Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference #68712

HeroicKatora · 2020-01-31T20:09:37Z

Usually, references to the interior are only created by the Deref and
DerefMut impl of the guards Ref and RefMut. Note that RefCell
already has to cope with leaks of such guards which, when it occurs,
effectively makes it impossible to ever acquire a mutable guard or any
guard for Ref and RefMut respectively. It is already safe to use
this to create a reference to the inner of the ref cell that lives as
long as the reference to the RefCell itself, e.g.

fn leak(r: &RefCell<usize>) -> Option<&usize> {
    let guard = r.try_borrow().ok()?;
    let leaked = Box::leak(Box::new(guard));
    Some(&*leaked)
}

The newly added methods allow the same reference conversion without an
indirection over a leaked allocation. It's placed on the Ref/RefMut to
compose with both borrow and try_borrow directly.

Usually, references to the interior are only created by the `Deref` and `DerefMut` impl of the guards `Ref` and `RefMut`. Note that `RefCell` already has to cope with leaks of such guards which, when it occurs, effectively makes it impossible to ever acquire a mutable guard or any guard for `Ref` and `RefMut` respectively. It is already safe to use this to create a reference to the inner of the ref cell that lives as long as the reference to the `RefCell` itself, e.g. ```rust fn leak(r: &RefCell<usize>) -> Option<&usize> { let guard = r.try_borrow().ok()?; let leaked = Box::leak(Box::new(guard)); Some(&*leaked) } ``` The newly added methods allow the same reference conversion without an indirection over a leaked allocation and composing with both borrow and try_borrow without additional method combinations.

rust-highfive · 2020-01-31T20:09:48Z

r? @joshtriplett

(rust_highfive has picked a reviewer for you, use r? to override)

Dylan-DPC-zz · 2020-02-10T14:22:11Z

r? @dtolnay

dtolnay

Neat!

dtolnay · 2020-02-11T21:38:53Z

Could you please make a tracking issue and add in the issue number?

src/libcore/cell.rs

Mark-Simulacrum · 2020-02-11T22:45:42Z

Should we add similar functions to MutexGuard and RwLockReadGuard/RwLockWriteGuard?

I would also suggest adding forceful "deborrow" functions like parking_lot's https://docs.rs/lock_api/0.3.3/lock_api/struct.Mutex.html#method.force_unlock to accompany this API, but that can be done later, and potentially as a separate feature gate.

wirelessringo · 2020-02-21T20:28:22Z

Ping from triage. @HeroicKatora any updates on this? Thanks.

src/libcore/cell.rs

HeroicKatora · 2020-02-24T12:33:27Z

I've reworded the sections in question. The decision of MutexGuard and forceful deborrow seems like it should be provided in a separate PR.

dtolnay

Looks good! Thanks for including the MutexGuard, RwLockReadGuard/RwLockWriteGuard, and forceful deborrow suggestions in the tracking issue.

dtolnay · 2020-02-24T22:09:43Z

@bors r+

bors · 2020-02-24T22:09:44Z

📌 Commit 329022d has been approved by dtolnay

…tolnay Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference Usually, references to the interior are only created by the `Deref` and `DerefMut` impl of the guards `Ref` and `RefMut`. Note that `RefCell` already has to cope with leaks of such guards which, when it occurs, effectively makes it impossible to ever acquire a mutable guard or any guard for `Ref` and `RefMut` respectively. It is already safe to use this to create a reference to the inner of the ref cell that lives as long as the reference to the `RefCell` itself, e.g. ```rust fn leak(r: &RefCell<usize>) -> Option<&usize> { let guard = r.try_borrow().ok()?; let leaked = Box::leak(Box::new(guard)); Some(&*leaked) } ``` The newly added methods allow the same reference conversion without an indirection over a leaked allocation. It's placed on the `Ref`/`RefMut` to compose with both borrow and try_borrow directly.

@ghost

Rollup of 5 pull requests Successful merges: - #68712 (Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference) - #69209 (Miscellaneous cleanup to formatting) - #69381 (Allow getting `no_std` from the config file) - #69434 (rustc_metadata: Use binary search from standard library) - #69447 (Minor refactoring of statement parsing) Failed merges: r? @ghost

RalfJung · 2020-02-26T18:08:45Z

src/libcore/cell.rs

+    #[unstable(feature = "cell_leak", issue = "69099")]
+    pub fn leak(orig: Ref<'b, T>) -> &'b T {
+        // By forgetting this Ref we ensure that the borrow counter in the RefCell never goes back
+        // to UNUSED again. No further mutable references can be created from the original cell.


"No further mutable references" is not entirely correct -- there is get_mut which circumvents the borrow counter. (If we felt fancy, we could even make it reset the borrow counter.)

The comment would be more accurate if it were along the lines of 'no further RefMut can be created'. That said, having a reset_borrows(&mut self) would work as well and then the comment would become outdated. Since the method seems so natural, I'll try to come up with a PR in the next days and fix this comment then.

@dtolnay

…tolnay Add undo_leak to reset RefCell borrow state This method is complementary for the feature cell_leak added in an earlier PR. It allows *safely* reverting the effects of leaking a borrow guard by statically proving that such a guard could not longer exist. This was not added to the existing `get_mut` out of concern of impacting the complexity of the otherwise pure pointer cast and because the name `get_mut` poorly communicates the intent of resetting remaining borrows. This is a follow-up to rust-lang#68712 and uses the same tracking issue, rust-lang#69099, as these methods deal with the same mechanism and the idea came up [in a review comment](rust-lang#68712 (comment)). @dtolnay who reviewed the prior PR. cc @RalfJung

rust-highfive assigned joshtriplett Jan 31, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 31, 2020

rust-highfive assigned dtolnay and unassigned joshtriplett Feb 10, 2020

dtolnay approved these changes Feb 11, 2020

View reviewed changes

Mark-Simulacrum reviewed Feb 11, 2020

View reviewed changes

src/libcore/cell.rs Outdated Show resolved Hide resolved

HeroicKatora mentioned this pull request Feb 12, 2020

Tracking Issue for std::cell::{Ref, RefMut}::leak #69099

Open

3 tasks

Add tracking number, adjust documentation wording

99b4357

wirelessringo added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 21, 2020

dtolnay requested changes Feb 24, 2020

View reviewed changes

src/libcore/cell.rs Outdated Show resolved Hide resolved

src/libcore/cell.rs Outdated Show resolved Hide resolved

Address method comments

329022d

dtolnay approved these changes Feb 24, 2020

View reviewed changes

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Feb 24, 2020

Dylan-DPC-zz mentioned this pull request Feb 25, 2020

Rollup of 8 pull requests #69463

Closed

Dylan-DPC-zz mentioned this pull request Feb 26, 2020

Rollup of 5 pull requests #69484

Merged

RalfJung reviewed Feb 26, 2020

View reviewed changes

bors merged commit 86b9377 into rust-lang:master Feb 26, 2020

HeroicKatora mentioned this pull request Feb 27, 2020

Add undo_leak to reset RefCell borrow state #69528

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference #68712

Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference #68712

HeroicKatora commented Jan 31, 2020 •

edited

Loading

rust-highfive commented Jan 31, 2020

Dylan-DPC-zz commented Feb 10, 2020

dtolnay left a comment

dtolnay commented Feb 11, 2020

Mark-Simulacrum commented Feb 11, 2020

wirelessringo commented Feb 21, 2020

HeroicKatora commented Feb 24, 2020

dtolnay left a comment

dtolnay commented Feb 24, 2020

bors commented Feb 24, 2020

RalfJung Feb 26, 2020

HeroicKatora Feb 26, 2020 •

edited

Loading

Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference #68712

Add methods to 'leak' RefCell borrows as references with the lifetime of the original reference #68712

Conversation

HeroicKatora commented Jan 31, 2020 • edited Loading

rust-highfive commented Jan 31, 2020

Dylan-DPC-zz commented Feb 10, 2020

dtolnay left a comment

Choose a reason for hiding this comment

dtolnay commented Feb 11, 2020

Mark-Simulacrum commented Feb 11, 2020

wirelessringo commented Feb 21, 2020

HeroicKatora commented Feb 24, 2020

dtolnay left a comment

Choose a reason for hiding this comment

dtolnay commented Feb 24, 2020

bors commented Feb 24, 2020

RalfJung Feb 26, 2020

Choose a reason for hiding this comment

HeroicKatora Feb 26, 2020 • edited Loading

Choose a reason for hiding this comment

HeroicKatora commented Jan 31, 2020 •

edited

Loading

HeroicKatora Feb 26, 2020 •

edited

Loading