Rollup merge of #89729 - jkugelman:must-use-core-std-constructors, r=joshtriplett

Add #[must_use] to core and std constructors

Parent issue: #89692

r? ``@joshtriplett``

library/std/src/path.rs

@@ -1146,6 +1146,7 @@ impl PathBuf {
     /// let path = PathBuf::new();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
     #[inline]
     pub fn new() -> PathBuf {
         PathBuf { inner: OsString::new() }
@@ -1170,6 +1171,7 @@ impl PathBuf {
     ///
     /// [`with_capacity`]: OsString::with_capacity
     #[stable(feature = "path_buf_capacity", since = "1.44.0")]
+    #[must_use]
     #[inline]
     pub fn with_capacity(capacity: usize) -> PathBuf {
         PathBuf { inner: OsString::with_capacity(capacity) }