split try trait in twain

[bend-n]

https://rust-lang.zulipchat.com/#narrow/channel/219381-t-libs/topic/on.20.60Try.60.20for.20.60.26Option.60/with/582974682

currently Try for &Option is impossible, because from_output would need to return &Option from &Self::Output. but i think there should be some way to do Option::as_ref in a generic setting, and i think this is a good cause to split the traits up a bit, like so:

pub trait Branch {
    type Output;
    type Residual;
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
}
pub trait FromOutput<O = <Self as Branch>::Output>: Branch {
    fn from_output(output: O) -> Self;
}
pub trait FromResidual<R = <Self as Branch>::Residual>: Branch {
    fn from_residual(residual: R) -> Self;
}
pub trait Try = Branch + FromResidual + FromOutput;
pub trait Residual<O>: Sized {
    type TryType: Try<Output = O, Residual = Self>;
}

which then allows (this is not implemented in this PR.)

impl<'a, T, E> Branch for &'a Result<T, E> {
    type Output = &'a T;
    type Residual = Result<Infallible, &'a E>;

    fn branch(self) -> ControlFlow<Self::Residual, Self::Output> { … }
}

which then allows

fn as_ref<R>(x: R) -> <R::Residual as Residual<<R as Branch>::Output>>::TryType
where
    R: Branch<Residual: Residual<<R as Branch>::Output>>
 { …  }

which then allows

let x: Result<&i32, &()> = as_ref(&Ok::<i32, ()>(1));

which i think is a great boon to Try.

[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

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