Update src/design_notes/copy_ergonomics.md

Co-authored-by: Josh Triplett <[email protected]>

Author: Mark-Simulacrum

src/design_notes/copy_ergonomics.md

@@ -107,6 +107,7 @@ structures).
 This could plausibly be done with moved values as well, so long as the
 semantics match the syntax (e.g. `wants_ref(foo)` acts like `wants_ref(&{foo})`)
 similar to how one can pass `&mut` to something that only wants `&`.
+This would be a tradeoff: in some cases people may want the type-checker to flag such cases and require explicitly taking a reference, while in other cases people may want the compiler to automatically make such code work. We would want to consider and evaluate this tradeoff, and whether we can usefully separate such cases.
 
 ### History