Add a `default` and `value` attribute parameters

[JelteF]

This would allow the same flexibility that's provided by derive_new for From, TryFrom, FromStr and Constructor. It would also be nice to add new as a derive as well, so derive_more can be a drop-in replacement for derive_new.

[JelteF]

@ffuugoo @tyranron This is needed to make Error derives with a Backtrace really nice. Then you can do something like:

#[derive(From, Error, Display)]
struct SimpleError;

#[derive(From, Error, Display)]
#[display(fmt="Some error")]
struct BacktraceError {
   source: SimpleError,
	#[from(value=::std::backtrace::capture())]
   backtrace: Backtrace,
}

fn simple_failing_function() -> Result<(), SimpleError> {
    Err(SimpleError)
}

fn backtrace_failing_function() -> Result<(), BacktraceError> {
    simple_failing_function()?
}

[JelteF]

Possibly we would want this case to have a special syntax for usability, e.g. #[from(backtrace_capture)]

[tyranron]

@JelteF I'm agree with having derive_new in derive_more! 🤘

But regarding the syntax #[from(value=::std::backtrace::capture())], I still think it's more idiomatically to use the naming adopted by serde_derive: #[from(with = ::std::backtrace::capture())].

Also, it worth considering the syntax in smart-default. Having some implicit coercions, it's quite neat and handy in practice (we use it a lot aong with derive_more):

#[default = "localhost"]
host: Cow<'static, str>,

[jmjoy]

This is a good idea, so how is the progress now?

[JelteF]

Removing this from the 1.0 milestone, since it doesn't require breaking changes.

[ModProg]

Maybe it would also be nice to be able to define Self::new as an alias to Default without needing to specify that on every field.