Drop elaboration

Dynamic drops

According to the reference:

When an initialized variable or temporary goes out of scope, its destructor is run, or it is dropped. Assignment also runs the destructor of its left-hand operand, if it's initialized. If a variable has been partially initialized, only its initialized fields are dropped.

When building the MIR, the Drop and DropAndReplace terminators represent places where drops may occur. However, in this phase, the presence of these terminators does not guarantee that a destructor will run. That's because the target of a drop may be uninitialized (usually because it has been moved from) before the terminator is reached. In general, we cannot know at compile-time whether a variable is initialized.


#![allow(unused)]
fn main() {
let mut y = vec![];

{
    let x = vec![1, 2, 3];
    if std::process::id() % 2 == 0 {
        y = x; // conditionally move `x` into `y`
    }
} // `x` goes out of scope here. Should it be dropped?
}

In these cases, we need to keep track of whether a variable is initialized dynamically. The rules are laid out in detail in RFC 320: Non-zeroing dynamic drops.

Drop obligations

From the RFC:

When a local variable becomes initialized, it establishes a set of "drop obligations": a set of structural paths (e.g. a local a, or a path to a field b.f.y) that need to be dropped.

The drop obligations for a local variable x of struct-type T are computed from analyzing the structure of T. If T itself implements Drop, then x is a drop obligation. If T does not implement Drop, then the set of drop obligations is the union of the drop obligations of the fields of T.

When a structural path is moved from (and thus becomes uninitialized), any drop obligations for that path or its descendants (path.f, path.f.g.h, etc.) are released. Types with Drop implementations do not permit moves from individual fields, so there is no need to track initializedness through them.

When a local variable goes out of scope (Drop), or when a structural path is overwritten via assignment (DropAndReplace), we check for any drop obligations for that variable or path. Unless that obligation has been released by this point, its associated Drop implementation will be called. For enum types, only fields corresponding to the "active" variant need to be dropped. When processing drop obligations for such types, we first check the discriminant to determine the active variant. All drop obligations for variants besides the active one are ignored.

Here are a few interesting types to help illustrate these rules:


#![allow(unused)]
fn main() {
struct NoDrop(u8); // No `Drop` impl. No fields with `Drop` impls.

struct NeedsDrop(Vec<u8>); // No `Drop` impl but has fields with `Drop` impls.

struct ThinVec(*const u8); // Custom `Drop` impl. Individual fields cannot be moved from.

impl Drop for ThinVec {
    fn drop(&mut self) { /* ... */ }
}

enum MaybeDrop {
    Yes(NeedsDrop),
    No(NoDrop),
}
}

Drop elaboration

One valid model for these rules is to keep a boolean flag (a "drop flag") for every structural path that is used at any point in the function. This flag is set when its path is initialized and is cleared when the path is moved from. When a Drop occurs, we check the flags for every obligation associated with the target of the Drop and call the associated Drop impl for those that are still applicable.

This process—transforming the newly built MIR with its imprecise Drop and DropAndReplace terminators into one with drop flags—is known as drop elaboration. When a MIR statement causes a variable to become initialized (or uninitialized), drop elaboration inserts code that sets (or clears) the drop flag for that variable. It wraps Drop terminators in conditionals that check the newly inserted drop flags.

Drop elaboration also splits DropAndReplace terminators into a Drop of the target and a write of the newly dropped place. This is somewhat unrelated to what we've discussed above.

Once this is complete, Drop terminators in the MIR correspond to a call to the "drop glue" or "drop shim" for the type of the dropped place. The drop glue for a type calls the Drop impl for that type (if one exists), and then recursively calls the drop glue for all fields of that type.

Drop elaboration in rustc

The approach described above is more expensive than necessary. One can imagine a few optimizations:

  • Only paths that are the target of a Drop (or have the target as a prefix) need drop flags.
  • Some variables are known to initialized (or uninitialized) when they are dropped. These do not need drop flags.
  • If a set of paths are only dropped or moved from via a shared prefix, those paths can share a single drop flag.

A subset of these are implemented in rustc.

In the compiler, drop elaboration is split across several modules. The pass itself is defined here, but the main logic is defined elsewhere since it is also used to build drop shims.

Drop elaboration designates each Drop in the newly built MIR as one of four kinds:

  • Static, the target is always initialized.
  • Dead, the target is always uninitialized.
  • Conditional, the target is either wholly initialized or wholly uninitialized. It is not partly initialized.
  • Open, the target may be partly initialized.

For this, it uses a pair of dataflow analyses, MaybeInitializedPlaces and MaybeUninitializedPlaces. If a place is in one but not the other, then the initializedness of the target is known at compile-time (Dead or Static). In this case, drop elaboration does not add a flag for the target. It simply removes (Dead) or preserves (Static) the Drop terminator.

For Conditional drops, we know that the initializedness of the variable as a whole is the same as the initializedness of its fields. Therefore, once we generate a drop flag for the target of that drop, it's safe to call the drop glue for that target.

Open drops

Open drops are the most complex, since we need to break down a single Drop terminator into several different ones, one for each field of the target whose type has drop glue (Ty::needs_drop). We cannot call the drop glue for the target itself because that requires all fields of the target to be initialized. Remember, variables whose type has a custom Drop impl do not allow Open drops because their fields cannot be moved from.

This is accomplished by recursively categorizing each field as Dead, Static, Conditional or Open. Fields whose type does not have drop glue are automatically Dead and need not be considered during the recursion. When we reach a field whose kind is not Open, we handle it as we did above. If the field is also Open, the recursion continues.

It's worth noting how we handle Open drops of enums. Inside drop elaboration, each variant of the enum is treated like a field, with the invariant that only one of those "variant fields" can be initialized at any given time. In the general case, we do not know which variant is the active one, so we will have to call the drop glue for the enum (which checks the discriminant) or check the discriminant ourselves as part of an elaborated Open drop. However, in certain cases (within a match arm, for example) we do know which variant of an enum is active. This information is encoded in the MaybeInitializedPlaces and MaybeUninitializedPlaces dataflow analyses by marking all places corresponding to inactive variants as uninitialized.

Cleanup paths

TODO: Discuss drop elaboration and unwinding.

Aside: drop elaboration and const-eval

In Rust, functions that are eligible for evaluation at compile-time must be marked explicitly using the const keyword. This includes implementations of the Drop trait, which may or may not be const. Code that is eligible for compile-time evaluation may only call const functions, so any calls to non-const Drop implementations in such code must be forbidden.

A call to a Drop impl is encoded as a Drop terminator in the MIR. However, as we discussed above, a Drop terminator in newly built MIR does not necessarily result in a call to Drop::drop. The drop target may be uninitialized at that point. This means that checking for non-const Drops on the newly built MIR can result in spurious errors. Instead, we wait until after drop elaboration runs, which eliminates Dead drops (ones where the target is known to be uninitialized) to run these checks.