Fix defaults serialization and 'invalid type: unit value' deserialization error (#60)

[tyranron]

A second attempt to fix #60

Dupe of #95, but tries to follow the feedback.

Background

As @vorner wrote:

I think there are two issues here, aren't there?

First one is how to use defaults ‒ and the answer for that one is to use the serde's default handling (eg. the default attribute on fields).

The other problem is the annoying unit value error. I managed to work around that one by merging an empty source as the first one, eg config.merge(File::from_str("", FileFormat::Toml))?. Could something like this be already done as part of the constructor? I mean, make sure whatever is tried to be deserialized looks like an empty map to serde, not unit.

Solution

1. The first issue is solved by providing two new methods:

   • set_defaults(&mut self, val: &T) -> Result<&mut Self>, where T: Serialize, which allows to mutate defaults values of Config by serializing them from given value.
   • try_defaults_from(val: &T) -> Result<Self>, where T: Serialize, which is the brother of try_from() but operates with defaults (not overrides).

2. The second issue is solved by creating empty ValueKind::Table rather than ValueKind::Nil as @derekdreery pointed out.

Checklist

• 1. Code style conformed.
• 2. Tests are added.
• 3. Documentation is added/corrected.

[tyranron]

ping @mehcode

[mehcode]

@tyranron Thank you very much for the fix. I decided to take out the additional methods ( pending the big rewrite in #111; don't want to increase the API surface ) but your added tests still pass if you add #[serde(default)] to your struct.