internal/aliases: Adds an internal alias package.

Adds a transitional package for handling types.Alias
until GoVersion>=1.26 for x/tools.

Updates golang/go#65294

Change-Id: I7a58cb9ceb9945529baf14d33543dbebc23af542
Reviewed-on: https://go-review.googlesource.com/c/tools/+/559995
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Tim King <taking@google.com>

internal/aliases/aliases.go

@@ -0,0 +1,28 @@
+// Copyright 2024 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 aliases
+
+import (
+	"go/token"
+	"go/types"
+)
+
+// Package aliases defines backward compatible shims
+// for the types.Alias type representation added in 1.22.
+// This defines placeholders for x/tools until 1.26.
+
+// NewAlias creates a new TypeName in Package pkg that
+// is an alias for the type rhs.
+//
+// When GoVersion>=1.22 and GODEBUG=gotypesalias=1,
+// the Type() of the return value is a *types.Alias.
+func NewAlias(pos token.Pos, pkg *types.Package, name string, rhs types.Type) *types.TypeName {
+	if enabled() {
+		tname := types.NewTypeName(pos, pkg, name, nil)
+		newAlias(tname, rhs)
+		return tname
+	}
+	return types.NewTypeName(pos, pkg, name, rhs)
+}

internal/aliases/aliases_go121.go

@@ -0,0 +1,30 @@
+// Copyright 2024 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.
+
+//go:build !go1.22
+// +build !go1.22
+
+package aliases
+
+import (
+	"go/types"
+)
+
+// Alias is a placeholder for a go/types.Alias for <=1.21.
+// It will never be created by go/types.
+type Alias struct{}
+
+func (*Alias) String() string { panic("unreachable") }
+
+func (*Alias) Underlying() types.Type { panic("unreachable") }
+
+func (*Alias) Obj() *types.TypeName { panic("unreachable") }
+
+// Unalias returns the type t for go <=1.21.
+func Unalias(t types.Type) types.Type { return t }
+
+// Always false for go <=1.21. Ignores GODEBUG.
+func enabled() bool { return false }
+
+func newAlias(name *types.TypeName, rhs types.Type) *Alias { panic("unreachable") }

internal/aliases/aliases_go122.go

@@ -0,0 +1,72 @@
+// Copyright 2024 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.
+
+//go:build go1.22
+// +build go1.22
+
+package aliases
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"os"
+	"strings"
+	"sync"
+)
+
+// Alias is an alias of types.Alias.
+type Alias = types.Alias
+
+// Unalias is a wrapper of types.Unalias.
+func Unalias(t types.Type) types.Type { return types.Unalias(t) }
+
+// newAlias is an internal alias around types.NewAlias.
+// Direct usage is discouraged as the moment.
+// Try to use NewAlias instead.
+func newAlias(tname *types.TypeName, rhs types.Type) *Alias {
+	a := types.NewAlias(tname, rhs)
+	// TODO(go.dev/issue/65455): Remove kludgy workaround to set a.actual as a side-effect.
+	Unalias(a)
+	return a
+}
+
+// enabled returns true when types.Aliases are enabled.
+func enabled() bool {
+	// Use the gotypesalias value in GODEBUG if set.
+	godebug := os.Getenv("GODEBUG")
+	value := -1 // last set value.
+	for _, f := range strings.Split(godebug, ",") {
+		switch f {
+		case "gotypesalias=1":
+			value = 1
+		case "gotypesalias=0":
+			value = 0
+		}
+	}
+	switch value {
+	case 0:
+		return false
+	case 1:
+		return true
+	default:
+		return aliasesDefault()
+	}
+}
+
+// aliasesDefault reports if aliases are enabled by default.
+func aliasesDefault() bool {
+	// Dynamically check if Aliases will be produced from go/types.
+	aliasesDefaultOnce.Do(func() {
+		fset := token.NewFileSet()
+		f, _ := parser.ParseFile(fset, "a.go", "package p; type A = int", 0)
+		pkg, _ := new(types.Config).Check("p", fset, []*ast.File{f}, nil)
+		_, gotypesaliasDefault = pkg.Scope().Lookup("A").Type().(*types.Alias)
+	})
+	return gotypesaliasDefault
+}
+
+var gotypesaliasDefault bool
+var aliasesDefaultOnce sync.Once

internal/aliases/aliases_test.go

@@ -0,0 +1,75 @@
+// Copyright 2024 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 aliases_test
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"os"
+	"testing"
+
+	"golang.org/x/tools/internal/aliases"
+	"golang.org/x/tools/internal/testenv"
+)
+
+// Assert that Obj exists on Alias.
+var _ func(*aliases.Alias) *types.TypeName = (*aliases.Alias).Obj
+
+// TestNewAlias tests that alias.NewAlias creates an alias of a type
+// whose underlying and Unaliased type is *Named.
+// When gotypesalias=1 and GoVersion >= 1.22, the type will
+// be an *aliases.Alias.
+func TestNewAlias(t *testing.T) {
+	const source = `
+	package P
+
+	type Named int
+	`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "hello.go", source, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var conf types.Config
+	pkg, err := conf.Check("P", fset, []*ast.File{f}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	expr := `*Named`
+	tv, err := types.Eval(fset, pkg, 0, expr)
+	if err != nil {
+		t.Fatalf("Eval(%s) failed: %v", expr, err)
+	}
+
+	for _, godebug := range []string{"", "gotypesalias=1"} {
+		t.Run(godebug, func(t *testing.T) {
+			saved := os.Getenv("GODEBUG")
+			defer os.Setenv("GODEBUG", saved)
+			os.Setenv("GODEBUG", godebug) // non parallel.
+
+			A := aliases.NewAlias(token.NoPos, pkg, "A", tv.Type)
+			if got, want := A.Name(), "A"; got != want {
+				t.Errorf("Expected A.Name()==%q. got %q", want, got)
+			}
+
+			if got, want := A.Type().Underlying(), tv.Type; got != want {
+				t.Errorf("Expected A.Type().Underlying()==%q. got %q", want, got)
+			}
+			if got, want := aliases.Unalias(A.Type()), tv.Type; got != want {
+				t.Errorf("Expected Unalias(A)==%q. got %q", want, got)
+			}
+
+			if testenv.Go1Point() >= 22 && godebug == "gotypesalias=1" {
+				if _, ok := A.Type().(*aliases.Alias); !ok {
+					t.Errorf("Expected A.Type() to be a types.Alias(). got %q", A.Type())
+				}
+			}
+		})
+	}
+}