go/types: eliminate typeHashing global variable

This is a port of CL 345929 to go/types. It is also a step toward making
instantiation concurrency-safe.

Also fix some whitespace in instantiate.go.

Updates #47910

Change-Id: Icdeb227cb83eee15da6db90daab294c8c55db601
Reviewed-on: https://go-review.googlesource.com/c/go/+/346557
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/go/types/instantiate.go

@@ -121,8 +121,8 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) 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.
+			// 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 {
 				return named
 			}
@@ -135,6 +135,7 @@ func (check *Checker) instance(pos token.Pos, typ Type, targs []Type) Type {
 			check.typMap[h] = named
 		}
 		return named
+
 	case *Signature:
 		tparams := t.TParams()
 		if !check.validateTArgLen(pos, tparams.Len(), len(targs)) {

src/go/types/subst.go

@@ -256,8 +256,6 @@ func (subst *subster) typ(typ Type) Type {
 	return typ
 }
 
-var typeHashing = 0
-
 // typeHash returns a string representation of typ, which can be used as an exact
 // 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
@@ -266,19 +264,16 @@ func typeHash(typ Type, targs []Type) string {
 	assert(typ != nil)
 	var buf bytes.Buffer
 
-	assert(typeHashing == 0)
-	typeHashing++
-	w := newTypeWriter(&buf, nil)
+	h := newTypeHasher(&buf)
 	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.
-		w.typeName(named.obj)
-		w.typeList(targs)
+		h.typeName(named.obj)
+		h.typeList(targs)
 	} else {
 		assert(targs == nil)
-		w.typ(typ)
+		h.typ(typ)
 	}
-	typeHashing--
 
 	if debug {
 		// there should be no instance markers in type hashes

src/go/types/typestring.go

@@ -63,10 +63,15 @@ type typeWriter struct {
 	buf  *bytes.Buffer
 	seen map[Type]bool
 	qf   Qualifier
+	hash bool
 }
 
 func newTypeWriter(buf *bytes.Buffer, qf Qualifier) *typeWriter {
-	return &typeWriter{buf, make(map[Type]bool), qf}
+	return &typeWriter{buf, make(map[Type]bool), qf, false}
+}
+
+func newTypeHasher(buf *bytes.Buffer) *typeWriter {
+	return &typeWriter{buf, make(map[Type]bool), nil, true}
 }
 
 func (w *typeWriter) byte(b byte)                               { w.buf.WriteByte(b) }
@@ -208,7 +213,7 @@ func (w *typeWriter) typ(typ Type) {
 		// types. Write them to aid debugging, but don't write
 		// them when we need an instance hash: whether a type
 		// is fully expanded or not doesn't matter for identity.
-		if typeHashing == 0 && t.instPos != nil {
+		if !w.hash && t.instPos != nil {
 			w.byte(instanceMarker)
 		}
 		w.typeName(t.obj)
@@ -292,7 +297,7 @@ func (w *typeWriter) tParamList(list []*TypeParam) {
 
 func (w *typeWriter) typeName(obj *TypeName) {
 	if obj == nil {
-		assert(typeHashing == 0) // we need an object for type hashing
+		assert(!w.hash) // we need an object for type hashing
 		w.string("<Named w/o object>")
 		return
 	}
@@ -301,7 +306,7 @@ func (w *typeWriter) typeName(obj *TypeName) {
 	}
 	w.string(obj.name)
 
-	if typeHashing != 0 {
+	if w.hash {
 		// For local defined types, use the (original!) TypeName's scope
 		// numbers to disambiguate.
 		if typ, _ := obj.typ.(*Named); typ != nil {
@@ -335,7 +340,7 @@ func (w *typeWriter) tuple(tup *Tuple, variadic bool) {
 				w.string(", ")
 			}
 			// parameter names are ignored for type identity and thus type hashes
-			if typeHashing == 0 && v.name != "" {
+			if !w.hash && v.name != "" {
 				w.string(v.name)
 				w.byte(' ')
 			}
@@ -383,8 +388,8 @@ func (w *typeWriter) signature(sig *Signature) {
 	}
 
 	w.byte(' ')
-	if n == 1 && (typeHashing != 0 || sig.results.vars[0].name == "") {
-		// single unnamed result (if typeHashing, name must be ignored)
+	if n == 1 && (w.hash || sig.results.vars[0].name == "") {
+		// single unnamed result (if type hashing, name must be ignored)
 		w.typ(sig.results.vars[0].typ)
 		return
 	}