expand: join NameRef with Kind in Variable

Nameref variables always contain strings, so it's more appropriate to
describe them with a value kind instead of a boolean attribute applying
to all value kinds.

This also means we can fit Kind in the space left by the removed
boolean, recovering about a third of the extra weight added to Variable
by the previous commit.

name   old time/op    new time/op    delta
Run-8    1.45ms ± 1%    1.45ms ± 1%    ~     (p=0.937 n=6+6)

name   old alloc/op   new alloc/op   delta
Run-8    81.5kB ± 0%    80.8kB ± 0%  -0.88%  (p=0.002 n=6+6)

name   old allocs/op  new allocs/op  delta
Run-8       882 ± 0%       881 ± 0%    ~     (p=0.182 n=6+6)

expand/environ.go

@@ -46,11 +46,12 @@ type WriteEnviron interface {
 	Set(name string, vr Variable) error
 }
 
-type ValueKind int
+type ValueKind uint8
 
 const (
 	Unset ValueKind = iota
 	String
+	NameRef
 	Indexed
 	Associative
 )
@@ -68,11 +69,10 @@ type Variable struct {
 	Local    bool
 	Exported bool
 	ReadOnly bool
-	NameRef  bool // if true, Value must be string
 
 	Kind ValueKind
 
-	Str  string            // Used when Kind is String.
+	Str  string            // Used when Kind is String or NameRef.
 	List []string          // Used when Kind is Indexed.
 	Map  map[string]string // Used when Kind is Associative.
 }
@@ -109,7 +109,7 @@ const maxNameRefDepth = 100
 func (v Variable) Resolve(env Environ) (string, Variable) {
 	name := ""
 	for i := 0; i < maxNameRefDepth; i++ {
-		if !v.NameRef {
+		if v.Kind != NameRef {
 			return name, v
 		}
 		name = v.Str // keep name for the next iteration

expand/param.go

@@ -94,7 +94,7 @@ func (cfg *Config) paramExp(pe *syntax.ParamExp) (string, error) {
 		var strs []string
 		if pe.Names != 0 {
 			strs = cfg.namesByPrefix(pe.Param.Value)
-		} else if orig.NameRef {
+		} else if orig.Kind == NameRef {
 			strs = append(strs, orig.Str)
 		} else if vr.Kind == Indexed {
 			for i, e := range vr.List {

interp/interp.go

@@ -883,13 +883,13 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 		case "readonly":
 			modes = append(modes, "-r")
 		case "nameref":
-			modes = append(modes, "-n")
+			valType = "-n"
 		}
 		for _, opt := range x.Opts {
 			switch s := r.literal(opt); s {
-			case "-x", "-r", "-n":
+			case "-x", "-r":
 				modes = append(modes, s)
-			case "-a", "-A":
+			case "-a", "-A", "-n":
 				valType = s
 			case "-g":
 				global = true
@@ -919,8 +919,6 @@ func (r *Runner) cmd(ctx context.Context, cm syntax.Command) {
 						vr.Exported = true
 					case "-r":
 						vr.ReadOnly = true
-					case "-n":
-						vr.NameRef = true
 					}
 				}
 				r.setVar(name, as.Index, vr)

interp/test.go

@@ -12,6 +12,7 @@ import (
 
 	"golang.org/x/crypto/ssh/terminal"
 
+	"mvdan.cc/sh/v3/expand"
 	"mvdan.cc/sh/v3/syntax"
 )
 
@@ -175,7 +176,7 @@ func (r *Runner) unTest(ctx context.Context, op syntax.UnTestOperator, x string)
 	case syntax.TsVarSet:
 		return r.lookupVar(x).IsSet()
 	case syntax.TsRefVar:
-		return r.lookupVar(x).NameRef
+		return r.lookupVar(x).Kind == expand.NameRef
 	case syntax.TsNot:
 		return x == ""
 	default:

interp/vars.go

@@ -163,8 +163,6 @@ func (r *Runner) setVar(name string, index syntax.ArithmExpr, vr expand.Variable
 	if name2, var2 := cur.Resolve(r.Env); name2 != "" {
 		name = name2
 		cur = var2
-		vr.NameRef = false
-		cur.NameRef = false
 	}
 
 	if vr.Kind == expand.String && index == nil {
@@ -246,6 +244,9 @@ func (r *Runner) assignVal(as *syntax.Assign, valType string) expand.Variable {
 		s := r.literal(as.Value)
 		if !as.Append || !prev.IsSet() {
 			prev.Kind = expand.String
+			if valType == "-n" {
+				prev.Kind = expand.NameRef
+			}
 			prev.Str = s
 			return prev
 		}
@@ -265,6 +266,9 @@ func (r *Runner) assignVal(as *syntax.Assign, valType string) expand.Variable {
 	if as.Array == nil {
 		// don't return the zero value, as that's an unset variable
 		prev.Kind = expand.String
+		if valType == "-n" {
+			prev.Kind = expand.NameRef
+		}
 		prev.Str = ""
 		return prev
 	}