From bd899d02e9530577e2d12f80b8defe3ef40144cf Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Fri, 7 Jun 2019 14:12:54 -0700
Subject: [PATCH 1/2] Make `i*::signum` a `const fn`.

This uses a well-known branchless implementation of `signum`.
Its `const`-ness is unstable and requires `#![feature(const_int_sign)]`.
---
 src/libcore/num/mod.rs | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/src/libcore/num/mod.rs b/src/libcore/num/mod.rs
index dd7090623f5b3..304b2fc9ebb06 100644
--- a/src/libcore/num/mod.rs
+++ b/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
             }
         }
 

From f6611db1d59bbf5fb7d5cfbcccc629b6916d06a3 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Fri, 7 Jun 2019 16:23:33 -0700
Subject: [PATCH 2/2] Add `const`-ness tests for `i32::signum`

---
 src/test/run-pass/const-int-sign.rs | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/test/run-pass/const-int-sign.rs b/src/test/run-pass/const-int-sign.rs
index 9d656a0203069..fcd3ef4ea025b 100644
--- a/src/test/run-pass/const-int-sign.rs
+++ b/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);
 }