fix(encode): keep a []any of tables in the value-array form

Assisted-by: GLM 5.3

CHANGELOG.md

@@ -71,6 +71,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   cannot be carried verbatim by the literal form: a run of three single quotes,
   a control character, or a lone carriage return. Such values previously
   produced output that did not re-parse.
+- A `[]any` holding only tables marshals in the value-array form with inline
+  tables, keeping the type `Parse` produces for such an array. It previously
+  took the `[[header]]` form, so a round-trip changed the value's type from
+  `[]any` to `[]map[string]any`.
 - Decoding into a `uint` destination checks the type's platform width instead
   of only the fixed widths, so a 32-bit `uint` no longer truncates silently;
   decoding a finite float beyond the `float32` range is an overflow error

docs/API.md

@@ -331,6 +331,11 @@ tree, _ := interpres.Parse([]byte(`arr = [1, {a = 2}, "x"]`))
 out, _ := interpres.Marshal(tree) // arr = [1, {a = 2}, "x"]
 ```
 
+A `[]any` holding only tables keeps the value-array form as well, because that
+is the shape `Parse` gives a value array of inline tables; emitting it as
+`[[headers]]` would re-parse as `[]map[string]any` and change the value's type
+across a round-trip.
+
 ### Empty arrays
 
 A nil slice is always omitted. An empty (length 0) array of tables is always

encode.go

@@ -388,6 +388,13 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, ctx string) error
 			break
 		}
 	}
+	// A []any of tables is what Parse produces for a value array of inline
+	// tables; the [[header]] form would re-parse as []map[string]any and so
+	// change the value's Go type across a round-trip. The header form is
+	// reserved for typed table slices.
+	if v.Type().Elem().Kind() == reflect.Interface {
+		allTables = false
+	}
 	if allTables {
 		subs := make([]*tomlDoc, n)
 		for i := range n {

encode_test.go

@@ -597,6 +597,36 @@ func TestMarshalMixedArrayWithInlineTable(t *testing.T) {
 	}
 }
 
+// A []any of tables is what Parse produces for a value array of inline
+// tables; it must stay in the value-array form, or the output would re-parse
+// as []map[string]any and the round-trip would change the value's type.
+func TestMarshalValueArrayOfTablesStaysInline(t *testing.T) {
+	for _, doc := range []string{
+		"0=[{}]",
+		"a = [{x = 1}, {x = 2}]\n",
+		"b = [{x = 1}, 2, \"three\"]\n",
+	} {
+		tree, err := Parse([]byte(doc))
+		if err != nil {
+			t.Fatalf("%s: parse: %v", doc, err)
+		}
+		out, err := Marshal(tree)
+		if err != nil {
+			t.Fatalf("%s: marshal: %v", doc, err)
+		}
+		if bytes.HasPrefix(out, []byte("[[")) {
+			t.Errorf("%s: emitted the [[header]] form for a value array:\n%s", doc, out)
+		}
+		re, err := Parse(out)
+		if err != nil {
+			t.Fatalf("%s: re-parse: %v\ndoc:\n%s", doc, err, out)
+		}
+		if !tomlEqual(tree, re) {
+			t.Errorf("%s: round-trip changed the tree:\nwas: %#v\nnow: %#v\ndoc:\n%s", doc, tree, re, out)
+		}
+	}
+}
+
 func TestMarshalNestedInlineTables(t *testing.T) {
 	tree := map[string]any{
 		"mix": []any{

testdata/fuzz/FuzzParse/d0e5d6fc3b865cfa

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("0=[{}]")