Idea: where expression (as poor man’s specialisation)

[scottmcm]

@mina86

Thanks to limited scope and imperative style, this would avoid most of the complications of specialisation while still offering many of the benefits.

Same question as Idea: "Maybe Trait" Object and Bounds (an alternative form of specialization) - #16 by scottmcm -- how does this syntax resolve the existing problems that are caused by lifetimes being erased before monomorphization?