deserialize: flexible defaults for missing struct fields

[anguslees]

Currently I have to define a newtype (eg: "BoolDefaultTrue") every time I want an optional struct field to have a non-Default::default() default, which is clumsy. Ideally I could just say something like:

struct Foo {
    #[serde(default=true)]
    enabled: bool,
}

... and this would fill in the value true if it was missing from the serialised string. Super-awesome would be if serde also generated a Default trait implementation for Foo that respected the same values, but I could understand that being considered out of scope.

[oli-obk]

If you create a manual Default impl for Foo, serde still uses the Default impl of the fields` type.

Example: this impl doesn't cause the #[serde(default)] to set the field to true.

impl Default for Foo {
    fn default() -> Foo {
        Foo {
            enabled: true,
        }
    }
}

@erickt: is using the surrounding type's Default impl (if present) too much magic? Alternatively requiring that the type impls Default if any field is annotated would be a breaking change and maybe not what one expects when annotating a field.

relevant code location: https://github.com/serde-rs/serde/blob/master/serde_codegen/src/de.rs#L994

[erickt]

This was implemented in #238!