std: Make abs() panic on overflow in debug mode

Debug overflow checks for arithmetic negation landed in rust-lang#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 rust-lang#24708, the `wrapping_neg` function had not yet been
backported, so the implementation was changed in rust-lang#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 rust-lang#25378

src/libcore/num/mod.rs

@@ -569,7 +569,7 @@ macro_rules! int_impl {
         #[inline]
         pub fn abs(self) -> $T {
             if self.is_negative() {
-                self.wrapping_neg()
+                -self
             } else {
                 self
             }

src/test/run-pass/int-abs-overflow.rs

@@ -0,0 +1,21 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -Z force-overflow-checks=on
+
+use std::thread;
+
+fn main() {
+    assert!(thread::spawn(|| i8::min_value().abs()).join().is_err());
+    assert!(thread::spawn(|| i16::min_value().abs()).join().is_err());
+    assert!(thread::spawn(|| i32::min_value().abs()).join().is_err());
+    assert!(thread::spawn(|| i64::min_value().abs()).join().is_err());
+    assert!(thread::spawn(|| isize::min_value().abs()).join().is_err());
+}