This allows end users to use an arbitrary expression to decide whether
or not to serialize some field. This expression has access to all the
fields in the struct, but none of the internal state of the Serialize
implementation. For structs, serde implements this by creating a
temporary trait and implementing the struct for it. For struct variants,
the fields are copied by reference into a temporary struct first
before implementing the temporary trait.
This also fixes a bug where the serde_codegen wasn't making calls to
Serializer::serialize_{tuple,struct}_variant{,_elt}.
The only way to safely serialize a `Path` is to use
`.to_string_lossy()`, which replaces invalid UTF-8 characters with
the U+FFFD replacement character. Unfortunately this would lose
information, so for our default implementations, it'd be better
to punt and report an error, and leave it up to the user to
decide if they want to use the lossy encoding.
Unfortunately, we had no way for `Serializer`s to require some methods
on `Serializer::Error`, so there was no way before this patch for
the `Path` implementation to generically report that it cannot encode
this value. This adds that implementation.
breaking-change
Closes#57.
