hclwrite: Add (*Attribute).SetName() method

Reference: #340
Reference: #680

Similar to `(*hclwrite.Block).SetType()`, this new method enables in-place renaming of an attribute.

hclwrite/ast_attribute.go

@@ -49,3 +49,10 @@ func (a *Attribute) init(name string, expr *Expression) {
 func (a *Attribute) Expr() *Expression {
 	return a.expr.content.(*Expression)
 }
+
+// SetName updates the name of the attribute to a given name.
+func (a *Attribute) SetName(name string) {
+	nameTok := newIdentToken(name)
+	nameObj := newIdentifier(nameTok)
+	a.name.ReplaceWith(nameObj)
+}

hclwrite/ast_attribute_test.go

@@ -0,0 +1,78 @@
+package hclwrite
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/davecgh/go-spew/spew"
+	"github.com/google/go-cmp/cmp"
+	"github.com/hashicorp/hcl/v2"
+	"github.com/hashicorp/hcl/v2/hclsyntax"
+)
+
+func TestAttributeSetName(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		src     string
+		oldName string
+		newName string
+		want    Tokens
+	}{
+		{
+			"old = 123",
+			"old",
+			"new",
+			Tokens{
+				{
+					Type:         hclsyntax.TokenIdent,
+					Bytes:        []byte(`new`),
+					SpacesBefore: 0,
+				},
+				{
+					Type:         hclsyntax.TokenEqual,
+					Bytes:        []byte{'='},
+					SpacesBefore: 1,
+				},
+				{
+					Type:         hclsyntax.TokenNumberLit,
+					Bytes:        []byte(`123`),
+					SpacesBefore: 1,
+				},
+				{
+					Type:         hclsyntax.TokenEOF,
+					Bytes:        []byte{},
+					SpacesBefore: 0,
+				},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(fmt.Sprintf("%s %s in %s", test.oldName, test.newName, test.src), func(t *testing.T) {
+			t.Parallel()
+
+			f, diags := ParseConfig([]byte(test.src), "", hcl.Pos{Line: 1, Column: 1})
+
+			if len(diags) != 0 {
+				for _, diag := range diags {
+					t.Logf("- %s", diag.Error())
+				}
+				t.Fatalf("unexpected diagnostics")
+			}
+
+			attr := f.Body().GetAttribute(test.oldName)
+			attr.SetName(test.newName)
+			got := f.BuildTokens(nil)
+			format(got)
+
+			if !reflect.DeepEqual(got, test.want) {
+				diff := cmp.Diff(test.want, got)
+				t.Errorf("wrong result\ngot:  %s\nwant: %s\ndiff:\n%s", spew.Sdump(got), spew.Sdump(test.want), diff)
+			}
+		})
+	}
+}