impls. This is a [breaking-change] (for gated code) in that when you
implement `Fn` (`FnMut`) you must also implement `FnOnce`. This commit
demonstrates how to fix it.
explicit form `Fn<A,B>` and now should use `Fn(A) -> B` or
`Fn<A,Output=B>`, but in some cases we get duplicate error
reports. This is mildly annoying and arises because of the main error
and another error from the projection. Might be worth squashing those,
but seems like a separate problem.
detect UFCS drop and allow UFCS methods to have explicit type parameters.
Work towards #18875.
Since code could previously call the methods & implement the traits
manually, this is a
[breaking-change]
Closes#19586. Closes#19375.