swift-mirror/lib/AST/Requirement.cpp

//===--- Requirement.cpp - Generic requirement ----------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2022 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 Requirement class.
//
//===----------------------------------------------------------------------===//

#include "swift/AST/ASTContext.h"
#include "swift/AST/ConformanceLookup.h"
#include "swift/AST/Requirement.h"
#include "swift/AST/Decl.h"
#include "swift/AST/GenericParamList.h"
#include "swift/AST/GenericSignature.h"
#include "swift/AST/Module.h"
#include "swift/AST/Types.h"
#include "swift/Basic/Assertions.h"
#include "RequirementMachine/RequirementLowering.h"

using namespace swift;

bool Requirement::hasError() const {
  if (getFirstType()->hasError())
    return true;

  if (getKind() != RequirementKind::Layout && getSecondType()->hasError())
    return true;

  return false;
}

bool Requirement::isCanonical() const {
  if (!getFirstType()->isCanonical())
    return false;

  switch (getKind()) {
  case RequirementKind::SameShape:
  case RequirementKind::Conformance:
  case RequirementKind::SameType:
  case RequirementKind::Superclass:
    if (!getSecondType()->isCanonical())
      return false;
    break;

  case RequirementKind::Layout:
    break;
  }

  return true;
}

/// Get the canonical form of this requirement.
Requirement Requirement::getCanonical() const {
  Type firstType = getFirstType()->getCanonicalType();

  switch (getKind()) {
  case RequirementKind::SameShape:
  case RequirementKind::Conformance:
  case RequirementKind::SameType:
  case RequirementKind::Superclass: {
    Type secondType = getSecondType()->getCanonicalType();
    return Requirement(getKind(), firstType, secondType);
  }

  case RequirementKind::Layout:
    return Requirement(getKind(), firstType, getLayoutConstraint());
  }
  llvm_unreachable("Unhandled RequirementKind in switch");
}

ProtocolDecl *Requirement::getProtocolDecl() const {
  assert(getKind() == RequirementKind::Conformance);
  return getSecondType()->castTo<ProtocolType>()->getDecl();
}

CheckRequirementResult Requirement::checkRequirement(
    SmallVectorImpl<Requirement> &subReqs,
    bool allowMissing,
    SmallVectorImpl<ProtocolConformanceRef> *isolatedConformances
) const {
  if (hasError())
    return CheckRequirementResult::SubstitutionFailure;

  auto firstType = getFirstType();

  auto expandPackRequirement = [&](PackType *packType) {
    for (auto eltType : packType->getElementTypes()) {
      // FIXME: Doesn't seem right
      if (auto *expansionType = eltType->getAs<PackExpansionType>())
        eltType = expansionType->getPatternType();

      auto kind = getKind();
      if (kind == RequirementKind::Layout) {
        subReqs.emplace_back(kind, eltType,
                             getLayoutConstraint());
      } else {
        subReqs.emplace_back(kind, eltType,
                             getSecondType());
      }
    }
    return CheckRequirementResult::PackRequirement;
  };

  switch (getKind()) {
  case RequirementKind::Conformance: {
    if (auto packType = firstType->getAs<PackType>()) {
      return expandPackRequirement(packType);
    }

    auto *proto = getProtocolDecl();

    if (firstType->isTypeParameter())
      return CheckRequirementResult::RequirementFailure;

    auto conformance = lookupConformance(
        firstType, proto, allowMissing);
    if (!conformance)
      return CheckRequirementResult::RequirementFailure;

    // Collect isolated conformances.
    if (isolatedConformances) {
      conformance.forEachIsolatedConformance(
          [&](ProtocolConformanceRef isolatedConformance) {
            isolatedConformances->push_back(isolatedConformance);
            return false;
          });
    }

    auto condReqs = conformance.getConditionalRequirements();
    if (condReqs.empty())
      return CheckRequirementResult::Success;
    subReqs.append(condReqs.begin(), condReqs.end());
    return CheckRequirementResult::ConditionalConformance;
  }

  case RequirementKind::Layout: {
    if (auto packType = firstType->getAs<PackType>()) {
      return expandPackRequirement(packType);
    }

    if (auto *archetypeType = firstType->getAs<ArchetypeType>()) {
      auto layout = archetypeType->getLayoutConstraint();
      if (layout && layout.merge(getLayoutConstraint()))
        return CheckRequirementResult::Success;

      return CheckRequirementResult::RequirementFailure;
    }

    if (getLayoutConstraint()->isClass()) {
      if (firstType->satisfiesClassConstraint())
        return CheckRequirementResult::Success;

      return CheckRequirementResult::RequirementFailure;
    }

    // TODO: Statically check other layout constraints, once they can
    // be spelled in Swift.
    return CheckRequirementResult::Success;
  }

  case RequirementKind::Superclass:
    if (auto packType = firstType->getAs<PackType>()) {
      return expandPackRequirement(packType);
    }

    if (getSecondType()->isExactSuperclassOf(firstType))
      return CheckRequirementResult::Success;

    return CheckRequirementResult::RequirementFailure;

  case RequirementKind::SameType:
    if (firstType->isEqual(getSecondType()))
      return CheckRequirementResult::Success;

    return CheckRequirementResult::RequirementFailure;

  case RequirementKind::SameShape:
    if (firstType->getReducedShape() ==
        getSecondType()->getReducedShape())
      return CheckRequirementResult::Success;

    return CheckRequirementResult::RequirementFailure;
  }

  llvm_unreachable("Bad requirement kind");
}

bool Requirement::canBeSatisfied() const {
  switch (getKind()) {
  case RequirementKind::SameShape:
    llvm_unreachable("Same-shape requirements not supported here");

  case RequirementKind::Conformance:
    return getFirstType()->is<ArchetypeType>();

  case RequirementKind::Layout: {
    if (auto *archetypeType = getFirstType()->getAs<ArchetypeType>()) {
      auto layout = archetypeType->getLayoutConstraint();
      return (!layout || layout.merge(getLayoutConstraint()));
    }

    return false;
  }

  case RequirementKind::Superclass:
    return (getFirstType()->isBindableTo(getSecondType()) ||
            getSecondType()->isBindableTo(getFirstType()));

  case RequirementKind::SameType:
    return (getFirstType()->isBindableTo(getSecondType()) ||
            getSecondType()->isBindableTo(getFirstType()));
  }

  llvm_unreachable("Bad requirement kind");
}

/// Determine the canonical ordering of requirements.
static unsigned getRequirementKindOrder(RequirementKind kind) {
  switch (kind) {
  case RequirementKind::SameShape: return 4;
  case RequirementKind::Conformance: return 2;
  case RequirementKind::Superclass: return 0;
  case RequirementKind::SameType: return 3;
  case RequirementKind::Layout: return 1;
  }
  llvm_unreachable("unhandled kind");
}

/// Linear order on requirements in a generic signature.
int Requirement::compare(const Requirement &other) const {
  int compareLHS =
    compareDependentTypes(getFirstType(), other.getFirstType());

  if (compareLHS != 0)
    return compareLHS;

  int compareKind = (getRequirementKindOrder(getKind()) -
                     getRequirementKindOrder(other.getKind()));

  if (compareKind != 0)
    return compareKind;

  // We should only have multiple conformance requirements.
  if (getKind() != RequirementKind::Conformance) {
    ABORT([&](auto &out) {
      out << "Unordered generic requirements\n";
      out << "LHS: "; dump(out); out << "\n";
      out << "RHS: "; other.dump(out);
    });
  }

  int compareProtos =
    TypeDecl::compare(getProtocolDecl(), other.getProtocolDecl());
  ASSERT(compareProtos != 0 && "Duplicate conformance requirements");

  return compareProtos;
}

static std::optional<CheckRequirementsResult>
checkRequirementsImpl(ArrayRef<Requirement> requirements,
                      bool allowTypeParameters) {
  SmallVector<Requirement, 4> worklist(requirements.begin(), requirements.end());

  bool hadSubstFailure = false;

  while (!worklist.empty()) {
    auto req = worklist.pop_back_val();

    // Check preconditions.
    auto firstType = req.getFirstType();
    ASSERT((allowTypeParameters || !firstType->hasTypeParameter())
           && "must take a contextual type. if you really are ok with an "
            "indefinite answer (and usually YOU ARE NOT), then consider whether "
            "you really, definitely are ok with an indefinite answer, and "
            "use `checkRequirementsWithoutContext` instead");
    ASSERT(!firstType->hasTypeVariable());

    if (req.getKind() != RequirementKind::Layout) {
      auto secondType = req.getSecondType();
      ASSERT((allowTypeParameters || !secondType->hasTypeParameter())
             && "must take a contextual type. if you really are ok with an "
              "indefinite answer (and usually YOU ARE NOT), then consider whether "
              "you really, definitely are ok with an indefinite answer, and "
              "use `checkRequirementsWithoutContext` instead");
      ASSERT(!secondType->hasTypeVariable());
    }

    switch (req.checkRequirement(worklist, /*allowMissing=*/true)) {
    case CheckRequirementResult::Success:
    case CheckRequirementResult::ConditionalConformance:
    case CheckRequirementResult::PackRequirement:
      break;

    case CheckRequirementResult::RequirementFailure:
      // If a requirement failure was caused by a context-free type parameter,
      // then we can't definitely know whether it would have satisfied the
      // requirement without context.
      if (req.getFirstType()->isTypeParameter()) {
        return std::nullopt;
      }
      return CheckRequirementsResult::RequirementFailure;

    case CheckRequirementResult::SubstitutionFailure:
      hadSubstFailure = true;
      break;
    }
  }

  if (hadSubstFailure)
    return CheckRequirementsResult::SubstitutionFailure;

  return CheckRequirementsResult::Success;
}

CheckRequirementsResult
swift::checkRequirements(ArrayRef<Requirement> requirements) {
  // This entry point requires that there are no type parameters in any of the
  // requirements, so the underlying check should always produce a result.
  return checkRequirementsImpl(requirements, /*allow type parameters*/ false)
    .value();
}

std::optional<CheckRequirementsResult>
swift::checkRequirementsWithoutContext(ArrayRef<Requirement> requirements) {
  return checkRequirementsImpl(requirements, /*allow type parameters*/ true);
}

CheckRequirementsResult swift::checkRequirements(
    ArrayRef<Requirement> requirements,
    TypeSubstitutionFn substitutions, SubstOptions options) {
  SmallVector<Requirement, 4> substReqs;
  for (auto req : requirements) {
    substReqs.push_back(req.subst(substitutions,
                                  LookUpConformanceInModule(), options));
  }

  return checkRequirements(substReqs);
}

InverseRequirement::InverseRequirement(Type subject,
                                       ProtocolDecl *protocol,
                                       SourceLoc loc)
    : subject(subject), protocol(protocol), loc(loc) {
  // Ensure it's an invertible protocol.
  assert(protocol);
  assert(protocol->getKnownProtocolKind());
  assert(getInvertibleProtocolKind(*(protocol->getKnownProtocolKind())));
}

InvertibleProtocolKind InverseRequirement::getKind() const {
  return *getInvertibleProtocolKind(*(protocol->getKnownProtocolKind()));
}

/// Do these two ArrayRefs alias any of the same memory?
template<typename T>
bool arrayrefs_overlap(ArrayRef<T> A, ArrayRef<T> B) {
  if (A.empty() || B.empty())
    return false;

  const T *ABegin = A.data();
  const T *AEnd = ABegin + A.size();
  const T *BBegin = B.data();
  const T *BEnd = BBegin + B.size();

  return ABegin < BEnd && BBegin < AEnd;
}

void InverseRequirement::expandDefaults(
    ASTContext &ctx,
    ArrayRef<Type> gps,
    ArrayRef<StructuralRequirement> existingReqs,
    SmallVectorImpl<StructuralRequirement> &result,
    SmallVectorImpl<Type> &expandedGPs) {
  // If there are no subjects, there's nothing to expand.
  if (gps.empty())
    return;

  // Vectors can reallocate, so we mustn't be looking at an ArrayRef pointing
  // into the same span of memory that we're also mutating!
  ASSERT(!arrayrefs_overlap(existingReqs, {result.data(), result.size()}) &&
         "requirements are aliasing!");
  ASSERT(!arrayrefs_overlap(gps, {expandedGPs.data(), expandedGPs.size()}) &&
         "types are aliasing!");

  auto expandFor = [&](Type gp) {
    expandedGPs.push_back(gp);
    for (auto ip : InvertibleProtocolSet::allKnown()) {
      auto proto = ctx.getProtocol(getKnownProtocolKind(ip));

      result.push_back({{RequirementKind::Conformance, gp,
                         proto->getDeclaredInterfaceType()},
                         SourceLoc()});
    }
  };

  // Used for further expansion of defaults for dependent type members of gps.
  // Contains the root generic parameters of the ones we were asked to expand.
  llvm::SmallSetVector<CanType, 8> seenRoots;
  for (auto gp : gps) {
    // Value generics never have inverses (or the positive thereof).
    if (auto gpTy = gp->getAs<GenericTypeParamType>()) {
      if (gpTy->isValue()) {
        continue;
      }
    }

    // Each generic parameter is inferred to have a conformance requirement
    // to all invertible protocols, regardless of what other requirements exist.
    // We later cancel them out in applyInverses.
    expandFor(gp);
    seenRoots.insert(gp->getDependentMemberRoot()->getCanonicalType());
  }

  // Look for structural requirements stating type parameter G conforms to P.
  // If P has a primary associatedtype P.A, infer default requirements for G.A
  // For example, given protocol,
  //
  //    protocol P<A>: ~Copyable { associatedtype A: ~Copyable }
  //
  // For an initial gp [T] and structural requirements [T: P, T.A: P],
  // we proceed with one pass over the original structural requirements:
  //
  // 1. Expand new requirement 'T: Copyable' (already done earlier)
  // 2. Because of requirement 'T: P', infer requirement [T.A: Copyable]
  // 4. Because of requirement 'T.A: P', infer requirement [T.A.A: Copyable]
  // 5. Expansion stops, as no other structural requirements are relevant.
  //    Because Copyable & Escapable don't have associated types, we're done.
  if (ctx.LangOpts.hasFeature(Feature::SuppressedAssociatedTypesWithDefaults)) {
    // Help avoid duplicate expansions of the same member type.
    llvm::SmallSetVector<CanType, 8> dmtsExpanded;

    for (auto const& sreq : existingReqs) {
      auto &req = sreq.req;
      if (req.getKind() != RequirementKind::Conformance)
        continue;

      // Is this subject rooted in one we did expand defaults for?
      auto subject = req.getFirstType();
      auto subjectRoot = subject->getDependentMemberRoot()->getCanonicalType();
      if (!seenRoots.contains(subjectRoot))
        continue;

      // Given a structural requirement `Subject: P`,
      // for each primary associated type A of P, expand defaults for Subject.A
      auto *proto = req.getProtocolDecl();
      for (auto *ATD : proto->getPrimaryAssociatedTypes()) {
        auto dmt = DependentMemberType::get(subject, ATD);
        auto cleanDMT =
            rewriting::stripBoundDependentMemberTypes(dmt)->getCanonicalType();

        // Did we already expand for the same DMT?
        if (dmtsExpanded.contains(cleanDMT))
          continue;

        expandFor(dmt);
        dmtsExpanded.insert(cleanDMT);
      }
    }
  }
}

/// Linear order on inverse requirements in a generic signature.
int InverseRequirement::compare(const InverseRequirement &other) const {
  int compareLHS =
      compareDependentTypes(subject, other.subject);

  if (compareLHS != 0)
    return compareLHS;

  int compareProtos =
      TypeDecl::compare(protocol, other.protocol);
  assert(compareProtos != 0 && "Duplicate conformance requirements");

  return compareProtos;
}