Add readTypeOfValue (#3728)

This improves csv-import a bit since it allows us to skip allocation of
values in the struct when we only want the type.

go/types/leaf_sequence.go

@@ -121,8 +121,7 @@ func (seq leafSequence) typeOf() *Type {
 	count := dec.readCount()
 	ts := make([]*Type, count)
 	for i := uint64(0); i < count; i++ {
-		v := dec.readValue()
-		ts[i] = v.typeOf()
+		ts[i] = dec.readTypeOfValue()
 	}
 	return makeCompoundType(kind, makeCompoundType(UnionKind, ts...))
 }

go/types/struct.go

@@ -159,6 +159,10 @@ func (s Struct) WalkRefs(cb RefCallback) {
 
 func (s Struct) typeOf() *Type {
 	dec := s.decoder()
+	return readStructTypeOfValue(&dec)
+}
+
+func readStructTypeOfValue(dec *valueDecoder) *Type {
 	dec.skipKind()
 	name := dec.readString()
 	count := dec.readCount()
@@ -167,7 +171,7 @@ func (s Struct) typeOf() *Type {
 		typeFields[i] = StructField{
 			Name:     dec.readString(),
 			Optional: false,
-			Type:     dec.readValue().typeOf(),
+			Type:     dec.readTypeOfValue(),
 		}
 	}
 	return makeStructTypeQuickly(name, typeFields)

go/types/value_decoder.go

@@ -348,6 +348,42 @@ func (r *valueDecoder) skipValue() {
 	}
 }
 
+// readTypeOfValue is basically readValue().typeOf() but it ensures that we do
+// not allocate values where we do not need to.
+func (r *valueDecoder) readTypeOfValue() *Type {
+	k := r.peekKind()
+	switch k {
+	case BlobKind:
+		r.skipBlob()
+		return BlobType
+	case BoolKind:
+		r.skipKind()
+		r.skipBool()
+		return BoolType
+	case NumberKind:
+		r.skipKind()
+		r.skipNumber()
+		return NumberType
+	case StringKind:
+		r.skipKind()
+		r.skipString()
+		return StringType
+	case ListKind, MapKind, RefKind, SetKind:
+		// These do not decode the actual values anyway.
+		return r.readValue().typeOf()
+	case StructKind:
+		return readStructTypeOfValue(r)
+	case TypeKind:
+		r.skipKind()
+		r.skipType()
+		return TypeType
+	case CycleKind, UnionKind, ValueKind:
+		d.Panic("A value instance can never have type %s", k)
+	}
+
+	panic("not reachable")
+}
+
 func (r *valueDecoder) copyValue(w nomsWriter) {
 	start := r.pos()
 	r.skipValue()