[Pre-RFC] Partial Impl Blocks

[Jules-Bertholet]

I proposed something similar a while ago, though with a very different motivation:

Idea: partial impls

Motivation Currently, it’s impossible to restrict a default method body to a set of bounds more specific to those of the trait. (Prior discussion: Conditional default trait method impls). In addition, there are cases where, for correctness, implementing one trait places restrictions on implementations of other traits, but enforcing those restrictions is difficult at present. Finally, it is currently impossible to define trait methods with default bodies that are impossible to override. Explanat…