RFC 1440: drop-types-in-const

lang (drop | const-eval | static)

• Feature Name: drop_types_in_const
• Start Date: 2016-01-01
• RFC PR: rust-lang/rfcs#1440
• Rust Issue: rust-lang/rust#33156

Summary

Allow types with destructors to be used in static items, const items, and const functions.

Motivation

Some of the collection types do not allocate any memory when constructed empty (most notably Vec). With the change to make leaking safe, the restriction on static or const items with destructors is no longer required to be a hard error (as it is safe and accepted that these destructors may never run).

Allowing types with destructors to be directly used in const functions and stored in statics or consts will remove the need to have runtime-initialisation for global variables.

Detailed design

• Lift the restriction on types with destructors being used in static or const items.
• statics containing Drop-types will not run the destructor upon program/thread exit.
• consts containing Drop-types will run the destructor at the appropriate point in the program.
• (Optionally adding a lint that warn about the possibility of resource leak)
• Alloc instantiating structures with destructors in constant expressions,
• Allow const fn to return types with destructors.
• Disallow constant expressions that require destructors to run during compile-time constant evaluation (i.e: a drop(foo) in a const fn).

Examples

Assuming that RwLock and Vec have const fn new methods, the following example is possible and avoids runtime validity checks.

/// Logging output handler
trait LogHandler: Send + Sync {
    // ...
}
/// List of registered logging handlers
static S_LOGGERS: RwLock<Vec< Box<LogHandler> >> = RwLock::new( Vec::new() );

/// Just an empty byte vector.
const EMPTY_BYTE_VEC: Vec<u8> = Vec::new();

Disallowed code

static VAL: usize = (Vec::<u8>::new(), 0).1;	// The `Vec` would be dropped

const fn sample(_v: Vec<u8>) -> usize {
	0	// Discards the input vector, dropping it
}

Drawbacks

Destructors do not run on static items (by design), so this can lead to unexpected behavior when a type's destructor has effects outside the program (e.g. a RAII temporary folder handle, which deletes the folder on drop). However, this can already happen using the lazy_static crate.

A const item's destructor will run at each point where the const item is used. If a const item is never used, its destructor will never run. These behaviors may be unexpected.

Alternatives

• Runtime initialisation of a raw pointer can be used instead (as the lazy_static crate currently does on stable)
• On nightly, a bug related to static and UnsafeCell<Option<T>> can be used to remove the dynamic allocation.
• Both of these alternatives require runtime initialisation, and incur a checking overhead on subsequent accesses.
• Leaking of objects could be addressed by using C++-style .dtors support
• This is undesirable, as it introduces confusion around destructor execution order.

Unresolved questions

• TBD