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Outline panicking code for RefCell::borrow and RefCell::borrow_mut #115491

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Sep 4, 2023
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Outline panicking code for RefCell::borrow and RefCell::borrow_mut #115491

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26 changes: 24 additions & 2 deletions library/core/src/cell.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -753,6 +753,22 @@ impl Display for BorrowMutError {
}
}

// This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
#[inline(never)]
#[track_caller]
#[cold]
fn panic_already_borrowed(err: BorrowMutError) -> ! {
panic!("already borrowed: {:?}", err)
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@klensy klensy Sep 3, 2023 •
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Now this will print on release "already borrowed: BorrowMutError" instead of "already borrowed", which is useless.
Next one similar.

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That's done to match the existing output. BorrowMutError may also contain the location of the original borrower.

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@klensy klensy Sep 3, 2023 •
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https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=4db78fa88be92de81e74933887a7a140
Oh, my bad, with this pr and previously that code outputs "already borrowed: BorrowMutError".

Sad to not to have test coverage. Can you add tests with panic outputs?

Can this be simple strings like "already borrowed" instead? If user wants more info from error he will use try_borrow/try_borrow_mut instead of borrow/borrow_mut, that fn should be as slim as possible (but that cold code anyway, except pulling debug impl for error). And location available only via debug_refcell which require rebuild std anyway?

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It would make sense to me to use simpler strings with debug_refcell disabled.

}

// This ensures the panicking code is outlined from `borrow` for `RefCell`.
#[inline(never)]
#[track_caller]
#[cold]
fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
panic!("already mutably borrowed: {:?}", err)
}

// Positive values represent the number of `Ref` active. Negative values
// represent the number of `RefMut` active. Multiple `RefMut`s can only be
// active at a time if they refer to distinct, nonoverlapping components of a
Expand Down Expand Up @@ -934,7 +950,10 @@ impl<T: ?Sized> RefCell<T> {
#[inline]
#[track_caller]
pub fn borrow(&self) -> Ref<'_, T> {
self.try_borrow().expect("already mutably borrowed")
match self.try_borrow() {
Ok(b) => b,
Err(err) => panic_already_mutably_borrowed(err),
}
}

/// Immutably borrows the wrapped value, returning an error if the value is currently mutably
Expand Down Expand Up @@ -1027,7 +1046,10 @@ impl<T: ?Sized> RefCell<T> {
#[inline]
#[track_caller]
pub fn borrow_mut(&self) -> RefMut<'_, T> {
self.try_borrow_mut().expect("already borrowed")
match self.try_borrow_mut() {
Ok(b) => b,
Err(err) => panic_already_borrowed(err),
}
}

/// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
Expand Down