Specialize map[string]iface when unmarshaling kvs

name                               old time/op    new time/op    delta
UnmarshalDataset/config-8            12.3ms ± 0%    10.9ms ± 0%  -11.36%  (p=0.008 n=5+5)
UnmarshalDataset/canada-8            55.2ms ± 0%    55.4ms ± 1%     ~     (p=0.690 n=5+5)
UnmarshalDataset/citm_catalog-8      16.5ms ± 1%    15.7ms ± 0%   -4.43%  (p=0.008 n=5+5)
UnmarshalDataset/twitter-8           7.01ms ± 1%    6.63ms ± 0%   -5.34%  (p=0.008 n=5+5)
UnmarshalDataset/code-8              52.0ms ± 0%    48.7ms ± 2%   -6.43%  (p=0.008 n=5+5)
UnmarshalDataset/example-8            119µs ± 0%     110µs ± 3%   -6.81%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-8     432ns ± 1%     431ns ± 1%     ~     (p=0.222 n=5+5)
Unmarshal/SimpleDocument/map-8        573ns ± 1%     540ns ± 1%   -5.64%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-8     33.7µs ± 0%    33.6µs ± 0%     ~     (p=0.310 n=5+5)
Unmarshal/ReferenceFile/map-8        44.2µs ± 1%    41.7µs ± 1%   -5.63%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-8          7.28µs ± 1%    6.74µs ± 1%   -7.42%  (p=0.008 n=5+5)
[Geo mean]                            292µs          277µs        -4.91%

name                               old speed      new speed      delta
UnmarshalDataset/config-8          85.2MB/s ± 0%  96.1MB/s ± 0%  +12.82%  (p=0.008 n=5+5)
UnmarshalDataset/canada-8          39.9MB/s ± 0%  39.8MB/s ± 1%     ~     (p=0.690 n=5+5)
UnmarshalDataset/citm_catalog-8    33.9MB/s ± 1%  35.5MB/s ± 0%   +4.64%  (p=0.008 n=5+5)
UnmarshalDataset/twitter-8         63.1MB/s ± 1%  66.6MB/s ± 0%   +5.65%  (p=0.008 n=5+5)
UnmarshalDataset/code-8            51.6MB/s ± 0%  55.1MB/s ± 2%   +6.88%  (p=0.008 n=5+5)
UnmarshalDataset/example-8         68.3MB/s ± 0%  73.4MB/s ± 3%   +7.34%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-8  25.4MB/s ± 1%  25.5MB/s ± 1%     ~     (p=0.246 n=5+5)
Unmarshal/SimpleDocument/map-8     19.2MB/s ± 1%  20.4MB/s ± 1%   +5.99%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-8    156MB/s ± 0%   156MB/s ± 0%     ~     (p=0.310 n=5+5)
Unmarshal/ReferenceFile/map-8       119MB/s ± 1%   126MB/s ± 1%   +5.97%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-8        75.0MB/s ± 1%  81.0MB/s ± 1%   +8.01%  (p=0.008 n=5+5)
[Geo mean]                         56.1MB/s       59.0MB/s        +5.17%

name                               old alloc/op   new alloc/op   delta
UnmarshalDataset/config-8            5.26MB ± 0%    4.75MB ± 0%   -9.66%  (p=0.008 n=5+5)
UnmarshalDataset/canada-8            83.0MB ± 0%    83.0MB ± 0%   -0.00%  (p=0.008 n=5+5)
UnmarshalDataset/citm_catalog-8      34.7MB ± 0%    34.3MB ± 0%   -1.13%  (p=0.008 n=5+5)
UnmarshalDataset/twitter-8           12.7MB ± 0%    12.5MB ± 0%   -1.44%  (p=0.008 n=5+5)
UnmarshalDataset/code-8              15.3MB ± 0%    13.9MB ± 0%   -9.27%  (p=0.008 n=5+5)
UnmarshalDataset/example-8            186kB ± 0%     182kB ± 0%   -2.20%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-8      805B ± 0%      805B ± 0%     ~     (all equal)
Unmarshal/SimpleDocument/map-8       1.13kB ± 0%    1.12kB ± 0%   -1.41%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-8     20.9kB ± 0%    20.9kB ± 0%     ~     (all equal)
Unmarshal/ReferenceFile/map-8        36.4kB ± 0%    35.4kB ± 0%     ~     (p=0.079 n=4+5)
Unmarshal/HugoFrontMatter-8          7.20kB ± 0%    6.98kB ± 0%   -3.11%  (p=0.008 n=5+5)
[Geo mean]                            312kB          303kB        -2.86%

name                               old allocs/op  new allocs/op  delta
UnmarshalDataset/config-8              189k ± 0%      157k ± 0%  -16.80%  (p=0.029 n=4+4)
UnmarshalDataset/canada-8              782k ± 0%      782k ± 0%   -0.00%  (p=0.008 n=5+5)
UnmarshalDataset/citm_catalog-8        191k ± 0%      167k ± 0%  -12.75%  (p=0.000 n=4+5)
UnmarshalDataset/twitter-8            56.9k ± 0%     45.5k ± 0%  -20.02%  (p=0.016 n=5+4)
UnmarshalDataset/code-8                626k ± 0%      537k ± 0%  -14.22%  (p=0.008 n=5+5)
UnmarshalDataset/example-8            1.36k ± 0%     1.11k ± 0%  -18.53%  (p=0.008 n=5+5)
Unmarshal/SimpleDocument/struct-8      9.00 ± 0%      9.00 ± 0%     ~     (all equal)
Unmarshal/SimpleDocument/map-8         13.0 ± 0%      12.0 ± 0%   -7.69%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/struct-8        183 ± 0%       183 ± 0%     ~     (all equal)
Unmarshal/ReferenceFile/map-8           526 ± 0%       466 ± 0%  -11.41%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-8             126 ± 0%       112 ± 0%  -11.11%  (p=0.008 n=5+5)
[Geo mean]                            3.73k          3.34k       -10.51%

unmarshaler.go

@@ -230,15 +230,6 @@ func (d *decoder) fromParser(root reflect.Value) error {
 	return d.p.Error()
 }
 
-/*
-Rules for the unmarshal code:
-
-- The stack is used to keep track of which values need to be set where.
-- handle* functions <=> switch on a given ast.Kind.
-- unmarshalX* functions need to unmarshal a node of kind X.
-- An "object" is either a struct or a map.
-*/
-
 func (d *decoder) handleRootExpression(expr *ast.Node, v reflect.Value) error {
 	var x reflect.Value
 	var err error
@@ -400,6 +391,84 @@ func (d *decoder) handleArrayTableCollection(key ast.Iterator, v reflect.Value)
 	return d.handleArrayTable(key, v)
 }
 
+func (d *decoder) handleKeyValuePartMapStringInterface(key ast.Iterator, value *ast.Node, m map[string]interface{}) (reflect.Value, error) {
+	k := string(key.Node().Data)
+
+	newMap := false
+	if m == nil {
+		newMap = true
+		m = make(map[string]interface{}, 8)
+	}
+
+	set := false
+	v, ok := m[k]
+	if !ok || key.IsLast() {
+		set = true
+		v = nil
+	}
+
+	mv := reflect.ValueOf(&v).Elem()
+
+	x, err := d.handleKeyValueInner(key, value, mv)
+	if err != nil {
+		return reflect.Value{}, err
+	}
+	if x.IsValid() {
+		mv = x
+		set = true
+	}
+
+	if set {
+		m[k] = mv.Interface()
+	}
+
+	if newMap {
+		return reflect.ValueOf(m), nil
+	}
+
+	return reflect.Value{}, nil
+}
+
+func (d *decoder) handleKeyPartMapStringInterface(key ast.Iterator, m map[string]interface{}, nextFn handlerFn, makeFn valueMakerFn) (reflect.Value, error) {
+	newMap := false
+
+	k := string(key.Node().Data)
+	if m == nil {
+		newMap = true
+		m = make(map[string]interface{}, 8)
+	}
+
+	v, ok := m[k]
+	set := false
+
+	if !ok || v == nil {
+		set = true
+		v = makeFn().Interface()
+	}
+
+	mv := reflect.ValueOf(v)
+
+	x, err := nextFn(key, mv)
+	if err != nil {
+		return reflect.Value{}, err
+	}
+
+	if x.IsValid() {
+		mv = x
+		set = true
+	}
+
+	if set {
+		m[k] = mv.Interface()
+	}
+
+	if newMap {
+		return reflect.ValueOf(m), nil
+	}
+
+	return reflect.Value{}, nil
+}
+
 func (d *decoder) handleKeyPart(key ast.Iterator, v reflect.Value, nextFn handlerFn, makeFn valueMakerFn) (reflect.Value, error) {
 	var rv reflect.Value
 
@@ -416,6 +485,11 @@ func (d *decoder) handleKeyPart(key ast.Iterator, v reflect.Value, nextFn handle
 	case reflect.Map:
 		vt := v.Type()
 
+		if vt == mapStringInterfaceType {
+			m := v.Interface().(map[string]interface{})
+			return d.handleKeyPartMapStringInterface(key, m, nextFn, makeFn)
+		}
+
 		// Create the key for the map element. Convert to key type.
 		mk := reflect.ValueOf(string(key.Node().Data)).Convert(vt.Key())
 
@@ -999,6 +1073,11 @@ func (d *decoder) handleKeyValuePart(key ast.Iterator, value *ast.Node, v reflec
 	case reflect.Map:
 		vt := v.Type()
 
+		if vt == mapStringInterfaceType {
+			m := v.Interface().(map[string]interface{})
+			return d.handleKeyValuePartMapStringInterface(key, value, m)
+		}
+
 		mk := reflect.ValueOf(string(key.Node().Data))
 		mkt := stringType
 
@@ -1022,12 +1101,10 @@ func (d *decoder) handleKeyValuePart(key ast.Iterator, value *ast.Node, v reflec
 		if !mv.IsValid() {
 			set = true
 			mv = reflect.New(v.Type().Elem()).Elem()
-		} else {
+		} else if key.IsLast() {
-			if key.IsLast() {
+			var x interface{}
-				var x interface{}
+			mv = reflect.ValueOf(&x).Elem()
-				mv = reflect.ValueOf(&x).Elem()
+			set = true
-				set = true
-			}
 		}
 
 		nv, err := d.handleKeyValueInner(key, value, mv)