From c7928f5af0b9dc7b042d4c390a5a5381cc53de9a Mon Sep 17 00:00:00 2001
From: Luca Barbieri <luca@luca-barbieri.com>
Date: Sun, 21 Jul 2019 11:49:36 +0200
Subject: [PATCH] Expand comment on BorrowRef::new
---
src/libcore/cell.rs | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index ea803bd3a1f..d842bf6d711 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -1103,10 +1103,20 @@ impl<'b> BorrowRef<'b> {
fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
let b = borrow.get().wrapping_add(1);
if !is_reading(b) {
- // If there's currently a writing borrow, or if incrementing the
- // refcount would overflow into a writing borrow.
+ // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
+ // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
+ // due to Rust's reference aliasing rules
+ // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
+ // into isize::min_value() (the max amount of writing borrows) so we can't allow
+ // an additional read borrow because isize can't represent so many read borrows
+ // (this can only happen if you mem::forget more than a small constant amount of
+ // `Ref`s, which is not good practice)
None
} else {
+ // Incrementing borrow can result in a reading value (< 0) in these cases:
+ // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
+ // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
+ // is large enough to represent having one more read borrow
borrow.set(b);
Some(BorrowRef { borrow })
}