Drop Elaboration

We know where drops may happen, so we now only need to decide which drops to run at each relevant program point.

Dropping happens in-place by running the compiler-generated core::ptr::drop_in_place function on the place to be dropped. Instead of calling that function directly we use the drop_in_place!() built-in macro so that the borrow-checker can tell that the place has been deinitialized.

In this step we’ll replace every ensure_dropped!($place) with a series of appropriate calls to drop_in_place!($subplace).

For any subplace of $place that hasn’t been explicitly moved out, we insert a call to drop_in_place!. This can require adding extra booleans (“drop flags”) if different branches haven’t moved the same places:

#![allow(unused)]
fn main() {
let x = Struct {
    a: String::new(),
    b: String::new(),
};
if foo() {
    drop(move!(x.a));
} else {
    drop(move!(x.b));
}
ensure_dropped!(x.a);
x.a = "some other string".to_owned();
ensure_dropped!(x);
scope_end!(x);

// becomes:
let a_is_initialized = true;
let b_is_initialized = true;
if foo() {
    drop(move!(x.a));
    a_is_initialized = false;
} else {
    drop(move!(x.b));
    b_is_initialized = false;
}
if a_is_initialized {
    drop_in_place!(x.a);
}
x.a = "some other string".to_owned();
if b_is_initialized {
    drop_in_place!(x.b);
}
drop_in_place!(x.a);
scope_end!(x);
}

(unwind paths omitted for legibility)

After this step, all the code involved in dropping values is explicit.

Backlinks

• In-Place Drop
• Moving Out Of &mut