Rollup merge of rust-lang#63786 - tspiteri:const-abs, r=alexcrichton

Make `abs`, `wrapping_abs`, `overflowing_abs` const functions

This makes `abs`, `wrapping_abs` and `overflowing_abs` const functions like rust-lang#58044 makes `wrapping_neg` and `overflowing_neg` const functions.

`abs` is made const by returning `(self ^ -1) - -1` = `!self + 1` = `-self` for negative numbers and `(self ^ 0) - 0` = `self` for non-negative numbers. The subexpression `self >> ($BITS - 1)` evaluates to `-1` for negative numbers and `0` otherwise. The subtraction overflows when `self` is `min_value()`, as we would be subtracting `max_value() - -1`; this is when `abs` should overflow.

`wrapping_abs` and `overflowing_abs` make use of `wrapping_sub` and `overflowing_sub` instead of the subtraction operator.

src/libcore/num/mod.rs

@@ -1401,12 +1401,8 @@ $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_abs", since = "1.13.0")]
             #[inline]
-            pub fn wrapping_abs(self) -> Self {
-                if self.is_negative() {
-                    self.wrapping_neg()
-                } else {
-                    self
-                }
+            pub const fn wrapping_abs(self) -> Self {
+                (self ^ (self >> ($BITS - 1))).wrapping_sub(self >> ($BITS - 1))
             }
         }
 
@@ -1764,12 +1760,8 @@ $EndFeature, "
 ```"),
             #[stable(feature = "no_panic_abs", since = "1.13.0")]
             #[inline]
-            pub fn overflowing_abs(self) -> (Self, bool) {
-                if self.is_negative() {
-                    self.overflowing_neg()
-                } else {
-                    (self, false)
-                }
+            pub const fn overflowing_abs(self) -> (Self, bool) {
+                (self ^ (self >> ($BITS - 1))).overflowing_sub(self >> ($BITS - 1))
             }
         }
 
@@ -1973,15 +1965,11 @@ $EndFeature, "
             #[stable(feature = "rust1", since = "1.0.0")]
             #[inline]
             #[rustc_inherit_overflow_checks]
-            pub fn abs(self) -> Self {
-                if self.is_negative() {
-                    // Note that the #[inline] above means that the overflow
-                    // semantics of this negation depend on the crate we're being
-                    // inlined into.
-                    -self
-                } else {
-                    self
-                }
+            pub const fn abs(self) -> Self {
+                // Note that the #[inline] above means that the overflow
+                // semantics of the subtraction depend on the crate we're being
+                // inlined into.
+                (self ^ (self >> ($BITS - 1))) - (self >> ($BITS - 1))
             }
         }
 

src/test/ui/consts/const-int-overflowing-rpass.rs

@@ -18,6 +18,10 @@ const SHR_B: (u32, bool) = 0x10u32.overflowing_shr(132);
 const NEG_A: (u32, bool) = 0u32.overflowing_neg();
 const NEG_B: (u32, bool) = core::u32::MAX.overflowing_neg();
 
+const ABS_POS: (i32, bool) = 10i32.overflowing_abs();
+const ABS_NEG: (i32, bool) = (-10i32).overflowing_abs();
+const ABS_MIN: (i32, bool) = i32::min_value().overflowing_abs();
+
 fn main() {
     assert_eq!(ADD_A, (7, false));
     assert_eq!(ADD_B, (0, true));
@@ -36,4 +40,8 @@ fn main() {
 
     assert_eq!(NEG_A, (0, false));
     assert_eq!(NEG_B, (1, true));
+
+    assert_eq!(ABS_POS, (10, false));
+    assert_eq!(ABS_NEG, (10, false));
+    assert_eq!(ABS_MIN, (i32::min_value(), true));
 }

src/test/ui/consts/const-int-sign-rpass.rs

@@ -11,6 +11,9 @@ const SIGNUM_POS: i32 = 10i32.signum();
 const SIGNUM_NIL: i32 = 0i32.signum();
 const SIGNUM_NEG: i32 = (-42i32).signum();
 
+const ABS_A: i32 = 10i32.abs();
+const ABS_B: i32 = (-10i32).abs();
+
 fn main() {
     assert!(NEGATIVE_A);
     assert!(!NEGATIVE_B);
@@ -20,4 +23,7 @@ fn main() {
     assert_eq!(SIGNUM_POS, 1);
     assert_eq!(SIGNUM_NIL, 0);
     assert_eq!(SIGNUM_NEG, -1);
+
+    assert_eq!(ABS_A, 10);
+    assert_eq!(ABS_B, 10);
 }

src/test/ui/consts/const-int-wrapping-rpass.rs

@@ -18,6 +18,10 @@ const SHR_B: u32 = 128u32.wrapping_shr(128);
 const NEG_A: u32 = 5u32.wrapping_neg();
 const NEG_B: u32 = 1234567890u32.wrapping_neg();
 
+const ABS_POS: i32 = 10i32.wrapping_abs();
+const ABS_NEG: i32 = (-10i32).wrapping_abs();
+const ABS_MIN: i32 = i32::min_value().wrapping_abs();
+
 fn main() {
     assert_eq!(ADD_A, 255);
     assert_eq!(ADD_B, 199);
@@ -36,4 +40,8 @@ fn main() {
 
     assert_eq!(NEG_A, 4294967291);
     assert_eq!(NEG_B, 3060399406);
+
+    assert_eq!(ABS_POS, 10);
+    assert_eq!(ABS_NEG, 10);
+    assert_eq!(ABS_MIN, i32::min_value());
 }