Implicit argument conversions through ImplicitAs

A type converts to T where it is passed as an argument to a parameter of type T, when it implements std’s ImplicitAs<T>, whose convert(&self) -> T the compiler calls. std implements it for String and Path to cstr. Chosen by the user on 2026-10-23.

Calling C was the first case: rl::DrawText(name.as_cstr(), …) for every String, and format! needing a let first. The user rejected a rule special to String in the compiler: a type should declare its own conversions, as C++’s implicit constructors do.

A trait (chosen) over an #[implicit] attribute. Both were written out. The trait can be a bound, fn f<S: ImplicitAs<cstr>>(s: &S), and with the orphan rule counting a trait’s arguments (D-0087) it covers a conversion into a module’s own type from one it did not write, which had been the attribute’s one advantage.
The name. Carbon, a C++ successor, has the same feature under the same name. Rust’s Into was rejected: it is explicit (.into()) and takes self by value, consuming what it converts, while this borrows. Converts sounded like a conversion that owns the value.
Only arguments. Not let, a field, a return or an operator’s operand, so a borrowed result such as a cstr into a String cannot outlive the call it was made for. That is also why passing the String is safer than calling as_cstr() by hand.
Borrowed, once, and opt-in. The argument is the receiver of a &self method, so it is not moved and a temporary lives until the call returns. Never two conversions in a row. A method or constructor never converts on its own: C++ made conversion the default and explicit the opt-out, and its guidelines spend a rule on undoing that.
Not to choose or infer. An imported overload set is still chosen by exact types (§15.3), and a generic parameter is inferred from the argument’s own type (§8.1). Where a type converts to several types, the parameter’s type picks which.
No numeric conversions. §5.1 stands. Neither side of one would be a program’s own, so only std could declare one, and it declares none.

Spec: §7.2, §5.3, §14.4, §15.2, §15.3.