Eager drop design note

Observations

Any attempt to make drop run more eagerly will have to take borrows into account

The original proposal was to use "CFG dead" as a criteria, but it's pretty clear that this will not work well. Example:


#![allow(unused)]
fn main() {
{
    let x = String::new();
    let y = &x;
    // last use of x is here
    println!("{}", y);
    // but we use y here
    ...
}
}

Here, the fact that y (indirectly) uses x feels like an important thing to take into account.

Some destructors can be run "at any time"...

Some destructors have very significant side-effects. The most notable example is dropping a lock guard.

Others correspond solely to "releasing resources": freeing memory is the most common example, but another might be replacing an entry in a table because you are done using it.

...but sometimes that significance is only known at the call site

However, it can be hard to know what is significant. For a lock guard, for example, if the lock is just being used to guard the data, then moving the lock release early is actually desirable, because you want to release the lock as soon as you are doing changing the data. But sometimes you have a Mutex<()>, in which case the lock has extra semantics. It's hard to know for sure.

Smarter drop placement will mean that adding uses of a variable changes when its destructor runs

This is not necesarily a problem, but it's an obvious implication: right now, the drop always runs when we exit the scope, so adding further uses to a variable has no effect, but that would have to change. That could be surprising (e.g., adding a debug printout changes the time when a lock is released).

In contrast, if you add an early drop drop(foo) today, you get helpful error messages when you try to use it again.

In other words, it's useful to have the destructor occurring at a known time (sometimes...).

Today's drop rules are, however, a source of confusion

The semantics of let _ = <expr> have been known to caught a lot of confusion, particularly given the interaction of place expressions and value expresssions:

  • let _ = foo -- no effect
  • let _ = foo() -- immediately drops the result of invoking foo()
  • let _guard = foo -- moves foo into _guard and drops at the end of the block
  • let _guard = foo() -- moves foo() into _guard and drops at the end of the block

Another common source of confusion is the lifetimes of temporaries in match statements and the like:


#![allow(unused)]
fn main() {
match foo.lock().data.copy_out() {
    ...
} // lock released here!
}

let guard = foo; ...; drop(guard); has the advantage of explicitness, so does something like foo.with(|guard| ...)

Clarity for unsafe code can be quite important

There are known footguns today with the timing of destructors and unsafe code. For example, CString::new().as_ptr() is a common thing people try to do that does not work. Eager destructors would enable more motion, which might exacerbate the problem.

In addition, unsafe code means we might not be able to know the semantics associated with a destructor, such as what precisely a Mutex<()> guards, and moving a drop earlier will break some unsafe code in hard-to-detect ways.

Alternatives

  • Scoped methods
  • let blocks
  • "Defer" type constructs or scoped guard type constructs from other languages
    • Go
    • D
    • Python
  • Built-in macros or RAII/closure-based helpers in the standard library.
    • Note that the scopeguard crate offers macros like defer! that inject a let into the block.