Skip to content

Implement cell::clone_ref #14189

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
wants to merge 1 commit into from
Closed

Implement cell::clone_ref #14189

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
34 changes: 34 additions & 0 deletions src/libcore/cell.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -186,6 +186,25 @@ impl<'b, T> Deref<T> for Ref<'b, T> {
}
}

/// Copy a `Ref`.
///
/// The `RefCell` is already immutably borrowed, so this cannot fail.
///
/// A `Clone` implementation would interfere with the widespread
/// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
#[experimental]
pub fn clone_ref<'b, T>(orig: &Ref<'b, T>) -> Ref<'b, T> {
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I assume this doesn't just call orig.parent.borrow() to avoid code bloat from fail!?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

And to avoid checking if the borrow flag is set to WRITING. LLVM can optimize

let y = x.get();
x.set(y + 1);

into a single inc instruction.

// Since this Ref exists, we know the borrow flag
// is not set to WRITING.
let borrow = orig.parent.borrow.get();
debug_assert!(borrow != WRITING && borrow != UNUSED);
orig.parent.borrow.set(borrow + 1);

Ref {
parent: orig.parent,
}
}

/// Wraps a mutable borrowed reference to a value in a `RefCell` box.
pub struct RefMut<'b, T> {
parent: &'b RefCell<T>
Expand Down Expand Up @@ -307,4 +326,19 @@ mod test {
let _ = _b;
let _b = x.borrow_mut();
}

#[test]
fn clone_ref_updates_flag() {
let x = RefCell::new(0);
{
let b1 = x.borrow();
assert!(x.try_borrow_mut().is_none());
{
let _b2 = clone_ref(&b1);
assert!(x.try_borrow_mut().is_none());
}
assert!(x.try_borrow_mut().is_none());
}
assert!(x.try_borrow_mut().is_some());
}
}