The map visitor for Dynamic was expecting a &str for the key, but the
serde_json deserializer internally uses a Cow string, which can be
Borrowed or Owned. In the case of Owned, the serde_json key deserializer
is calling visit_string on the Visitor, which for &str will result in
the error:
Error("invalid type: string \"a\", expected a borrowed string", line: 0,
column: 0)
smartstring actually has its own Visitor implementation that handles
both cases, so we can use it instead of an explicit conversion.
