Backport total_cmp for Rust <1.62

Author: cuviper

build.rs

@@ -16,6 +16,7 @@ fn main() {
         ac.emit_expression_cfg("1f64.copysign(-1f64)", "has_copysign");
     }
     ac.emit_expression_cfg("1f64.is_subnormal()", "has_is_subnormal");
+    ac.emit_expression_cfg("1f64.total_cmp(&2f64)", "has_total_cmp");
 
     ac.emit_expression_cfg("1u32.to_ne_bytes()", "has_int_to_from_bytes");
     ac.emit_expression_cfg("3.14f64.to_ne_bytes()", "has_float_to_from_bytes");

src/float.rs

@@ -2268,17 +2268,31 @@ pub trait TotalOrder {
     fn total_cmp(&self, other: &Self) -> Ordering;
 }
 macro_rules! totalorder_impl {
-    ($T:ident) => {
+    ($T:ident, $I:ident, $U:ident, $bits:expr) => {
         impl TotalOrder for $T {
             #[inline]
+            #[cfg(has_total_cmp)]
             fn total_cmp(&self, other: &Self) -> Ordering {
+                // Forward to the core implementation
                 Self::total_cmp(&self, other)
             }
+            #[inline]
+            #[cfg(not(has_total_cmp))]
+            fn total_cmp(&self, other: &Self) -> Ordering {
+                // Backport the core implementation (since 1.62)
+                let mut left = self.to_bits() as $I;
+                let mut right = other.to_bits() as $I;
+
+                left ^= (((left >> ($bits - 1)) as $U) >> 1) as $I;
+                right ^= (((right >> ($bits - 1)) as $U) >> 1) as $I;
+
+                left.cmp(&right)
+            }
         }
     };
 }
-totalorder_impl!(f64);
-totalorder_impl!(f32);
+totalorder_impl!(f64, i64, u64, 64);
+totalorder_impl!(f32, i32, u32, 32);
 
 #[cfg(test)]
 mod tests {