improve float to_degrees/to_radians rounding comments and impl

* revise comments explaining why we are using literal or expression
* add unspecified precision comments as we don't guarantee precision
* use expression in `f128::to_degrees()`
* make `f64::to_degrees()` impl consistent with other functions

Author: karolzwolak

library/core/src/num/f128.rs

@@ -630,6 +630,13 @@ impl f128 {
 
     /// Converts radians to degrees.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// #![feature(f128)]
     /// # // FIXME(f16_f128): remove when `eqtf2` is available
@@ -645,13 +652,22 @@ impl f128 {
     #[unstable(feature = "f128", issue = "116909")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
     pub const fn to_degrees(self) -> Self {
-        // Use a literal for better precision.
-        const PIS_IN_180: f128 = 57.2957795130823208767981548141051703324054724665643215491602_f128;
+        // The division here is correctly rounded with respect to the true value of 180/π.
+        // Although π is irrational and already rounded, the double rounding happens
+        // to produce correct result for f128.
+        const PIS_IN_180: f128 = 180.0 / consts::PI;
         self * PIS_IN_180
     }
 
     /// Converts degrees to radians.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// #![feature(f128)]
     /// # // FIXME(f16_f128): remove when `eqtf2` is available
@@ -668,7 +684,8 @@ impl f128 {
     #[unstable(feature = "f128", issue = "116909")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
     pub const fn to_radians(self) -> f128 {
-        // Use a literal for better precision.
+        // Use a literal to avoid double rounding, consts::PI is already rounded,
+        // and dividing would round again.
         const RADS_PER_DEG: f128 =
             0.0174532925199432957692369076848861271344287188854172545609719_f128;
         self * RADS_PER_DEG

library/core/src/num/f16.rs

@@ -625,6 +625,13 @@ impl f16 {
 
     /// Converts radians to degrees.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// #![feature(f16)]
     /// # // FIXME(f16_f128): extendhfsf2, truncsfhf2, __gnu_h2f_ieee, __gnu_f2h_ieee missing for many platforms
@@ -640,13 +647,21 @@ impl f16 {
     #[unstable(feature = "f16", issue = "116909")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
     pub const fn to_degrees(self) -> Self {
-        // Use a literal for better precision.
+        // Use a literal to avoid double rounding, consts::PI is already rounded,
+        // and dividing would round again.
         const PIS_IN_180: f16 = 57.2957795130823208767981548141051703_f16;
         self * PIS_IN_180
     }
 
     /// Converts degrees to radians.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// #![feature(f16)]
     /// # // FIXME(f16_f128): extendhfsf2, truncsfhf2, __gnu_h2f_ieee, __gnu_f2h_ieee missing for many platforms
@@ -663,7 +678,8 @@ impl f16 {
     #[unstable(feature = "f16", issue = "116909")]
     #[must_use = "this returns the result of the operation, without modifying the original"]
     pub const fn to_radians(self) -> f16 {
-        // Use a literal for better precision.
+        // Use a literal to avoid double rounding, consts::PI is already rounded,
+        // and dividing would round again.
         const RADS_PER_DEG: f16 = 0.017453292519943295769236907684886_f16;
         self * RADS_PER_DEG
     }

library/core/src/num/f32.rs

@@ -839,6 +839,13 @@ impl f32 {
 
     /// Converts radians to degrees.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// let angle = std::f32::consts::PI;
     ///
@@ -852,13 +859,21 @@ impl f32 {
     #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_degrees(self) -> f32 {
-        // Use a constant for better precision.
+        // Use a literal to avoid double rounding, consts::PI is already rounded,
+        // and dividing would round again.
         const PIS_IN_180: f32 = 57.2957795130823208767981548141051703_f32;
         self * PIS_IN_180
     }
 
     /// Converts degrees to radians.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// let angle = 180.0f32;
     ///
@@ -872,6 +887,9 @@ impl f32 {
     #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_radians(self) -> f32 {
+        // The division here is correctly rounded with respect to the true value of π/180.
+        // Although π is irrational and already rounded, the double rounding happens
+        // to produce correct result for f32.
         const RADS_PER_DEG: f32 = consts::PI / 180.0;
         self * RADS_PER_DEG
     }

library/core/src/num/f64.rs

@@ -856,6 +856,13 @@ impl f64 {
 
     /// Converts radians to degrees.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// let angle = std::f64::consts::PI;
     ///
@@ -869,14 +876,22 @@ impl f64 {
     #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const 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)
+        // The division here is correctly rounded with respect to the true value of 180/π.
+        // Although π is irrational and already rounded, the double rounding happens
+        // to produce correct result for f64.
+        const PIS_IN_180: f64 = 180.0 / consts::PI;
+        self * PIS_IN_180
     }
 
     /// Converts degrees to radians.
     ///
+    /// # Unspecified precision
+    ///
+    /// The precision of this function is non-deterministic. This means it varies by platform,
+    /// Rust version, and can even differ within the same execution from one invocation to the next.
+    ///
+    /// # Examples
+    ///
     /// ```
     /// let angle = 180.0_f64;
     ///
@@ -890,6 +905,9 @@ impl f64 {
     #[rustc_const_stable(feature = "const_float_methods", since = "1.85.0")]
     #[inline]
     pub const fn to_radians(self) -> f64 {
+        // The division here is correctly rounded with respect to the true value of π/180.
+        // Although π is irrational and already rounded, the double rounding happens
+        // to produce correct result for f64.
         const RADS_PER_DEG: f64 = consts::PI / 180.0;
         self * RADS_PER_DEG
     }