Decode: don't crash on embedded nil pointers (#808)

Also has the perks of reducing the overhead of FindByIndex:

```
name                                old time/op    new time/op    delta
UnmarshalDataset/config-32            17.0ms ± 1%    17.0ms ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32            71.6ms ± 1%    71.4ms ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/citm_catalog-32      24.2ms ± 3%    23.5ms ± 2%  -3.03%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-32           9.37ms ± 1%    9.09ms ± 2%  -2.97%  (p=0.032 n=5+5)
UnmarshalDataset/code-32              75.4ms ± 2%    74.9ms ± 0%    ~     (p=0.222 n=5+5)
UnmarshalDataset/example-32            147µs ±10%     136µs ± 1%  -7.14%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-32     512ns ± 2%     500ns ± 0%  -2.35%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-32        721ns ± 2%     702ns ± 1%  -2.68%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-32     40.1µs ± 0%    39.6µs ± 0%  -1.30%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-32        62.3µs ± 1%    60.6µs ± 0%  -2.83%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-32          10.8µs ± 1%    10.5µs ± 1%  -2.86%  (p=0.008 n=5+5)

name                                old speed      new speed      delta
UnmarshalDataset/config-32          61.8MB/s ± 1%  61.8MB/s ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32          30.8MB/s ± 1%  30.8MB/s ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/citm_catalog-32    23.0MB/s ± 3%  23.8MB/s ± 2%  +3.09%  (p=0.032 n=5+5)
UnmarshalDataset/twitter-32         47.2MB/s ± 1%  48.6MB/s ± 2%  +3.09%  (p=0.032 n=5+5)
UnmarshalDataset/code-32            35.6MB/s ± 2%  35.9MB/s ± 0%    ~     (p=0.222 n=5+5)
UnmarshalDataset/example-32         55.3MB/s ±10%  59.4MB/s ± 1%  +7.36%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-32  21.5MB/s ± 2%  22.0MB/s ± 0%  +2.41%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/map-32     15.2MB/s ± 2%  15.7MB/s ± 1%  +2.74%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-32    131MB/s ± 0%   132MB/s ± 0%  +1.31%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-32      84.1MB/s ± 1%  86.6MB/s ± 0%  +2.91%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-32        50.6MB/s ± 1%  52.1MB/s ± 1%  +2.93%  (p=0.008 n=5+5)

name                                old alloc/op   new alloc/op   delta
UnmarshalDataset/config-32            5.86MB ± 0%    5.86MB ± 0%    ~     (p=0.579 n=5+5)
UnmarshalDataset/canada-32            83.0MB ± 0%    83.0MB ± 0%    ~     (p=0.651 n=5+5)
UnmarshalDataset/citm_catalog-32      34.7MB ± 0%    34.7MB ± 0%    ~     (p=0.548 n=5+5)
UnmarshalDataset/twitter-32           12.7MB ± 0%    12.7MB ± 0%    ~     (p=1.000 n=5+5)
UnmarshalDataset/code-32              22.2MB ± 0%    22.2MB ± 0%    ~     (p=0.841 n=5+5)
UnmarshalDataset/example-32            186kB ± 0%     186kB ± 0%    ~     (p=0.111 n=5+5)
Unmarshal/SimpleDocument/struct-32      805B ± 0%      805B ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/map-32       1.13kB ± 0%    1.13kB ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/struct-32     20.9kB ± 0%    20.9kB ± 0%    ~     (p=0.643 n=5+5)
Unmarshal/ReferenceFile/map-32        38.3kB ± 0%    38.3kB ± 0%    ~     (p=0.397 n=5+5)
Unmarshal/HugoFrontMatter-32          7.44kB ± 0%    7.44kB ± 0%    ~     (all equal)

name                                old allocs/op  new allocs/op  delta
UnmarshalDataset/config-32              227k ± 0%      227k ± 0%    ~     (p=1.000 n=5+5)
UnmarshalDataset/canada-32              782k ± 0%      782k ± 0%    ~     (all equal)
UnmarshalDataset/citm_catalog-32        192k ± 0%      192k ± 0%    ~     (p=0.968 n=4+5)
UnmarshalDataset/twitter-32            56.9k ± 0%     56.9k ± 0%    ~     (p=0.429 n=4+5)
UnmarshalDataset/code-32               1.05M ± 0%     1.05M ± 0%    ~     (p=0.556 n=4+5)
UnmarshalDataset/example-32            1.36k ± 0%     1.36k ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/struct-32      9.00 ± 0%      9.00 ± 0%    ~     (all equal)
Unmarshal/SimpleDocument/map-32         13.0 ± 0%      13.0 ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/struct-32        183 ± 0%       183 ± 0%    ~     (all equal)
Unmarshal/ReferenceFile/map-32           642 ± 0%       642 ± 0%    ~     (all equal)
Unmarshal/HugoFrontMatter-32             141 ± 0%       141 ± 0%    ~     (all equal)
```

Fixes #807

Author: pelletier

unmarshaler.go

@@ -123,7 +123,7 @@ type decoder struct {
 	stashedExpr bool
 
 	// Skip expressions until a table is found. This is set to true when a
-	// table could not be create (missing field in map), so all KV expressions
+	// table could not be created (missing field in map), so all KV expressions
 	// need to be skipped.
 	skipUntilTable bool
 
@@ -483,7 +483,7 @@ func (d *decoder) handleKeyPart(key ast.Iterator, v reflect.Value, nextFn handle
 		d.errorContext.Struct = t
 		d.errorContext.Field = path
 
-		f := v.FieldByIndex(path)
+		f := fieldByIndex(v, path)
 		x, err := nextFn(key, f)
 		if err != nil || d.skipUntilTable {
 			return reflect.Value{}, err
@@ -1071,7 +1071,7 @@ func (d *decoder) handleKeyValuePart(key ast.Iterator, value *ast.Node, v reflec
 		d.errorContext.Struct = t
 		d.errorContext.Field = path
 
-		f := v.FieldByIndex(path)
+		f := fieldByIndex(v, path)
 		x, err := d.handleKeyValueInner(key, value, f)
 		if err != nil {
 			return reflect.Value{}, err
@@ -1135,6 +1135,21 @@ func initAndDereferencePointer(v reflect.Value) reflect.Value {
 	return elem
 }
 
+// Same as reflect.Value.FieldByIndex, but creates pointers if needed.
+func fieldByIndex(v reflect.Value, path []int) reflect.Value {
+	for i, x := range path {
+		v = v.Field(x)
+
+		if i < len(path)-1 && v.Kind() == reflect.Pointer {
+			if v.IsNil() {
+				v.Set(reflect.New(v.Type().Elem()))
+			}
+			v = v.Elem()
+		}
+	}
+	return v
+}
+
 type fieldPathsMap = map[string][]int
 
 var globalFieldPathsCache atomic.Value // map[danger.TypeID]fieldPathsMap
@@ -1192,7 +1207,11 @@ func forEachField(t reflect.Type, path []int, do func(name string, path []int))
 		}
 
 		if f.Anonymous && name == "" {
-			forEachField(f.Type, fieldPath, do)
+			t2 := f.Type
+			if t2.Kind() == reflect.Pointer {
+				t2 = t2.Elem()
+			}
+			forEachField(t2, fieldPath, do)
 			continue
 		}
 

unmarshaler_test.go

@@ -2451,6 +2451,21 @@ answer = 42
 	require.Error(t, err)
 }
 
+func TestIssue807(t *testing.T) {
+	type A struct {
+		Name string `toml:"name"`
+	}
+
+	type M struct {
+		*A
+	}
+
+	var m M
+	err := toml.Unmarshal([]byte(`name = 'foo'`), &m)
+	require.NoError(t, err)
+	require.Equal(t, "foo", m.Name)
+}
+
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {
 		desc string