add missing safety comments

Author: RalfJung

core/src/num/f128.rs

@@ -1378,6 +1378,7 @@ impl f128 {
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     #[must_use = "method returns a new number and does not mutate the original value"]
     pub const fn copysign(self, sign: f128) -> f128 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::copysignf128(self, sign) }
     }
 }

core/src/num/f16.rs

@@ -1352,6 +1352,7 @@ impl f16 {
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     #[must_use = "method returns a new number and does not mutate the original value"]
     pub const fn copysign(self, sign: f16) -> f16 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::copysignf16(self, sign) }
     }
 }

core/src/num/f32.rs

@@ -1449,6 +1449,7 @@ impl f32 {
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     #[inline]
     pub const fn abs(self) -> f32 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::fabsf32(self) }
     }
 
@@ -1509,6 +1510,7 @@ impl f32 {
     #[stable(feature = "copysign", since = "1.35.0")]
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     pub const fn copysign(self, sign: f32) -> f32 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::copysignf32(self, sign) }
     }
 }

core/src/num/f64.rs

@@ -1449,6 +1449,7 @@ impl f64 {
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     #[inline]
     pub const fn abs(self) -> f64 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::fabsf64(self) }
     }
 
@@ -1509,6 +1510,7 @@ impl f64 {
     #[rustc_const_unstable(feature = "const_float_methods", issue = "130843")]
     #[inline]
     pub const fn copysign(self, sign: f64) -> f64 {
+        // SAFETY: this is actually a safe intrinsic
         unsafe { intrinsics::copysignf64(self, sign) }
     }
 }