Add drop_leak and drop_leak_mut

[davidspies]

I'm trying to implement a StaticRef type which can be built from Rc<RefCell<T>> without needing a lifetime (when dropped, it decrements the refcount and the borrow count). But I find that undo_leak doesn't quite do what I want it to do in the presence of multiple StaticRefs, so I'd like to add drop_leak.

Here's the implementation of StaticRef:

#![feature(cell_leak)]

use std::{
    cell::{Ref, RefCell},
    ops::Deref,
    rc::Rc,
};

pub struct StaticRef<T, S = T>(Holder<S>, *const T);

impl<T> StaticRef<T> {
    pub fn borrow_from(orig: Rc<RefCell<T>>) -> Self {
        let x: *const T = Ref::leak(orig.borrow());
        StaticRef(Holder(orig), x)
    }
}
impl<S, T> StaticRef<T, S> {
    pub fn map<T2, F: for<'b> FnOnce(&'b T) -> &'b T2>(self, op: F) -> StaticRef<T2, S> {
        let StaticRef(holder, x) = self;
        StaticRef(holder, op(unsafe { &*x }))
    }
}

struct Holder<S>(Rc<RefCell<S>>);

impl<S> Drop for Holder<S> {
    fn drop(&mut self) {
        unsafe { self.0.drop_leak() }
    }
}

impl<S, T> Deref for StaticRef<T, S> {
    type Target = T;

    fn deref<'b>(&'b self) -> &'b T {
        unsafe { &*self.1 }
    }
}

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @kennytm (or someone else) soon.

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[davidspies]

How do I run just the two doctests locally? It seems to take forever to build and test any changes

[SkiFire13]

I guess #69099 should be updated to include this PR

[kennytm]

How do I run just the two doctests locally? It seems to take forever to build and test any changes

i think it's easier to copy the entire RefCell into its own crate outside of libstd during development.

[JohnTitor]

We should add a panic section to describe when this panics.

[JohnTitor]

Ditto.

[JohnCSimon]

Ping from triage
@davidspies can you please address the reviewer comments? Thank you.

[crlf0710]

@davidspies Ping from triage, any updates on this?

[crlf0710]

Triage: I'm closing this due to inactivity. Feel free to reopen or create a new PR when you're ready to work on this again, thanks!