diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 56ea51226f9d5..2caf8832294c6 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1498,6 +1498,80 @@ impl PathBuf {
         true
     }
 
+    /// Updates [`self.extension`] to `Some(extension)` or to `None` if
+    /// `extension` is empty.
+    ///
+    /// Returns `false` and does nothing if [`self.file_name`] is [`None`],
+    /// returns `true` and updates the extension otherwise.
+    ///
+    /// If [`self.extension`] is [`None`], the extension is added; otherwise
+    /// it is replaced.
+    ///
+    /// If `extension` is the empty string, [`self.extension`] will be [`None`]
+    /// afterwards, not `Some("")`.
+    ///
+    /// # Caveats
+    ///
+    /// The new `extension` may contain dots and will be used in its entirety,
+    /// but only the part after the final dot will be reflected in
+    /// [`self.extension`].
+    ///
+    /// If the file stem contains internal dots and `extension` is empty, part
+    /// of the old file stem will be considered the new [`self.extension`].
+    ///
+    /// See the examples below.
+    ///
+    /// [`self.file_name`]: Path::file_name
+    /// [`self.extension`]: Path::extension
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::path::{Path, PathBuf};
+    ///
+    /// let mut p = PathBuf::from("/feel/the");
+    ///
+    /// p.add_extension("formatted");
+    /// assert_eq!(Path::new("/feel/the.formatted"), p.as_path());
+    ///
+    /// p.add_extension("dark.side");
+    /// assert_eq!(Path::new("/feel/the.formatted.dark.side"), p.as_path());
+    ///
+    /// p.set_extension("cookie");
+    /// assert_eq!(Path::new("/feel/the.formatted.dark.cookie"), p.as_path());
+    ///
+    /// p.set_extension("");
+    /// assert_eq!(Path::new("/feel/the.formatted.dark"), p.as_path());
+    /// p.set_extension("");
+    /// assert_eq!(Path::new("/feel/the.formatted"), p.as_path());
+    /// p.set_extension("");
+    /// assert_eq!(Path::new("/feel/the"), p.as_path());
+    /// p.set_extension("");
+    /// assert_eq!(Path::new("/feel/the"), p.as_path());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn add_extension<S: AsRef<OsStr>>(&mut self, extension: S) -> bool {
+        self._add_extension(extension.as_ref())
+    }
+
+    fn _add_extension(&mut self, extension: &OsStr) -> bool {
+        if self.file_stem().is_none() {
+            return false;
+        }
+
+        // add the new extension, if any
+        let v = self.as_mut_vec();
+        let new = extension.as_encoded_bytes();
+        if !new.is_empty() {
+            v.reserve_exact(new.len() + 1);
+            v.push(b'.');
+            v.extend_from_slice(new);
+        }
+
+        true
+    }
+
+
     /// Yields a mutable reference to the underlying [`OsString`] instance.
     ///
     /// # Examples