Fix an accuracy regression in f32::to_degrees

rust-lang#47919 regressed some test cases for `f32::to_degrees`, while improving others. This change satisfies all previous test cases and should be more accurate than both previous implementations. The `f32` to `f64` cast is lossless, `f64::to_degrees` should provide more accuracy than a native `f32::to_degrees`, and conversion back to `f32` will be as accurate a conversion as possible. It can be hard to reason about floating-point accuracy, but given that this passes all the tests, I feel confident it's an improvement.
varkor · Feb 28, 2018 · 2d11f04 · 2d11f04
1 parent 0ff9872
commit 2d11f04
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Showing 2 changed files with 5 additions and 3 deletions.
diff --git a/src/libcore/num/f32.rs b/src/libcore/num/f32.rs
@@ -239,9 +239,7 @@ impl Float for f32 {
     /// Converts to degrees, assuming the number is in radians.
     #[inline]
     fn to_degrees(self) -> f32 {
-        // Use a constant for better precision.
-        const PIS_IN_180: f32 = 57.2957795130823208767981548141051703_f32;
-        self * PIS_IN_180
+        (self as f64).to_degrees() as f32
     }
 
     /// Converts to radians, assuming the number is in degrees.

diff --git a/src/libstd/f32.rs b/src/libstd/f32.rs
@@ -1532,6 +1532,10 @@ mod tests {
         assert_eq!(inf.to_degrees(), inf);
         assert_eq!(neg_inf.to_degrees(), neg_inf);
         assert_eq!(1_f32.to_degrees(), 57.2957795130823208767981548141051703);
+        assert_eq!(f32::consts::FRAC_PI_6.to_degrees(), 30.0f32);
+        assert_eq!(f32::consts::FRAC_PI_3.to_degrees(), 60.0f32);
+        assert_eq!(30.0f32.to_degrees().to_radians(), 30.0f32);
+        assert_eq!(30.0f32.to_radians().to_degrees(), 30.0f32);
     }
 
     #[test]