Rollup merge of #47919 - varkor:to_degrees-precision, r=rkruppe Use constant for 180/π in to_degrees The current `f32|f64.to_degrees` implementation uses a division to calculate `180/π`, which causes a loss of precision. Using a constant is still not perfect (implementing a maximally-precise algorithm would come with a high performance cost), but improves precision with a minimal change. As per the discussion in #29944, this fixes #29944 (the costs of improving the precision further would not outweigh the gains).

2 files changed, 6 insertions, 1 deletions

diff --git a/src/libcore/num/f32.rs b/src/libcore/num/f32.rs
index 207df84d080..3586fa5442f 100644
--- a/src/libcore/num/f32.rs
+++ b/src/libcore/num/f32.rs
@@ -239,7 +239,9 @@ impl Float for f32 {
     /// Converts to degrees, assuming the number is in radians.
     #[inline]
     fn to_degrees(self) -> f32 {
-        self * (180.0f32 / consts::PI)
+        // Use a constant for better precision.
+        const PIS_IN_180: f32 = 57.2957795130823208767981548141051703_f32;
+        self * PIS_IN_180
     }
 
     /// Converts to radians, assuming the number is in degrees.
diff --git a/src/libcore/num/f64.rs b/src/libcore/num/f64.rs
index 9206132e8b4..64c0d508b38 100644
--- a/src/libcore/num/f64.rs
+++ b/src/libcore/num/f64.rs
@@ -237,6 +237,9 @@ impl Float for f64 {
     /// Converts to degrees, assuming the number is in radians.
     #[inline]
     fn to_degrees(self) -> f64 {
+        // The division here is correctly rounded with respect to the true
+        // value of 180/π. (This differs from f32, where a constant must be
+        // used to ensure a correctly rounded result.)
         self * (180.0f64 / consts::PI)
     }