Add guide for writing polymorphic code

[kud1ing]

I think a guide would be nice that explains what to choose when writing polymorphic code (sort of the inverse of https://cheats.rs/#generics-constraints which explains what the syntax means if you see it in the wild).

See also:

• https://stackoverflow.com/questions/32059370/when-is-it-appropriate-to-use-an-associated-type-versus-a-generic-type

Functions (arguments/return values)

dyn Trait

dynamic binding (trait object) => no monomorphization

• advantages:
  • smaller binaries
• disadvantages:
  • runtime overhead (vtable)
  • the trait must be object-safe:
  • return type must not be Self
  • there are no generic type parameters.

impl Trait

static binding => monomorphization

• advantages:
  • no runtime overhead
• disadvantages:
  • larger binaries
  • implies that all return values are of the same type
  • can not be used in traits at return position

generics

static binding? monomorphization?

• advantages:
• disadvantages:

Data structures

• enums:
  • static binding (?)
  • advantages:
  • disadvantages:
  • the set of possible values can not be extended externally
• generics with associated types
  • advantages:
  • code using that structure gets simpler
  • disadvantages:
• generics without associated types
  • advantages:
  • disadvantages:

This may or may not fit https://cheats.rs/#generics-constraints

[ralfbiedert]

Is that when defining functions?

[kud1ing]

Good question. I've updated the text above.

[ralfbiedert]

Also closing this one as it seems to have stalled and I'm not quite sure how I would address this.

This is purely for "issue hygiene" and doesn't mean I'm opposed to the item. Please re-open / comment if there is any progress!