bson/bsoncodec: check kind encoder cache size at startup

This check will guard against changes to the reflect.Kind constants.

Author: charlievieth

bson/bsoncodec/codec_cache.go

@@ -82,6 +82,13 @@ func (c *typeDecoderCache) Clone() *typeDecoderCache {
 	return cc
 }
 
+func init() {
+	if s := reflect.Kind(len(kindEncoderCache{}.entries)).String(); s != "kind27" {
+		panic("The capacity of kindEncoderCache is too small.\n" +
+			"This is due to a new type being added to reflect.Kind.")
+	}
+}
+
 // atomic.Value requires that all calls to Store() have the same concrete type
 // so we wrap the ValueEncoder with a kindEncoderCacheEntry to ensure the type
 // is always the same (since different concrete types may implement the