Rust trait object safety — dyn Trait requires object-safe traits

Trying to use dyn Trait gives error 'the trait X cannot be made into an object'. The trait compiles fine when used with generics but not with dynamic dispatch.

(1) Object safety rules: trait methods cannot return Self, cannot have generic type parameters, cannot use Self in argument position (except as receiver). (2) Fix: use associated types instead of generics where possible. (3) For methods returning Self: add where Self: Sized bound to exclude them from vtable. (4) Use Box<dyn Trait> for heap-allocated trait objects. (5) Alternative: use enum dispatch (enum with variants for each impl) — faster than dyn dispatch and allows non-object-safe traits. (6) The enum_dispatch crate automates this pattern.

Dynamic dispatch uses a vtable (function pointer table). The compiler must know the size and signature of every method at compile time. Generic methods would need infinite vtable entries, and Self-returning methods can't know the concrete size.