How is a trait implemented, and where?

Decided

D-0031, settled by the language’s author: part of spec §18. Older text cites it as 18.31.

Only by impl Trait for Type { … }, written with the trait or with the type — Rust’s orphan rule. A module can then add behaviour to a type it did not write (an editor implementing Inspect for an engine’s structs, or for f32), and two libraries can never both implement a third’s trait for a fourth’s type. A struct no longer lists traits after :, which is reserved for inheritance (18.32). Drop and Copy become prelude traits, stricter than others: the struct’s own module, every instance, no bounds on Drop, never called by name — they are its C++ destructor and copy constructor. A type’s own method wins over a trait’s; two traits providing one name are chosen between as Trait::method(&v). An implementation for every type at once (impl<T> X for T) is not allowed (§6.1, §7.6)

Widened by r78: where an impl may be written now also counts the trait’s type arguments, as Rust’s rule does (D-0087), so impl ImplicitAs<Vec2> for rl::Vector2 belongs with Vec2; and an imported header’s types count as declared by the module carrying its recipe (D-0088).