Derive `Arbitrary` for types with lifetimes

[mcy]

derive(Arbitrary) currently won't derive Arbitrary if the type has a lifetime parameter in it. Most of the time this is fine, because Arbitrary: 'static. However, it seems like it would be possible to implement Arbitrary if all of the type parameters were set to 'static. This is somewhat inconsistent with Cow<'static, impl Arbitrary>: Arbitrary.

I haven't looked into how hard this would be, but I'm willing to mail a change adding this (maybe behind a Cargo feature?) if maintainers would consider it a useful addition.

[fitzgen]

Yeah, if the lifetime is 'static then the derive should handle it.

The derive's code is in ./derive/src/lib.rs. I'm not sure off the top of my head what would need to change in there to support this.

[mcy]

I think I didn't phrase what I meant correctly. I want the following behavior:

#[derive(Arbitrary)]
struct Foo<'a, 'b, ...> { ... }
// Generated impl:
impl Arbitrary for Foo<'static, 'static, ...> { ... }

I believe that this should be "easy", by blindly rewriting every lifetime into 'static. I would have liked to write

#[derive(Arbitrary)]
type StaticFoo = Foo<'static, 'static, ...>;

I feel this is less surprising syntax but not something proc macros can do AFAIK, since they don't have type information available.

[frewsxcv]

Done in #63