I think a new keyword, something like `stable`, is needed for specifying that
an argument passed to a trait function is guaranteed to be logically unchanged
after the function call. For example:
trait Foo {
fn foo(stable self);
}
impl Foo for int {
fn foo(&self) {} // OK
}
impl Foo for uint {
fn foo(self) {} // OK
}
impl Foo for Box<int> {
fn foo(stable self) {} // OK (implicitly clones self)
}
fn main() {
let x: Box<int> = box 42;
x.foo(); // `x` is implicitly cloned
x.foo(); // OK
}
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