Improve floating point documentation:

- Refine the "NaN as a special value" top level explanation of f32
- Refine `const NAN` docstring.
- Refine `fn is_sign_positive` and `fn is_sign_negative` docstrings.
- Refine `fn min` and `fn max` docstrings.
- Refine `fn trunc` docstrings.
- Refine `fn powi` docstrings.
- Refine `fn copysign` docstrings.
- Reword `NaN` and `NAN` as plain "NaN", unless they refer to the specific `const NAN`.
- Reword "a number" to `self` in function docstrings to clarify.
- Remove "Returns NAN if the number is NAN" as this is told to be the default behavior in the top explanation.
- Remove "propagating NaNs", as full propagation (preservation of payloads) is not guaranteed.

Author: golddranks

library/core/src/num/f32.rs

@@ -418,6 +418,15 @@ impl f32 {
     pub const MAX_10_EXP: i32 = 38;
 
     /// Not a Number (NaN).
+    ///
+    /// Note that IEEE-745 doesn't define just a single NaN value;
+    /// a plethora of bit patterns are considered to be NaN.
+    /// Furthermore, the standard makes a difference
+    /// between a "signaling" and a "quiet" NaN,
+    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
+    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
+    /// and the stability of its representation over Rust versions
+    /// and target platforms isn't guaranteed.
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     pub const NAN: f32 = 0.0_f32 / 0.0_f32;
     /// Infinity (∞).
@@ -427,7 +436,7 @@ impl f32 {
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     pub const NEG_INFINITY: f32 = -1.0_f32 / 0.0_f32;
 
-    /// Returns `true` if this value is `NaN`.
+    /// Returns `true` if this value is NaN.
     ///
     /// ```
     /// let nan = f32::NAN;
@@ -476,7 +485,7 @@ impl f32 {
         self.abs_private() == Self::INFINITY
     }
 
-    /// Returns `true` if this number is neither infinite nor `NaN`.
+    /// Returns `true` if this number is neither infinite nor NaN.
     ///
     /// ```
     /// let f = 7.0f32;
@@ -527,7 +536,7 @@ impl f32 {
     }
 
     /// Returns `true` if the number is neither zero, infinite,
-    /// [subnormal], or `NaN`.
+    /// [subnormal], or NaN.
     ///
     /// ```
     /// let min = f32::MIN_POSITIVE; // 1.17549435e-38f32
@@ -582,8 +591,12 @@ impl f32 {
         }
     }
 
-    /// Returns `true` if `self` has a positive sign, including `+0.0`, `NaN`s with
-    /// positive sign bit and positive infinity.
+    /// Returns `true` if `self` has a positive sign, including `+0.0`, NaNs with
+    /// positive sign bit and positive infinity. Note that IEEE-745 doesn't assign any
+    /// meaning to the sign bit in case of a NaN, and as Rust doesn't guarantee that
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
     ///
     /// ```
     /// let f = 7.0_f32;
@@ -600,8 +613,12 @@ impl f32 {
         !self.is_sign_negative()
     }
 
-    /// Returns `true` if `self` has a negative sign, including `-0.0`, `NaN`s with
-    /// negative sign bit and negative infinity.
+    /// Returns `true` if `self` has a negative sign, including `-0.0`, NaNs with
+    /// negative sign bit and negative infinity. Note that IEEE-745 doesn't assign any
+    /// meaning to the sign bit in case of a NaN, and as Rust doesn't guarantee that
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
     ///
     /// ```
     /// let f = 7.0f32;
@@ -674,17 +691,17 @@ impl f32 {
 
     /// Returns the maximum of the two numbers.
     ///
-    /// Follows the IEEE-754 2008 semantics for maxNum, except for handling of signaling NaNs.
-    /// This matches the behavior of libm’s fmax.
+    /// If one of the arguments is NaN, then the other argument is returned.
+    /// This follows the IEEE-754 2008 semantics for maxNum, except for handling of signaling NaNs;
+    /// this function handles all NaNs the same way and avoids maxNum's problems with associativity.
+    /// This also matches the behavior of libm’s fmax.
     ///
     /// ```
     /// let x = 1.0f32;
     /// let y = 2.0f32;
     ///
     /// assert_eq!(x.max(y), y);
     /// ```
-    ///
-    /// If one of the arguments is NaN, then the other argument is returned.
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -694,25 +711,25 @@ impl f32 {
 
     /// Returns the minimum of the two numbers.
     ///
-    /// Follows the IEEE-754 2008 semantics for minNum, except for handling of signaling NaNs.
-    /// This matches the behavior of libm’s fmin.
+    /// If one of the arguments is NaN, then the other argument is returned.
+    /// This follows the IEEE-754 2008 semantics for minNum, except for handling of signaling NaNs;
+    /// this function handles all NaNs the same way and avoids minNum's problems with associativity.
+    /// This also matches the behavior of libm’s fmin.
     ///
     /// ```
     /// let x = 1.0f32;
     /// let y = 2.0f32;
     ///
     /// assert_eq!(x.min(y), x);
     /// ```
-    ///
-    /// If one of the arguments is NaN, then the other argument is returned.
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn min(self, other: f32) -> f32 {
         intrinsics::minnumf32(self, other)
     }
 
-    /// Returns the maximum of the two numbers, propagating NaNs.
+    /// Returns the maximum of the two numbers.
     ///
     /// This returns NaN when *either* argument is NaN, as opposed to
     /// [`f32::max`] which only returns NaN when *both* arguments are NaN.
@@ -744,7 +761,7 @@ impl f32 {
         }
     }
 
-    /// Returns the minimum of the two numbers, propagating NaNs.
+    /// Returns the minimum of the two numbers.
     ///
     /// This returns NaN when *either* argument is NaN, as opposed to
     /// [`f32::min`] which only returns NaN when *both* arguments are NaN.

library/core/src/num/f64.rs

@@ -417,6 +417,15 @@ impl f64 {
     pub const MAX_10_EXP: i32 = 308;
 
     /// Not a Number (NaN).
+    ///
+    /// Note that IEEE-745 doesn't define just a single NaN value;
+    /// a plethora of bit patterns are considered to be NaN.
+    /// Furthermore, the standard makes a difference
+    /// between a "signaling" and a "quiet" NaN,
+    /// and allows inspecting its "payload" (the unspecified bits in the bit pattern).
+    /// This constant isn't guaranteed to equal to any specific NaN bitpattern,
+    /// and the stability of its representation over Rust versions
+    /// and target platforms isn't guaranteed.
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     pub const NAN: f64 = 0.0_f64 / 0.0_f64;
     /// Infinity (∞).
@@ -426,7 +435,7 @@ impl f64 {
     #[stable(feature = "assoc_int_consts", since = "1.43.0")]
     pub const NEG_INFINITY: f64 = -1.0_f64 / 0.0_f64;
 
-    /// Returns `true` if this value is `NaN`.
+    /// Returns `true` if this value is NaN.
     ///
     /// ```
     /// let nan = f64::NAN;
@@ -475,7 +484,7 @@ impl f64 {
         self.abs_private() == Self::INFINITY
     }
 
-    /// Returns `true` if this number is neither infinite nor `NaN`.
+    /// Returns `true` if this number is neither infinite nor NaN.
     ///
     /// ```
     /// let f = 7.0f64;
@@ -526,7 +535,7 @@ impl f64 {
     }
 
     /// Returns `true` if the number is neither zero, infinite,
-    /// [subnormal], or `NaN`.
+    /// [subnormal], or NaN.
     ///
     /// ```
     /// let min = f64::MIN_POSITIVE; // 2.2250738585072014e-308f64
@@ -581,8 +590,12 @@ impl f64 {
         }
     }
 
-    /// Returns `true` if `self` has a positive sign, including `+0.0`, `NaN`s with
-    /// positive sign bit and positive infinity.
+    /// Returns `true` if `self` has a positive sign, including `+0.0`, NaNs with
+    /// positive sign bit and positive infinity. Note that IEEE-745 doesn't assign any
+    /// meaning to the sign bit in case of a NaN, and as Rust doesn't guarantee that
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
     ///
     /// ```
     /// let f = 7.0_f64;
@@ -608,8 +621,12 @@ impl f64 {
         self.is_sign_positive()
     }
 
-    /// Returns `true` if `self` has a negative sign, including `-0.0`, `NaN`s with
-    /// negative sign bit and negative infinity.
+    /// Returns `true` if `self` has a negative sign, including `-0.0`, NaNs with
+    /// negative sign bit and negative infinity. Note that IEEE-745 doesn't assign any
+    /// meaning to the sign bit in case of a NaN, and as Rust doesn't guarantee that
+    /// the bit pattern of NaNs are conserved over arithmetic operations, the result of
+    /// `is_sign_positive` on a NaN might produce an unexpected result in some cases.
+    /// See [explanation of NaN as a special value](f32) for more info.
     ///
     /// ```
     /// let f = 7.0_f64;
@@ -690,17 +707,17 @@ impl f64 {
 
     /// Returns the maximum of the two numbers.
     ///
-    /// Follows the IEEE-754 2008 semantics for maxNum, except for handling of signaling NaNs.
-    /// This matches the behavior of libm’s fmax.
+    /// If one of the arguments is NaN, then the other argument is returned.
+    /// This follows the IEEE-754 2008 semantics for maxNum, except for handling of signaling NaNs;
+    /// this function handles all NaNs the same way and avoids maxNum's problems with associativity.
+    /// This also matches the behavior of libm’s fmax.
     ///
     /// ```
     /// let x = 1.0_f64;
     /// let y = 2.0_f64;
     ///
     /// assert_eq!(x.max(y), y);
     /// ```
-    ///
-    /// If one of the arguments is NaN, then the other argument is returned.
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -710,25 +727,25 @@ impl f64 {
 
     /// Returns the minimum of the two numbers.
     ///
-    /// Follows the IEEE-754 2008 semantics for minNum, except for handling of signaling NaNs.
-    /// This matches the behavior of libm’s fmin.
+    /// If one of the arguments is NaN, then the other argument is returned.
+    /// This follows the IEEE-754 2008 semantics for minNum, except for handling of signaling NaNs;
+    /// this function handles all NaNs the same way and avoids minNum's problems with associativity.
+    /// This also matches the behavior of libm’s fmin.
     ///
     /// ```
     /// let x = 1.0_f64;
     /// let y = 2.0_f64;
     ///
     /// assert_eq!(x.min(y), x);
     /// ```
-    ///
-    /// If one of the arguments is NaN, then the other argument is returned.
     #[must_use = "this returns the result of the comparison, without modifying either input"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn min(self, other: f64) -> f64 {
         intrinsics::minnumf64(self, other)
     }
 
-    /// Returns the maximum of the two numbers, propagating NaNs.
+    /// Returns the maximum of the two numbers.
     ///
     /// This returns NaN when *either* argument is NaN, as opposed to
     /// [`f64::max`] which only returns NaN when *both* arguments are NaN.
@@ -760,7 +777,7 @@ impl f64 {
         }
     }
 
-    /// Returns the minimum of the two numbers, propagating NaNs.
+    /// Returns the minimum of the two numbers.
     ///
     /// This returns NaN when *either* argument is NaN, as opposed to
     /// [`f64::min`] which only returns NaN when *both* arguments are NaN.

library/core/src/primitive_docs.rs

@@ -977,10 +977,20 @@ mod prim_tuple {}
 ///   like `1.0 / 0.0`.
 /// - [NaN (not a number)](#associatedconstant.NAN): this value results from
 ///   calculations like `(-1.0).sqrt()`. NaN has some potentially unexpected
-///   behavior: it is unequal to any float, including itself! It is also neither
-///   smaller nor greater than any float, making it impossible to sort. Lastly,
-///   it is considered infectious as almost all calculations where one of the
-///   operands is NaN will also result in NaN.
+///   behavior:
+///   - It is unequal to any float, including itself!
+///   - It is also neither smaller nor greater than any float, making it
+///     impossible to sort by the default comparison operation. This is the
+///     reason `f32` doesn't implement the `Ord` and `Eq` traits.
+///   - It is also considered *infectious* as almost all calculations where one
+///     of the operands is NaN will also result in NaN. The explanations on this
+///     page only explicitly document behavior on NaN operands if this default
+///     is *not* observed by the operation.
+///   - Lastly, there are multiple bit patterns that are considered NaN.
+///     Rust does not currently guarantee that the bit patterns of NaN are
+///     preserved over arithmetic operations,
+///     so there may be some surprising results upon inspecting the bit patterns,
+///     as the same calculations might produce NaNs with different bit patterns.
 ///
 /// For more information on floating point numbers, see [Wikipedia][wikipedia].
 ///

library/std/src/f32.rs

@@ -30,7 +30,7 @@ pub use core::f32::{
 #[cfg(not(test))]
 #[cfg_attr(bootstrap, lang = "f32_runtime")]
 impl f32 {
-    /// Returns the largest integer less than or equal to a number.
+    /// Returns the largest integer less than or equal to `self`.
     ///
     /// # Examples
     ///
@@ -51,7 +51,7 @@ impl f32 {
         unsafe { intrinsics::floorf32(self) }
     }
 
-    /// Returns the smallest integer greater than or equal to a number.
+    /// Returns the smallest integer greater than or equal to `self`.
     ///
     /// # Examples
     ///
@@ -70,7 +70,7 @@ impl f32 {
         unsafe { intrinsics::ceilf32(self) }
     }
 
-    /// Returns the nearest integer to a number. Round half-way cases away from
+    /// Returns the nearest integer to `self`. Round half-way cases away from
     /// `0.0`.
     ///
     /// # Examples
@@ -90,7 +90,8 @@ impl f32 {
         unsafe { intrinsics::roundf32(self) }
     }
 
-    /// Returns the integer part of a number.
+    /// Returns the integer part of `self`.
+    /// This means that non-integer numbers are always truncated towards zero.
     ///
     /// # Examples
     ///
@@ -111,7 +112,7 @@ impl f32 {
         unsafe { intrinsics::truncf32(self) }
     }
 
-    /// Returns the fractional part of a number.
+    /// Returns the fractional part of `self`.
     ///
     /// # Examples
     ///
@@ -132,8 +133,7 @@ impl f32 {
         self - self.trunc()
     }
 
-    /// Computes the absolute value of `self`. Returns `NAN` if the
-    /// number is `NAN`.
+    /// Computes the absolute value of `self`.
     ///
     /// # Examples
     ///
@@ -161,7 +161,7 @@ impl f32 {
     ///
     /// - `1.0` if the number is positive, `+0.0` or `INFINITY`
     /// - `-1.0` if the number is negative, `-0.0` or `NEG_INFINITY`
-    /// - `NAN` if the number is `NAN`
+    /// - NaN if the number is NaN
     ///
     /// # Examples
     ///
@@ -185,8 +185,10 @@ impl f32 {
     /// `sign`.
     ///
     /// Equal to `self` if the sign of `self` and `sign` are the same, otherwise
-    /// equal to `-self`. If `self` is a `NAN`, then a `NAN` with the sign of
-    /// `sign` is returned.
+    /// equal to `-self`. If `self` is a NaN, then a NaN with the sign bit of
+    /// `sign` is returned. Note, however, that conserving the sign bit on NaN
+    /// across arithmetical operations is not generally guaranteed.
+    /// See [explanation of NaN as a special value](primitive@f32) for more info.
     ///
     /// # Examples
     ///
@@ -299,7 +301,9 @@ impl f32 {
 
     /// Raises a number to an integer power.
     ///
-    /// Using this function is generally faster than using `powf`
+    /// Using this function is generally faster than using `powf`.
+    /// It might have different sequence of rounding operations than `powf`,
+    /// so the results are not guaranteed to agree.
     ///
     /// # Examples
     ///