Rollup merge of rust-lang#89794 - jkugelman:must-use-to_value-conversions, r=joshtriplett

Add #[must_use] to to_value conversions

`NonNull<T>::cast` snuck in when I wasn't looking. What a scamp!

Parent issue: rust-lang#89692

r? ````@joshtriplett````

Author: JohnTitor

library/core/src/char/methods.rs

@@ -328,9 +328,11 @@ impl char {
     ///
     /// ```should_panic
     /// // this panics
-    /// '1'.to_digit(37);
+    /// let _ = '1'.to_digit(37);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_digit(self, radix: u32) -> Option<u32> {
         assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");

library/core/src/ptr/non_null.rs

@@ -242,6 +242,8 @@ impl<T: ?Sized> NonNull<T> {
     /// The pointer can be later reconstructed with [`NonNull::from_raw_parts`].
     #[unstable(feature = "ptr_metadata", issue = "81513")]
     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn to_raw_parts(self) -> (NonNull<()>, <T as super::Pointee>::Metadata) {
         (self.cast(), super::metadata(self.as_ptr()))
@@ -386,6 +388,8 @@ impl<T: ?Sized> NonNull<T> {
     /// ```
     #[stable(feature = "nonnull_cast", since = "1.27.0")]
     #[rustc_const_stable(feature = "const_nonnull_cast", since = "1.36.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn cast<U>(self) -> NonNull<U> {
         // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null

library/std/src/ffi/c_str.rs

@@ -1342,6 +1342,8 @@ impl CStr {
     /// assert_eq!(cstr.to_bytes(), b"foo");
     /// ```
     #[inline]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn to_bytes(&self) -> &[u8] {
         let bytes = self.to_bytes_with_nul();
@@ -1367,6 +1369,8 @@ impl CStr {
     /// assert_eq!(cstr.to_bytes_with_nul(), b"foo\0");
     /// ```
     #[inline]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn to_bytes_with_nul(&self) -> &[u8] {
         unsafe { &*(&self.inner as *const [c_char] as *const [u8]) }
@@ -1437,6 +1441,8 @@ impl CStr {
     ///     Cow::Owned(String::from("Hello �World")) as Cow<'_, str>
     /// );
     /// ```
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "cstr_to_str", since = "1.4.0")]
     pub fn to_string_lossy(&self) -> Cow<'_, str> {
         String::from_utf8_lossy(self.to_bytes())

library/std/src/ffi/os_str.rs

@@ -576,6 +576,8 @@ impl OsStr {
     /// assert_eq!(os_str.to_str(), Some("foo"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_str(&self) -> Option<&str> {
         self.inner.to_str()
@@ -627,6 +629,8 @@ impl OsStr {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_string_lossy(&self) -> Cow<'_, str> {
         self.inner.to_string_lossy()
@@ -644,6 +648,8 @@ impl OsStr {
     /// assert_eq!(os_string, OsString::from("foo"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_os_string(&self) -> OsString {
         OsString { inner: self.inner.to_owned() }

library/std/src/net/ip.rs

@@ -426,6 +426,8 @@ impl IpAddr {
     /// assert_eq!(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1)).to_canonical().is_loopback(), true);
     /// ```
     #[inline]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[rustc_const_unstable(feature = "const_ip", issue = "76205")]
     #[unstable(feature = "ip", issue = "27709")]
     pub const fn to_canonical(&self) -> IpAddr {
@@ -901,6 +903,8 @@ impl Ipv4Addr {
     /// ```
     #[rustc_const_stable(feature = "const_ipv4", since = "1.50.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn to_ipv6_compatible(&self) -> Ipv6Addr {
         let [a, b, c, d] = self.octets();
@@ -926,6 +930,8 @@ impl Ipv4Addr {
     /// ```
     #[rustc_const_stable(feature = "const_ipv4", since = "1.50.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn to_ipv6_mapped(&self) -> Ipv6Addr {
         let [a, b, c, d] = self.octets();
@@ -1648,6 +1654,8 @@ impl Ipv6Addr {
     /// ```
     #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
     #[unstable(feature = "ip", issue = "27709")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn to_ipv4_mapped(&self) -> Option<Ipv4Addr> {
         match self.octets() {
@@ -1685,6 +1693,8 @@ impl Ipv6Addr {
     /// ```
     #[rustc_const_stable(feature = "const_ipv6", since = "1.50.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub const fn to_ipv4(&self) -> Option<Ipv4Addr> {
         if let [0, 0, 0, 0, 0, 0 | 0xffff, ab, cd] = self.segments() {
@@ -1708,9 +1718,11 @@ impl Ipv6Addr {
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1).is_loopback(), false);
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1).to_canonical().is_loopback(), true);
     /// ```
-    #[inline]
     #[rustc_const_unstable(feature = "const_ipv6", issue = "76205")]
     #[unstable(feature = "ip", issue = "27709")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
+    #[inline]
     pub const fn to_canonical(&self) -> IpAddr {
         if let Some(mapped) = self.to_ipv4_mapped() {
             return IpAddr::V4(mapped);

library/std/src/path.rs

@@ -1952,6 +1952,8 @@ impl Path {
     /// assert_eq!(path.to_str(), Some("foo.txt"));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_str(&self) -> Option<&str> {
         self.inner.to_str()
@@ -1978,6 +1980,8 @@ impl Path {
     /// Had `path` contained invalid unicode, the `to_string_lossy` call might
     /// have returned `"fo�.txt"`.
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[inline]
     pub fn to_string_lossy(&self) -> Cow<'_, str> {
         self.inner.to_string_lossy()
@@ -1994,6 +1998,8 @@ impl Path {
     /// assert_eq!(path_buf, std::path::PathBuf::from("foo.txt"));
     /// ```
     #[rustc_conversion_suggestion]
+    #[must_use = "this returns the result of the operation, \
+                  without modifying the original"]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn to_path_buf(&self) -> PathBuf {
         PathBuf::from(self.inner.to_os_string())