Rollup merge of rust-lang#89720 - jkugelman:must-use-math-operations, r=joshtriplett

Add #[must_use] to math and bit manipulation methods

Also tidied up a few other nearby `#[must_use]`s.

Parent issue: rust-lang#89692

Author: matthiaskrgr

library/core/src/num/f32.rs

@@ -643,6 +643,8 @@ impl f32 {
     ///
     /// assert!(abs_difference <= f32::EPSILON);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "f32_deg_rad_conversions", since = "1.7.0")]
     #[inline]
     pub fn to_degrees(self) -> f32 {
@@ -660,6 +662,8 @@ impl f32 {
     ///
     /// assert!(abs_difference <= f32::EPSILON);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "f32_deg_rad_conversions", since = "1.7.0")]
     #[inline]
     pub fn to_radians(self) -> f32 {
@@ -719,6 +723,8 @@ impl f32 {
     /// * Not be `NaN`
     /// * Not be infinite
     /// * Be representable in the return type `Int`, after truncating off its fractional part
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_approx_unchecked_to", since = "1.44.0")]
     #[inline]
     pub unsafe fn to_int_unchecked<Int>(self) -> Int
@@ -747,6 +753,8 @@ impl f32 {
     /// assert_eq!((12.5f32).to_bits(), 0x41480000);
     ///
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -809,6 +817,8 @@ impl f32 {
     /// let bytes = 12.5f32.to_be_bytes();
     /// assert_eq!(bytes, [0x41, 0x48, 0x00, 0x00]);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -825,6 +835,8 @@ impl f32 {
     /// let bytes = 12.5f32.to_le_bytes();
     /// assert_eq!(bytes, [0x00, 0x00, 0x48, 0x41]);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -854,6 +866,8 @@ impl f32 {
     ///     }
     /// );
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]

library/core/src/num/f64.rs

@@ -658,6 +658,8 @@ impl f64 {
     ///
     /// assert!(abs_difference < 1e-10);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_degrees(self) -> f64 {
@@ -676,6 +678,8 @@ impl f64 {
     ///
     /// assert!(abs_difference < 1e-10);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_radians(self) -> f64 {
@@ -735,6 +739,8 @@ impl f64 {
     /// * Not be `NaN`
     /// * Not be infinite
     /// * Be representable in the return type `Int`, after truncating off its fractional part
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_approx_unchecked_to", since = "1.44.0")]
     #[inline]
     pub unsafe fn to_int_unchecked<Int>(self) -> Int
@@ -763,6 +769,8 @@ impl f64 {
     /// assert_eq!((12.5f64).to_bits(), 0x4029000000000000);
     ///
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_bits_conv", since = "1.20.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -825,6 +833,8 @@ impl f64 {
     /// let bytes = 12.5f64.to_be_bytes();
     /// assert_eq!(bytes, [0x40, 0x29, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -841,6 +851,8 @@ impl f64 {
     /// let bytes = 12.5f64.to_le_bytes();
     /// assert_eq!(bytes, [0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x29, 0x40]);
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]
@@ -870,6 +882,8 @@ impl f64 {
     ///     }
     /// );
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "float_to_from_bytes", since = "1.40.0")]
     #[rustc_const_unstable(feature = "const_float_bits_conv", issue = "72447")]
     #[inline]