RequirementMachine: A term rewriting system for reasoning about generic signatures

include/swift/AST/ASTContext.h

@@ -111,6 +111,7 @@ namespace swift {
   class TupleTypeElt;
   class EnumElementDecl;
   class ProtocolDecl;
+  class RequirementMachine;
   class SubstitutableType;
   class SourceManager;
   class ValueDecl;
@@ -1132,6 +1133,11 @@ class ASTContext final {
   GenericSignatureBuilder *getOrCreateGenericSignatureBuilder(
                                                      CanGenericSignature sig);
 
+  /// Retrieve or create a term rewriting system for answering queries on
+  /// type parameters written against the given generic signature.
+  RequirementMachine *getOrCreateRequirementMachine(
+      CanGenericSignature sig);
+
   /// Retrieve a generic signature with a single unconstrained type parameter,
   /// like `<T>`.
   CanGenericSignature getSingleGenericParameterSignature() const;

include/swift/AST/LayoutConstraint.h

@@ -284,6 +284,10 @@ class LayoutConstraint {
   bool operator!=(LayoutConstraint rhs) const {
     return !(*this == rhs);
   }
+
+  /// Defines a somewhat arbitrary linear order on layout constraints.
+  /// -1 if this < rhs, 0 if this == rhs, 1 if this > rhs.
+  int compare(LayoutConstraint rhs) const;
 };
 
 // Permit direct uses of isa/cast/dyn_cast on LayoutConstraint.
@@ -313,12 +317,6 @@ struct LayoutConstraintLoc {
 
   bool isError() const;
 
-  // FIXME: We generally shouldn't need to build LayoutConstraintLoc without
-  // a location.
-  static LayoutConstraintLoc withoutLoc(LayoutConstraint Layout) {
-    return LayoutConstraintLoc(Layout, SourceLoc());
-  }
-
   /// Get the representative location of this type, for diagnostic
   /// purposes.
   SourceLoc getLoc() const { return Loc; }
@@ -328,13 +326,7 @@ struct LayoutConstraintLoc {
   bool hasLocation() const { return Loc.isValid(); }
   LayoutConstraint getLayoutConstraint() const { return Layout; }
 
-  void setLayoutConstraint(LayoutConstraint value) {
-    Layout = value;
-  }
-
   bool isNull() const { return Layout.isNull(); }
-
-  LayoutConstraintLoc clone(ASTContext &ctx) const { return *this; }
 };
 
 /// Checks if ID is a name of a layout constraint and returns this

include/swift/AST/LayoutConstraintKind.h

@@ -14,6 +14,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/Support/DataTypes.h"
+
 #ifndef SWIFT_LAYOUT_CONSTRAINTKIND_H
 #define SWIFT_LAYOUT_CONSTRAINTKIND_H
 

include/swift/AST/ProtocolGraph.h

@@ -0,0 +1,113 @@
+//===--- ProtocolGraph.h - Collects information about protocols -*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_PROTOCOLGRAPH_H
+#define SWIFT_PROTOCOLGRAPH_H
+
+#include "swift/AST/Requirement.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/TinyPtrVector.h"
+
+namespace swift {
+
+class ProtocolDecl;
+class AssociatedTypeDecl;
+
+namespace rewriting {
+
+/// Stores cached information about a protocol.
+struct ProtocolInfo {
+  /// All immediately-inherited protocols.
+  ArrayRef<ProtocolDecl *> Inherited;
+
+  /// Transitive closure of inherited protocols; does not include the protocol
+  /// itself. Computed by ProtocolGraph::computeInheritedProtocols().
+  llvm::TinyPtrVector<const ProtocolDecl *> AllInherited;
+
+  /// Transitive closure of inherited associated types together with all
+  /// associated types from the protocol itself. Computed by
+  /// ProtocolGraph::computeInheritedAssociatedTypes().
+  llvm::TinyPtrVector<AssociatedTypeDecl *> AssociatedTypes;
+
+  /// The protocol's requirement signature.
+  ArrayRef<Requirement> Requirements;
+
+  /// Used by ProtocolGraph::computeProtocolDepth() to detect circularity.
+  unsigned Mark : 1;
+
+  /// Longest chain of protocol refinements, including this one. Greater than
+  /// zero on valid code, might be zero if there's a cycle. Computed by
+  /// ProtocolGraph::computeLinearOrder().
+  unsigned Depth : 31;
+
+  /// Index of the protocol in the linear order. Computed by
+  /// ProtocolGraph::computeLinearOrder().
+  unsigned Index : 32;
+
+  ProtocolInfo() {
+    Mark = 0;
+    Depth = 0;
+    Index = 0;
+  }
+
+  ProtocolInfo(ArrayRef<ProtocolDecl *> inherited,
+               llvm::TinyPtrVector<AssociatedTypeDecl *> &&types,
+               ArrayRef<Requirement> reqs)
+    : Inherited(inherited),
+      AssociatedTypes(types),
+      Requirements(reqs) {
+    Mark = 0;
+    Depth = 0;
+    Index = 0;
+  }
+};
+
+/// Stores cached information about all protocols transtively
+/// referenced from a set of generic requirements.
+///
+/// Out-of-line methods are documented in ProtocolGraph.cpp.
+struct ProtocolGraph {
+  llvm::DenseMap<const ProtocolDecl *, ProtocolInfo> Info;
+  std::vector<const ProtocolDecl *> Protocols;
+  bool Debug = false;
+
+  void visitRequirements(ArrayRef<Requirement> reqs);
+
+  const ProtocolInfo &getProtocolInfo(
+      const ProtocolDecl *proto) const;
+
+  void addProtocol(const ProtocolDecl *proto);
+
+  void computeTransitiveClosure();
+
+  void computeLinearOrder();
+
+  void computeInheritedAssociatedTypes();
+
+  void computeInheritedProtocols();
+
+  int compareProtocols(const ProtocolDecl *lhs,
+                       const ProtocolDecl *rhs) const;
+
+  bool inheritsFrom(const ProtocolDecl *thisProto,
+                    const ProtocolDecl *otherProto) const;
+
+private:
+  unsigned computeProtocolDepth(const ProtocolDecl *proto);
+};
+
+} // end namespace rewriting
+
+} // end namespace swift
+
+#endif

include/swift/AST/RequirementMachine.h

@@ -0,0 +1,62 @@
+//===--- RequirementMachine.h - Generics with term rewriting ----*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_REQUIREMENTMACHINE_H
+#define SWIFT_REQUIREMENTMACHINE_H
+
+namespace swift {
+
+class ASTContext;
+class AssociatedTypeDecl;
+class CanGenericSignature;
+class CanType;
+class GenericSignature;
+class ProtocolDecl;
+class Requirement;
+
+namespace rewriting {
+
+class Term;
+
+Term getTermForType(CanType paramType, const ProtocolDecl *proto);
+
+} // end namespace rewriting
+
+/// Wraps a rewrite system with higher-level operations in terms of
+/// generic signatures and interface types.
+class RequirementMachine final {
+  friend class ASTContext;
+
+  struct Implementation;
+
+  ASTContext &Context;
+  Implementation *Impl;
+
+  explicit RequirementMachine(ASTContext &ctx);
+
+  RequirementMachine(const RequirementMachine &) = delete;
+  RequirementMachine(RequirementMachine &&) = delete;
+  RequirementMachine &operator=(const RequirementMachine &) = delete;
+  RequirementMachine &operator=(RequirementMachine &&) = delete;
+
+  void addGenericSignature(CanGenericSignature sig);
+
+  bool isComplete() const;
+  void markComplete();
+
+public:
+  ~RequirementMachine();
+};
+
+} // end namespace swift
+
+#endif