Auto merge of #25441 - alexcrichton:debug-panic-neg, r=aturon

Debug overflow checks for arithmetic negation landed in #24500, at which time
the `abs` method on signed integers was changed to using `wrapping_neg` to
ensure that the function never panicked. This implied that `abs` of `INT_MIN`
would return `INT_MIN`, another negative value. When this change was back-ported
to beta, however, in #24708, the `wrapping_neg` function had not yet been
backported, so the implementation was changed in #24785 to `!self + 1`. This
change had the unintended side effect of enabling debug overflow checks for the
`abs` function. Consequently, the current state of affairs is that the beta
branch checks for overflow in debug mode for `abs` and the nightly branch does
not.

This commit alters the behavior of nightly to have `abs` always check for
overflow in debug mode. This change is more consistent with the way the standard
library treats overflow as well, and it is also not a breaking change as it's
what the beta branch currently does (albeit if by accident).

cc #25378

1 files changed, 12 insertions, 3 deletions

diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index 011830ddb78..bd7286dfa3f 100644
--- a/src/libcore/num/mod.rs
+++ b/src/libcore/num/mod.rs
@@ -563,13 +563,22 @@ macro_rules! int_impl {
             acc
         }
 
-        /// Computes the absolute value of `self`. `Int::min_value()` will be
-        /// returned if the number is `Int::min_value()`.
+        /// Computes the absolute value of `self`.
+        ///
+        /// # Overflow behavior
+        ///
+        /// The absolute value of `i32::min_value()` cannot be represented as an
+        /// `i32`, and attempting to calculate it will cause an overflow. This
+        /// means that code in debug mode will trigger a panic on this case and
+        /// optimized code will return `i32::min_value()` without a panic.
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
         pub fn abs(self) -> $T {
             if self.is_negative() {
-                self.wrapping_neg()
+                // Note that the #[inline] above means that the overflow
+                // semantics of this negation depend on the crate we're being
+                // inlined into.
+                -self
             } else {
                 self
             }