go/types/objectpath: add Encoder type, to amortize Scope.Names

This change adds a new Encoder type with For method, that
is functionally equivalent to the old For method but avoids
the surprising cost of repeated calls to Scope.Names, which
allocates and sorts a slice.

See golang/go#58668
Fixes golang/go#51017

Change-Id: I328e60c60f9bc312af253a0509aa029c41960d41
Reviewed-on: https://go-review.googlesource.com/c/tools/+/470678
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Tim King <taking@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go/types/objectpath/objectpath.go

@@ -113,6 +113,20 @@ const (
 	opObj    = 'O' // .Obj()		 (Named, TypeParam)
 )
 
+// For is equivalent to new(Encoder).For(obj).
+//
+// It may be more efficient to reuse a single Encoder across several calls.
+func For(obj types.Object) (Path, error) {
+	return new(Encoder).For(obj)
+}
+
+// An Encoder amortizes the cost of encoding the paths of multiple objects.
+// The zero value of an Encoder is ready to use.
+type Encoder struct {
+	scopeNamesMemo   map[*types.Scope][]string      // memoization of Scope.Names()
+	namedMethodsMemo map[*types.Named][]*types.Func // memoization of namedMethods()
+}
+
 // For returns the path to an object relative to its package,
 // or an error if the object is not accessible from the package's Scope.
 //
@@ -145,24 +159,7 @@ const (
 //	.Type().Field(0)					(field Var X)
 //
 // where p is the package (*types.Package) to which X belongs.
-func For(obj types.Object) (Path, error) {
-	return newEncoderFor()(obj)
-}
-
-// An encoder amortizes the cost of encoding the paths of multiple objects.
-// Nonexported pending approval of proposal 58668.
-type encoder struct {
-	scopeNamesMemo   map[*types.Scope][]string      // memoization of Scope.Names()
-	namedMethodsMemo map[*types.Named][]*types.Func // memoization of namedMethods()
-}
-
-// Exposed to gopls via golang.org/x/tools/internal/typesinternal
-// pending approval of proposal 58668.
-//
-//go:linkname newEncoderFor
-func newEncoderFor() func(types.Object) (Path, error) { return new(encoder).For }
-
-func (enc *encoder) For(obj types.Object) (Path, error) {
+func (enc *Encoder) For(obj types.Object) (Path, error) {
 	pkg := obj.Pkg()
 
 	// This table lists the cases of interest.
@@ -341,7 +338,7 @@ func appendOpArg(path []byte, op byte, arg int) []byte {
 // This function is just an optimization that avoids the general scope walking
 // approach. You are expected to fall back to the general approach if this
 // function fails.
-func (enc *encoder) concreteMethod(meth *types.Func) (Path, bool) {
+func (enc *Encoder) concreteMethod(meth *types.Func) (Path, bool) {
 	// Concrete methods can only be declared on package-scoped named types. For
 	// that reason we can skip the expensive walk over the package scope: the
 	// path will always be package -> named type -> method. We can trivially get
@@ -730,23 +727,8 @@ func namedMethods(named *types.Named) []*types.Func {
 	return methods
 }
 
-// scopeNames is a memoization of scope.Names. Callers must not modify the result.
-func (enc *encoder) scopeNames(scope *types.Scope) []string {
-	m := enc.scopeNamesMemo
-	if m == nil {
-		m = make(map[*types.Scope][]string)
-		enc.scopeNamesMemo = m
-	}
-	names, ok := m[scope]
-	if !ok {
-		names = scope.Names() // allocates and sorts
-		m[scope] = names
-	}
-	return names
-}
-
 // namedMethods is a memoization of the namedMethods function. Callers must not modify the result.
-func (enc *encoder) namedMethods(named *types.Named) []*types.Func {
+func (enc *Encoder) namedMethods(named *types.Named) []*types.Func {
 	m := enc.namedMethodsMemo
 	if m == nil {
 		m = make(map[*types.Named][]*types.Func)
@@ -758,5 +740,19 @@ func (enc *encoder) namedMethods(named *types.Named) []*types.Func {
 		m[named] = methods
 	}
 	return methods
+}
 
+// scopeNames is a memoization of scope.Names. Callers must not modify the result.
+func (enc *Encoder) scopeNames(scope *types.Scope) []string {
+	m := enc.scopeNamesMemo
+	if m == nil {
+		m = make(map[*types.Scope][]string)
+		enc.scopeNamesMemo = m
+	}
+	names, ok := m[scope]
+	if !ok {
+		names = scope.Names() // allocates and sorts
+		m[scope] = names
+	}
+	return names
 }

gopls/internal/lsp/source/methodsets/methodsets.go

@@ -57,7 +57,6 @@ import (
 	"golang.org/x/tools/go/types/objectpath"
 	"golang.org/x/tools/gopls/internal/lsp/safetoken"
 	"golang.org/x/tools/internal/typeparams"
-	"golang.org/x/tools/internal/typesinternal"
 )
 
 // An Index records the non-empty method sets of all package-level
@@ -232,7 +231,7 @@ func (b *indexBuilder) build(fset *token.FileSet, pkg *types.Package) *Index {
 		return gobPosition{b.string(posn.Filename), posn.Offset, len(obj.Name())}
 	}
 
-	objectpathFor := typesinternal.NewObjectpathFunc()
+	objectpathFor := new(objectpath.Encoder).For
 
 	// setindexInfo sets the (Posn, PkgPath, ObjectPath) fields for each method declaration.
 	setIndexInfo := func(m *gobMethod, method *types.Func) {

gopls/internal/lsp/source/xrefs/xrefs.go

@@ -19,7 +19,6 @@ import (
 	"golang.org/x/tools/go/types/objectpath"
 	"golang.org/x/tools/gopls/internal/lsp/protocol"
 	"golang.org/x/tools/gopls/internal/lsp/source"
-	"golang.org/x/tools/internal/typesinternal"
 )
 
 // Index constructs a serializable index of outbound cross-references
@@ -42,7 +41,7 @@ func Index(files []*source.ParsedGoFile, pkg *types.Package, info *types.Info) [
 		return objects
 	}
 
-	objectpathFor := typesinternal.NewObjectpathFunc()
+	objectpathFor := new(objectpath.Encoder).For
 
 	for fileIndex, pgf := range files {