support for layered config using a serde Deserializer as a source #7
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This allows a serde Deserializer, representing a config file, to be used as a value source, via the builder API, which is also new in this commit. To use the serde feature, you have to put the `#[conf(serde)]` tag on your struct. We also introduce a `ConfSerde` extension trait, which is implemented when `#[conf(serde)]` is annotated, and provides implementation details for this layered config feature on that struct. The priority order is args > env > serde > defaults We also support this in subcommands, via the `SubcommandsSerde` extension trait. To further adjust the serde system, most fields have serde options like `serde(rename)` and `serde(skip)`. Structs have a `serde(allow_unknown_fields)` option. Parameter and Repeat fields have a `serde(use_value_parser)` option, which can be useful if you just want serde to read a string and then forward it to the value parser.
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This allows a serde Deserializer, representing a config file, to be used as a value source, via the new
ConfSerdeextension trait.To use it you have to put the
#[conf(serde)]tag on your struct.The priority order is args > env > serde > defaults.
You can use this with a config file in any serde supported format. Or the data can come from another source, such as a json Value that you loaded and modified, or a figment::Value for instance.
I am very happy that this approach avoids creating any specific dependency on code that opens a file or reads any particular format, and that the error reporting is still comprehensive in the way that I want. This is at least proof of concept for a way that I would be happy for this to work in
conf.Still needs a lot more tests and documentation, and we don't really support any serde options right now, but we have a plan for that.