internal/gcimporter: use Alias.Rhs, not unsafe hack

Unfortunately we need an aliases.Rhs shim to handle the
three cases (before, at, after go1.22).

(Alias.Rhs was recently added in CL CL 581615.)

Updates golang/go#66559

Change-Id: I25a35c14f3ef5ddb77712afcce17f960dd181b5c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/581635
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/aliases/aliases_go121.go

@@ -15,11 +15,10 @@ import (
 // It will never be created by go/types.
 type Alias struct{}
 
-func (*Alias) String() string { panic("unreachable") }
-
+func (*Alias) String() string         { panic("unreachable") }
 func (*Alias) Underlying() types.Type { panic("unreachable") }
-
-func (*Alias) Obj() *types.TypeName { panic("unreachable") }
+func (*Alias) Obj() *types.TypeName   { panic("unreachable") }
+func Rhs(alias *Alias) types.Type     { panic("unreachable") }
 
 // Unalias returns the type t for go <=1.21.
 func Unalias(t types.Type) types.Type { return t }

internal/aliases/aliases_go122.go

@@ -17,6 +17,17 @@ import (
 // Alias is an alias of types.Alias.
 type Alias = types.Alias
 
+// Rhs returns the type on the right-hand side of the alias declaration.
+func Rhs(alias *Alias) types.Type {
+	if alias, ok := any(alias).(interface{ Rhs() types.Type }); ok {
+		return alias.Rhs() // go1.23+
+	}
+
+	// go1.22's Alias didn't have the Rhs method,
+	// so Unalias is the best we can do.
+	return Unalias(alias)
+}
+
 // Unalias is a wrapper of types.Unalias.
 func Unalias(t types.Type) types.Type { return types.Unalias(t) }
 

internal/gcimporter/iexport.go

@@ -21,7 +21,6 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"unsafe"
 
 	"golang.org/x/tools/go/types/objectpath"
 	"golang.org/x/tools/internal/aliases"
@@ -529,7 +528,7 @@ func (p *iexporter) doDecl(obj types.Object) {
 			if alias, ok := t.(*aliases.Alias); ok {
 				// Preserve materialized aliases,
 				// even of non-exported types.
-				t = aliasRHS(alias)
+				t = aliases.Rhs(alias)
 			}
 			w.typ(t, obj.Pkg())
 			break
@@ -1331,19 +1330,3 @@ func (e internalError) Error() string { return "gcimporter: " + string(e) }
 func internalErrorf(format string, args ...interface{}) error {
 	return internalError(fmt.Sprintf(format, args...))
 }
-
-// aliasRHS removes exactly one Alias constructor.
-func aliasRHS(alias *aliases.Alias) types.Type {
-	// TODO(adonovan): if proposal #66559 is accepted, this will
-	// become Alias.RHS(alias). In the meantime, we must punch
-	// through the drywall.
-	type go123Alias struct {
-		_   *types.TypeName
-		_   *types.TypeParamList
-		RHS types.Type
-		_   types.Type
-	}
-	var raw *go123Alias
-	*(**aliases.Alias)(unsafe.Pointer(&raw)) = alias
-	return raw.RHS
-}