->getDepth() != 1)
        return Type();
      Type substGenericParam = genericParam->withDepth(depth);
      if (genericParam->isParameterPack()) {
        substGenericParam = PackType::getSingletonPackExpansion(
            substGenericParam);
      }
      return substGenericParam;
    },
    [substConcreteType, conformance, &ctx](
        InFlightSubstitution &IFS, Type type, ProtocolDecl *proto)
          -> ProtocolConformanceRef {
      // There are two special cases where we must use the Self conformance
      // given to us:
      // 1) The hard-coded DistributedActor conformance to Actor.
      // 2) A reparented protocol conformance representing the default witness table.
      //
      // In both cases, the conforming type is a type parameter, so we must
      // use the supplied conformance.
      //
      // In every other situation, we do the lookup again, even for the Self
      // conformance we already do have, to avoid having to re-create the right
      // Inherited / SpecializedProtocolConformance sugar by hand.
      if (type->isEqual(ctx.TheSelfType) &&
          substConcreteType->isEqual(ctx.TheSelfType) &&
          conformance) {
        return ProtocolConformanceRef(conformance);
      }

      return lookupConformance(type.subst(IFS), proto);
    });

  // If the requirement itself is non-generic, the witness thunk signature
  // is that of the conformance context.
  if (!covariantSelf &&
      reqSig.getGenericParams().size() == 1 &&
      reqSig.getRequirements().size() == 1) {
    witnessThunkSig = conformanceDC->getGenericSignatureOfContext()
        .getCanonicalSignature();
    if (witnessThunkSig) {
      reqToWitnessThunkSigMap = reqToWitnessThunkSigMap.subst(
          witnessThunkSig.getGenericEnvironment()
              ->getForwardingSubstitutionMap());
    }
    return;
  }

  // Construct a generic signature by collecting the constraints
  // from the requirement and the context of the conformance together,
  // because both define the capabilities of the requirement.
  SmallVector<GenericTypeParamType *, 2> genericParamTypes;

  // If the conforming type is a class, add a class-constrained 'Self'
  // parameter.
  if (covariantSelf) {
    genericParamTypes.push_back(ctx.TheSelfType);
  }

  // Now, add all generic parameters from the conforming type.
  if (conformanceSig) {
    for (auto param : conformanceSig.getGenericParams()) {
      auto substParam = conformanceToWitnessThunkGenericParamFn(param);
      genericParamTypes.push_back(substParam);
    }
  }

  // Next, add requirements.
  SmallVector<Requirement, 2> requirements;
  if (covariantSelf) {
    Requirement reqt(RequirementKind::Superclass, ctx.TheSelfType, substConcreteType);
    requirements.push_back(reqt);
  }

  if (conformanceSig) {
    for (auto &rawReq : conformanceSig.getRequirements()) {
      auto req = rawReq.subst(conformanceToWitnessThunkTypeFn,
                              LookUpConformanceInModule());
      requirements.push_back(req);
    }
  }

  // Finally, add the generic parameters from the requirement.
  for (auto genericParam : reqSig.getGenericParams().slice(1)) {
    // The only depth that makes sense is depth == 1, the generic parameters
    // of the requirement itself. Anything else is from invalid code.
    if (genericParam->getDepth() != 1) {
      return;
    }

    // Create an equivalent generic parameter at the next depth.
    auto substGenericParam = genericParam->withDepth(depth);
    genericParamTypes.push_back(substGenericParam);
  }

  ++NumRequirementEnvironments;

  // Next, add each of the requirements (mapped from the requirement's
  // interface types into the abstract type parameters).
  for (auto &rawReq : reqSig.getRequirements()) {
    auto req = rawReq.subst(reqToWitnessThunkSigMap);
    requirements.push_back(req);
  }

  witnessThunkSig = buildGenericSignature(ctx, GenericSignature(),
                                          std::move(genericParamTypes),
                                          std::move(requirements),
                                          DefaultRequirementOptions());
  reqToWitnessThunkSigMap = reqToWitnessThunkSigMap.subst(
      witnessThunkSig.getGenericEnvironment()
          ->getForwardingSubstitutionMap());
}