generic fields cannot be defaulted (or skipped)

[colin-kiegel]

#[derive(Builder)]
struct Generic<T> {
    #[builder(setter(skip))] // <-- implicit default
    pub ipsum: T,
    #[builder(default)] // <-- explicit default
    pub dolor: T,
}

fails with

error[E0277]: the trait bound `T: std::default::Default` is not satisfied
|
| #[derive(Builder)]
|          ^^^^^^^ the trait `std::default::Default` is not implemented for `T` 
= help: consider adding a `where T: std::default::Default` bound
= note: required by `std::default::Default::default`

[colin-kiegel]

The reason is that we never emit bounds like T: Default (cmp. #91)

Current workarounds are:

• add a T: Default bound on the target type (e.g. struct Generic<T: Default>)
• use a custom default #[builder(default="magic_generic_fn_in_scope()")]

[TedDriggs]

I looked at this when fixing #91, but I couldn't find a way to express "this is fine if a) a struct-level default was specified, b) an explicit field-level default was specified, or c) this field was set explicitly at runtime" which made me suspect it's not possible to handle ergonomically through the type system.

[botev]

This is even worse when we have something like:

Generic<T> {
list: Vec<T>
}

Where for certain cases I know T does not implement Default, so I can't enforce it, however obviously Vec<T> does... For such generic types, I can not even use the builder.

[TedDriggs]

Using an explicit default seems like a simple enough workaround that we don’t need to keep this issue open.