From 6430351e7457649d7b6cb50c33a4ab83e33944f8 Mon Sep 17 00:00:00 2001
From: Vinzent Steinberg <Vinzent.Steinberg@gmail.com>
Date: Tue, 10 Apr 2018 15:26:14 +0200
Subject: [PATCH 1/3] =?UTF-8?q?Use=20constant=20for=20180/=CF=80=20in=20f3?=
 =?UTF-8?q?2::to=5Fdegrees?=
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The current `f32::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.

This is a backport from [`std`].

[`std`]: https://github.com/rust-lang/rust/pull/47919/commits/e34c31bf02eb0f0ff4dd43ae72e0eae53f2ac519
---
 src/float.rs | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/float.rs b/src/float.rs
index 072a91b..dbd11d4 100644
--- a/src/float.rs
+++ b/src/float.rs
@@ -771,7 +771,9 @@ impl FloatCore for f32 {
     #[inline]
     #[cfg(not(feature = "std"))]
     fn to_degrees(self) -> Self {
-        self * (180.0 / f32::consts::PI)
+        // Use a constant for better precision.
+        const PIS_IN_180: f32 = 57.2957795130823208767981548141051703_f32;
+        self * PIS_IN_180
     }
 
     #[inline]
@@ -841,6 +843,9 @@ impl FloatCore for f64 {
     #[inline]
     #[cfg(not(feature = "std"))]
     fn to_degrees(self) -> Self {
+        // 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 * (180.0 / f64::consts::PI)
     }
 
@@ -2008,4 +2013,9 @@ mod tests {
             assert!((Float::to_radians(deg) - rad).abs() < 1e-5);
         }
     }
+
+    #[test]
+    fn to_degrees_rounding() {
+        assert_eq!(1_f32.to_degrees(), 57.2957795130823208767981548141051703);
+    }
 }

From 9ca219c677a68227ce8a483b9e919910d8b833da Mon Sep 17 00:00:00 2001
From: Vinzent Steinberg <Vinzent.Steinberg@gmail.com>
Date: Tue, 10 Apr 2018 19:56:39 +0200
Subject: [PATCH 2/3] Avoid test failure with Rust 1.8 by limiting to no-std
 builds

---
 src/float.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/float.rs b/src/float.rs
index dbd11d4..830d556 100644
--- a/src/float.rs
+++ b/src/float.rs
@@ -2015,6 +2015,9 @@ mod tests {
     }
 
     #[test]
+    // This fails with the forwarded `std` implementation in Rust 1.8.
+    // To avoid the failure, the test is limited to `no_std` builds.
+    #[cfg(not(feature = "std"))]
     fn to_degrees_rounding() {
         assert_eq!(1_f32.to_degrees(), 57.2957795130823208767981548141051703);
     }

From 6aaff332d353eb4fb785521a27fe558441e462b0 Mon Sep 17 00:00:00 2001
From: Josh Stone <cuviper@gmail.com>
Date: Fri, 4 May 2018 12:19:23 -0700
Subject: [PATCH 3/3] Explicitly test FloatCore in to_degrees_rounding

---
 src/float.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/float.rs b/src/float.rs
index 830d556..8df3254 100644
--- a/src/float.rs
+++ b/src/float.rs
@@ -2019,6 +2019,8 @@ mod tests {
     // To avoid the failure, the test is limited to `no_std` builds.
     #[cfg(not(feature = "std"))]
     fn to_degrees_rounding() {
-        assert_eq!(1_f32.to_degrees(), 57.2957795130823208767981548141051703);
+        use float::FloatCore;
+
+        assert_eq!(FloatCore::to_degrees(1_f32), 57.2957795130823208767981548141051703);
     }
 }