go/types: implement deduplication of instances using the Environment

Implement deduplication of type instances via the Environment by
recording a map of unique IDs for *Named types. This avoids an issue
with the existing type hash, where qualified names alone were not
sufficient to differentiate two *Named types that have the same fully
qualified name but which are distinct pointers. It also allows us to
drop the scope accounting for local types.

A subtle bug is also fixed in subst.go, where the instance t was passed
to typeHash rather than t.orig.

Change-Id: I85639ccc1c9bfee470babd2fc85375484c8ed0b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/344390
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/check.go

@@ -89,7 +89,7 @@ type Checker struct {
 	nextID  uint64                 // unique Id for type parameters (first valid Id is 1)
 	objMap  map[Object]*declInfo   // maps package-level objects and (non-interface) methods to declaration info
 	impMap  map[importKey]*Package // maps (import path, source directory) to (complete or fake) package
-	typMap  map[string]*Named      // maps an instantiated named type hash to a *Named type
+	env     *Environment           // for deduplicating identical instances
 
 	// pkgPathMap maps package names to the set of distinct import paths we've
 	// seen for that name, anywhere in the import graph. It is used for
@@ -192,7 +192,7 @@ func NewChecker(conf *Config, fset *token.FileSet, pkg *Package, info *Info) *Ch
 		version: version,
 		objMap:  make(map[Object]*declInfo),
 		impMap:  make(map[importKey]*Package),
-		typMap:  make(map[string]*Named),
+		env:     NewEnvironment(),
 	}
 }
 

src/go/types/decl.go

@@ -316,7 +316,7 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 		}
 
 	case *Named:
-		t.expand(check.typMap)
+		t.expand(check.env)
 		// don't touch the type if it is from a different package or the Universe scope
 		// (doing so would lead to a race condition - was issue #35049)
 		if t.obj.pkg != check.pkg {

src/go/types/environment.go

@@ -0,0 +1,56 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import "sync"
+
+// An Environment is an opaque type checking environment. It may be used to
+// share identical type instances across type-checked packages or calls to
+// Instantiate.
+//
+// It is safe for concurrent use.
+type Environment struct {
+	mu      sync.Mutex
+	typeMap map[string]*Named // type hash -> instance
+	nextID  int               // next unique ID
+	seen    map[*Named]int    // assigned unique IDs
+}
+
+// NewEnvironment creates a new Environment.
+func NewEnvironment() *Environment {
+	return &Environment{
+		typeMap: make(map[string]*Named),
+		seen:    make(map[*Named]int),
+	}
+}
+
+// TODO(rfindley): move Environment.typeHash here.
+
+// typeForHash returns the recorded type for the type hash h, if it exists.
+// If no type exists for h and n is non-nil, n is recorded for h.
+func (env *Environment) typeForHash(h string, n *Named) *Named {
+	env.mu.Lock()
+	defer env.mu.Unlock()
+	if existing := env.typeMap[h]; existing != nil {
+		return existing
+	}
+	if n != nil {
+		env.typeMap[h] = n
+	}
+	return n
+}
+
+// idForType returns a unique ID for the pointer n.
+func (env *Environment) idForType(n *Named) int {
+	env.mu.Lock()
+	defer env.mu.Unlock()
+	id, ok := env.seen[n]
+	if !ok {
+		id = env.nextID
+		env.seen[n] = id
+		env.nextID++
+	}
+	return id
+}

src/go/types/instantiate.go

@@ -13,19 +13,6 @@ import (
 	"go/token"
 )
 
-// An Environment is an opaque type checking environment. It may be used to
-// share identical type instances across type checked packages or calls to
-// Instantiate.
-//
-// Currently, Environment is just a placeholder and has no effect on
-// instantiation.
-type Environment struct {
-	// Environment is currently un-implemented, because our instantiatedHash
-	// logic doesn't correctly handle Named type identity across multiple
-	// packages.
-	// TODO(rfindley): implement this.
-}
-
 // Instantiate instantiates the type typ with the given type arguments targs.
 // typ must be a *Named or a *Signature type, and its number of type parameters
 // must match the number of provided type arguments. The result is a new,
@@ -44,7 +31,7 @@ type Environment struct {
 // TODO(rfindley): change this function to also return an error if lengths of
 // tparams and targs do not match.
 func Instantiate(env *Environment, typ Type, targs []Type, validate bool) (Type, error) {
-	inst := (*Checker)(nil).instance(token.NoPos, typ, targs)
+	inst := (*Checker)(nil).instance(token.NoPos, typ, targs, env)
 
 	var err error
 	if validate {
@@ -84,7 +71,7 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, posList
 		}()
 	}
 
-	inst := check.instance(pos, typ, targs)
+	inst := check.instance(pos, typ, targs, check.env)
 
 	assert(len(posList) <= len(targs))
 	check.later(func() {
@@ -116,23 +103,26 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, posList
 // instance creates a type or function instance using the given original type
 // typ and arguments targs. For Named types the resulting instance will be
 // unexpanded.
-func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) Type {
+func (check *Checker) instance(pos token.Pos, typ Type, targs []Type, env *Environment) Type {
 	switch t := typ.(type) {
 	case *Named:
-		h := typeHash(t, targs)
-		if check != nil {
-			// typ may already have been instantiated with identical type arguments.
-			// In that case, re-use the existing instance.
-			if named := check.typMap[h]; named != nil {
+		var h string
+		if env != nil {
+			h = env.typeHash(t, targs)
+			// typ may already have been instantiated with identical type arguments. In
+			// that case, re-use the existing instance.
+			if named := env.typeForHash(h, nil); named != nil {
 				return named
 			}
 		}
 		tname := NewTypeName(pos, t.obj.pkg, t.obj.name, nil)
 		named := check.newNamed(tname, t, nil, nil, nil) // methods and tparams are set when named is loaded
 		named.targs = NewTypeList(targs)
 		named.instPos = &pos
-		if check != nil {
-			check.typMap[h] = named
+		if env != nil {
+			// It's possible that we've lost a race to add named to the environment.
+			// In this case, use whichever instance is recorded in the environment.
+			named = env.typeForHash(h, named)
 		}
 		return named
 
@@ -144,7 +134,7 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) Type {
 		if tparams.Len() == 0 {
 			return typ // nothing to do (minor optimization)
 		}
-		sig := check.subst(pos, typ, makeSubstMap(tparams.list(), targs), nil).(*Signature)
+		sig := check.subst(pos, typ, makeSubstMap(tparams.list(), targs), env).(*Signature)
 		// If the signature doesn't use its type parameters, subst
 		// will not make a copy. In that case, make a copy now (so
 		// we can set tparams to nil w/o causing side-effects).

src/go/types/instantiate_test.go

@@ -0,0 +1,72 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	. "go/types"
+	"testing"
+)
+
+func TestInstantiateEquality(t *testing.T) {
+	const src = genericPkg + "p; type T[P any] int"
+
+	pkg, err := pkgFor(".", src, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	T := pkg.Scope().Lookup("T").Type().(*Named)
+
+	// Instantiating the same type twice should result in pointer-equivalent
+	// instances.
+	env := NewEnvironment()
+	res1, err := Instantiate(env, T, []Type{Typ[Int]}, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res2, err := Instantiate(env, T, []Type{Typ[Int]}, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if res1 != res2 {
+		t.Errorf("first instance (%s) not pointer-equivalent to second instance (%s)", res1, res2)
+	}
+}
+
+func TestInstantiateNonEquality(t *testing.T) {
+	const src = genericPkg + "p; type T[P any] int"
+
+	pkg1, err := pkgFor(".", src, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	pkg2, err := pkgFor(".", src, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// We consider T1 and T2 to be distinct types, so their instances should not
+	// be deduplicated by the environment.
+	T1 := pkg1.Scope().Lookup("T").Type().(*Named)
+	T2 := pkg2.Scope().Lookup("T").Type().(*Named)
+
+	env := NewEnvironment()
+	res1, err := Instantiate(env, T1, []Type{Typ[Int]}, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res2, err := Instantiate(env, T2, []Type{Typ[Int]}, false)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if res1 == res2 {
+		t.Errorf("instance from pkg1 (%s) is pointer-equivalent to instance from pkg2 (%s)", res1, res2)
+	}
+	if Identical(res1, res2) {
+		t.Errorf("instance from pkg1 (%s) is identical to instance from pkg2 (%s)", res1, res2)
+	}
+}

src/go/types/named.go

@@ -242,27 +242,33 @@ func (n *Named) setUnderlying(typ Type) {
 
 // expand ensures that the underlying type of n is instantiated.
 // The underlying type will be Typ[Invalid] if there was an error.
-func (n *Named) expand(typMap map[string]*Named) *Named {
+func (n *Named) expand(env *Environment) *Named {
 	if n.instPos != nil {
 		// n must be loaded before instantiation, in order to have accurate
 		// tparams. This is done implicitly by the call to n.TParams, but making it
 		// explicit is harmless: load is idempotent.
 		n.load()
 		var u Type
 		if n.check.validateTArgLen(*n.instPos, n.tparams.Len(), n.targs.Len()) {
-			if typMap == nil {
+			// TODO(rfindley): handling an optional Checker and Environment here (and
+			// in subst) feels overly complicated. Can we simplify?
+			if env == nil {
 				if n.check != nil {
-					typMap = n.check.typMap
+					env = n.check.env
 				} else {
 					// If we're instantiating lazily, we might be outside the scope of a
 					// type-checking pass. In that case we won't have a pre-existing
-					// typMap, but don't want to create a duplicate of the current instance
-					// in the process of expansion.
-					h := typeHash(n.orig, n.targs.list())
-					typMap = map[string]*Named{h: n}
+					// environment, but don't want to create a duplicate of the current
+					// instance in the process of expansion.
+					env = NewEnvironment()
 				}
+				h := env.typeHash(n.orig, n.targs.list())
+				// add the instance to the environment to avoid infinite recursion.
+				// addInstance may return a different, existing instance, but we
+				// shouldn't return that instance from expand.
+				env.typeForHash(h, n)
 			}
-			u = n.check.subst(*n.instPos, n.orig.underlying, makeSubstMap(n.TParams().list(), n.targs.list()), typMap)
+			u = n.check.subst(*n.instPos, n.orig.underlying, makeSubstMap(n.TParams().list(), n.targs.list()), env)
 		} else {
 			u = Typ[Invalid]
 		}

src/go/types/subst.go

@@ -43,8 +43,8 @@ func (m substMap) lookup(tpar *TypeParam) Type {
 // that it doesn't modify the incoming type. If a substitution took place, the
 // result type is different from the incoming type.
 //
-// If the given typMap is non-nil, it is used in lieu of check.typMap.
-func (check *Checker) subst(pos token.Pos, typ Type, smap substMap, typMap map[string]*Named) Type {
+// If the given environment is non-nil, it is used in lieu of check.env.
+func (check *Checker) subst(pos token.Pos, typ Type, smap substMap, env *Environment) Type {
 	if smap.empty() {
 		return typ
 	}
@@ -64,27 +64,27 @@ func (check *Checker) subst(pos token.Pos, typ Type, smap substMap, typMap map[s
 
 	if check != nil {
 		subst.check = check
-		if typMap == nil {
-			typMap = check.typMap
+		if env == nil {
+			env = check.env
 		}
 	}
-	if typMap == nil {
+	if env == nil {
 		// If we don't have a *Checker and its global type map,
 		// use a local version. Besides avoiding duplicate work,
 		// the type map prevents infinite recursive substitution
 		// for recursive types (example: type T[P any] *T[P]).
-		typMap = make(map[string]*Named)
+		env = NewEnvironment()
 	}
-	subst.typMap = typMap
+	subst.env = env
 
 	return subst.typ(typ)
 }
 
 type subster struct {
-	pos    token.Pos
-	smap   substMap
-	check  *Checker // nil if called via Instantiate
-	typMap map[string]*Named
+	pos   token.Pos
+	smap  substMap
+	check *Checker // nil if called via Instantiate
+	env   *Environment
 }
 
 func (subst *subster) typ(typ Type) Type {
@@ -217,25 +217,25 @@ func (subst *subster) typ(typ Type) Type {
 		}
 
 		// before creating a new named type, check if we have this one already
-		h := typeHash(t, newTArgs)
+		h := subst.env.typeHash(t.orig, newTArgs)
 		dump(">>> new type hash: %s", h)
-		if named, found := subst.typMap[h]; found {
+		if named := subst.env.typeForHash(h, nil); named != nil {
 			dump(">>> found %s", named)
 			return named
 		}
 
-		// Create a new named type and populate typMap to avoid endless recursion.
-		// The position used here is irrelevant because validation only occurs on t
-		// (we don't call validType on named), but we use subst.pos to help with
-		// debugging.
+		// Create a new named type and populate the environment to avoid endless
+		// recursion. The position used here is irrelevant because validation only
+		// occurs on t (we don't call validType on named), but we use subst.pos to
+		// help with debugging.
 		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
 		t.load()
 		// It's ok to provide a nil *Checker because the newly created type
 		// doesn't need to be (lazily) expanded; it's expanded below.
 		named := (*Checker)(nil).newNamed(tname, t.orig, nil, t.tparams, t.methods) // t is loaded, so tparams and methods are available
 		named.targs = NewTypeList(newTArgs)
-		subst.typMap[h] = named
-		t.expand(subst.typMap) // must happen after typMap update to avoid infinite recursion
+		subst.env.typeForHash(h, named)
+		t.expand(subst.env) // must happen after env update to avoid infinite recursion
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, newTArgs)
@@ -260,14 +260,16 @@ func (subst *subster) typ(typ Type) Type {
 // type hash: types that are identical produce identical string representations.
 // If typ is a *Named type and targs is not empty, typ is printed as if it were
 // instantiated with targs.
-func typeHash(typ Type, targs []Type) string {
+func (env *Environment) typeHash(typ Type, targs []Type) string {
+	assert(env != nil)
 	assert(typ != nil)
 	var buf bytes.Buffer
 
-	h := newTypeHasher(&buf)
+	h := newTypeHasher(&buf, env)
 	if named, _ := typ.(*Named); named != nil && len(targs) > 0 {
 		// Don't use WriteType because we need to use the provided targs
 		// and not any targs that might already be with the *Named type.
+		h.typePrefix(named)
 		h.typeName(named.obj)
 		h.typeList(targs)
 	} else {