Add is_empty() for Path and OsStr

Related to #30259

Rebase of #30623

Author: ticki

src/libstd/ffi/os_str.rs

@@ -277,6 +277,12 @@ impl OsStr {
         self.to_bytes().and_then(|b| CString::new(b).ok())
     }
 
+    /// Checks if the string is empty.
+    #[unstable(feature = "osstr_is_empty", reason = "recently added", issue = "30259")]
+    pub fn is_empty(&self) -> bool {
+        self.inner.inner.is_empty()
+    }
+
     /// Gets the underlying byte representation.
     ///
     /// Note: it is *crucial* that this API is private, to avoid

src/libstd/path.rs

@@ -1897,6 +1897,24 @@ impl Path {
     pub fn is_dir(&self) -> bool {
         fs::metadata(self).map(|m| m.is_dir()).unwrap_or(false)
     }
+
+    /// Checks if the path buffer is empty. On Windows, it will return false if the inner string is
+    /// invalid unicode. On Unix, this is a no-op.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(os_extras)]
+    /// use std::path::Path;
+    ///
+    /// let path = Path::new("/tmp/foo.rs");
+    ///
+    /// assert!(!path.is_empty());
+    /// ```
+    #[unstable(feature = "path_is_empty", reason = "recently added", issue = "30259")]
+    pub fn is_empty(&self) -> bool {
+        self.inner.is_empty()
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -3239,6 +3257,17 @@ mod tests {
         }
     }
 
+    #[test]
+    pub fn is_empty() {
+        let path = Path::new("/tmp/foo.rs");
+        let mut path_buf = PathBuf::new();
+
+        assert!(!path.is_empty());
+        assert!(path_buf.is_empty());
+        path_buf.push("catsarecute");
+        assert!(!path_buf.is_empty());
+    }
+
     #[test]
     pub fn test_set_extension() {
         macro_rules! tfe(