fix(decode): allocate embedded pointer maps and settle case collisions

Assisted-by: GLM 5.3

decode.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding"
 	"fmt"
+	"maps"
 	"reflect"
 	"slices"
 	"strings"
@@ -337,7 +338,8 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 	if len(schema.required) > 0 {
 		seen = make(map[string]bool, len(tbl))
 	}
-	for key, val := range tbl {
+	for _, key := range d.tableKeys(tbl) {
+		val := tbl[key]
 		// A key that is already lowercase, which document keys usually are,
 		// hits the map directly; only a miss pays for the case fold.
 		resolved := key
@@ -349,12 +351,15 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 		if !ok {
 			if schema.embedMaps != nil {
 				// Leftover keys land in an untagged embedded map, the inverse
-				// of the encoder inlining that map's entries.
+				// of the encoder inlining that map's entries. The assign call
+				// rather than assignMap itself lets it allocate the embedded
+				// pointer the field may be, the way any other destination is
+				// reached.
 				mv, err := fieldByIndex(dst, schema.embedMaps[0])
 				if err != nil {
 					return newDecodeError(key, err)
 				}
-				if err := d.assignMap(map[string]any{key: val}, mv); err != nil {
+				if err := d.assign(map[string]any{key: val}, mv); err != nil {
 					return newDecodeError(key, err)
 				}
 			}
@@ -379,6 +384,18 @@ func (d *decoder) assignStruct(tbl map[string]any, dst reflect.Value) error {
 	return nil
 }
 
+// tableKeys returns the keys of tbl in the order the document wrote them
+// when the node index knows it, and in sorted order otherwise, the order a
+// hand-built tree or a node-free parse offers. The order settles which of
+// two keys that differ only in case wins one field: the same key wins every
+// run, instead of whichever a map iteration happened to hand out.
+func (d *decoder) tableKeys(tbl map[string]any) []string {
+	if node := d.nodeOf(tbl); node != nil {
+		return node.Keys()
+	}
+	return slices.Sorted(maps.Keys(tbl))
+}
+
 func (d *decoder) assignMap(tbl map[string]any, dst reflect.Value) error {
 	if dst.Type().Key().Kind() != reflect.String {
 		return fmt.Errorf("interpres: map key must be a string, got %s", dst.Type().Key())
@@ -499,7 +516,7 @@ func (d *decoder) setLocalTimeValue(t time.Time, dst reflect.Value) error {
 				t.Hour(), t.Minute(), t.Second(), t.Nanosecond(), d.loc)))
 			return nil
 		}
-		return fmt.Errorf("interpres: cannot assign local date-time to time.Time; set Decoder.LocalTimeLocation to choose the zone")
+		return fmt.Errorf("interpres: cannot assign local date-time to time.Time; set LocalTimeLocation to choose the zone")
 	}
 	return fmt.Errorf("interpres: cannot assign local date-time to %s", dst.Type())
 }

decode_test.go

@@ -1690,3 +1690,85 @@ func TestErrorMessagesGolden(t *testing.T) {
 		})
 	}
 }
+
+// TestLocalTimeLocationLeavesOffsetsAlone pins that the option's zone is
+// used for local date-times only: an offset date-time keeps the offset the
+// document wrote.
+func TestLocalTimeLocationLeavesOffsetsAlone(t *testing.T) {
+	var cfg struct {
+		Stamp time.Time `toml:"stamp"`
+	}
+	err := Unmarshal([]byte("stamp = 1979-05-27T07:32:00-07:00\n"), &cfg,
+		LocalTimeLocation(time.FixedZone("Prague", 2*60*60)))
+	if err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if _, off := cfg.Stamp.Zone(); off != -7*60*60 {
+		t.Errorf("offset = %d, want the document's -07:00", off/3600)
+	}
+}
+
+// TestUnmarshalCaseCollisionIsDeterministic pins that two keys differing
+// only in case, both matching one field, resolve the same way on every run
+// and on both decode paths.
+func TestUnmarshalCaseCollisionIsDeterministic(t *testing.T) {
+	type cfg struct {
+		Host string `toml:"host"`
+	}
+	in := []byte("Host = \"upper\"\nhost = \"lower\"\n")
+	// The tree path iterates a map, so pin the winner across many runs.
+	var want string
+	for range 50 {
+		var viaTree cfg
+		if err := treeDecodeInto(in, &viaTree); err != nil {
+			t.Fatalf("tree decode: %v", err)
+		}
+		if want == "" {
+			want = viaTree.Host
+		} else if viaTree.Host != want {
+			t.Fatalf("tree decode is not deterministic: %q then %q", want, viaTree.Host)
+		}
+	}
+	var targeted cfg
+	if err := Unmarshal(in, &targeted); err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if targeted.Host != want {
+		t.Errorf("targeted Host = %q, tree %q", targeted.Host, want)
+	}
+}
+
+// TestUnmarshalEmbeddedPointerMap pins that leftover keys reach an embedded
+// pointer to a map, allocating it, rather than panicking on the pointer.
+func TestUnmarshalEmbeddedPointerMap(t *testing.T) {
+	type Extra map[string]int
+	type cfg struct {
+		*Extra
+		Name string `toml:"name"`
+	}
+	var c cfg
+	err := Unmarshal([]byte("name = \"x\"\nrogue = 7\n"), &c)
+	if err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if c.Extra == nil || (*c.Extra)["rogue"] != 7 {
+		t.Errorf("embedded map = %v, want rogue allocated and filled", c.Extra)
+	}
+}
+
+// TestUnmarshalIgnoresEncodeTagOptions pins that the emission-only tag
+// options change nothing on the decode side.
+func TestUnmarshalIgnoresEncodeTagOptions(t *testing.T) {
+	type cfg struct {
+		Name string `toml:"name,omitempty"`
+		Port int    `toml:"port,omitzero,comment=The port"`
+	}
+	var c cfg
+	err := Unmarshal([]byte("name = \"x\"\nport = 8080\n"), &c)
+	if err != nil {
+		t.Fatalf("unmarshal: %v", err)
+	}
+	if c.Name != "x" || c.Port != 8080 {
+		t.Errorf("cfg = %+v, want both fields filled", c)
+	}
+}