Auto merge of rust-lang#61635 - ecstatic-morse:const-signum, r=oli-obk

Make `i*::signum` a `const fn`.

Ticks a box in rust-lang#53718.

This uses a well-known branchless implementation of `signum`: `(n > 0) as i32 - (n < 0) as i32`.

Here's a [playground](https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=747cf191c4974bf66c9d75e509ae6e6e) comparing the two techniques. On x86 in release mode, the branchless implementation is able to replace a `mov` and `cmov` with a `sar` and `add`, so this should be a bit faster as well.

~~This is marked as a draft since I think I'll need to add `#[rustc_const_unstable]` somewhere. Perhaps the reviewer can point me in the right direction.~~

Author: bors

src/libcore/num/mod.rs

@@ -1993,13 +1993,10 @@ assert_eq!((-10", stringify!($SelfT), ").signum(), -1);",
 $EndFeature, "
 ```"),
             #[stable(feature = "rust1", since = "1.0.0")]
+            #[rustc_const_unstable(feature = "const_int_sign")]
             #[inline]
-            pub fn signum(self) -> Self {
-                match self {
-                    n if n > 0 =>  1,
-                    0          =>  0,
-                    _          => -1,
-                }
+            pub const fn signum(self) -> Self {
+                (self > 0) as Self - (self < 0) as Self
             }
         }
 

src/test/run-pass/const-int-sign.rs

@@ -1,11 +1,21 @@
+#![feature(const_int_sign)]
+
 const NEGATIVE_A: bool = (-10i32).is_negative();
 const NEGATIVE_B: bool = 10i32.is_negative();
-const POSITIVE_A: bool= (-10i32).is_positive();
-const POSITIVE_B: bool= 10i32.is_positive();
+const POSITIVE_A: bool = (-10i32).is_positive();
+const POSITIVE_B: bool = 10i32.is_positive();
+
+const SIGNUM_POS: i32 = 10i32.signum();
+const SIGNUM_NIL: i32 = 0i32.signum();
+const SIGNUM_NEG: i32 = (-42i32).signum();
 
 fn main() {
     assert!(NEGATIVE_A);
     assert!(!NEGATIVE_B);
     assert!(!POSITIVE_A);
     assert!(POSITIVE_B);
+
+    assert_eq!(SIGNUM_POS, 1);
+    assert_eq!(SIGNUM_NIL, 0);
+    assert_eq!(SIGNUM_NEG, -1);
 }