diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index caae8f924d2b1..8d565e26a1609 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1519,6 +1519,74 @@ impl PathBuf {
         true
     }
 
+    /// Append [`self.extension`] with `extension`.
+    ///
+    /// Returns `false` and does nothing if [`self.file_name`] is [`None`],
+    /// returns `true` and updates the extension otherwise.
+    ///
+    /// # Caveats
+    ///
+    /// The appended `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`].
+    ///
+    /// See the examples below.
+    ///
+    /// [`self.file_name`]: Path::file_name
+    /// [`self.extension`]: Path::extension
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(path_add_extension)]
+    ///
+    /// 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.add_extension("");
+    /// assert_eq!(Path::new("/feel/the.formatted.dark"), p.as_path());
+    /// ```
+    #[unstable(feature = "path_add_extension", issue = "127292")]
+    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 {
+        let file_name = match self.file_name() {
+            None => return false,
+            Some(f) => f.as_encoded_bytes(),
+        };
+
+        let new = extension;
+        if !new.is_empty() {
+            // truncate until right after the file name
+            // this is necessary for trimming the trailing slash
+            let end_file_name = file_name[file_name.len()..].as_ptr().addr();
+            let start = self.inner.as_encoded_bytes().as_ptr().addr();
+            self.inner.truncate(end_file_name.wrapping_sub(start));
+
+            // append the new extension
+            self.inner.reserve_exact(new.len() + 1);
+            self.inner.push(OsStr::new("."));
+            self.inner.push(new);
+        }
+
+        true
+    }
+
     /// Yields a mutable reference to the underlying [`OsString`] instance.
     ///
     /// # Examples
@@ -2656,6 +2724,32 @@ impl Path {
         new_path
     }
 
+    /// Creates an owned [`PathBuf`] like `self` but with the extension added.
+    ///
+    /// See [`PathBuf::add_extension`] for more details.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(path_add_extension)]
+    ///
+    /// use std::path::{Path, PathBuf};
+    ///
+    /// let path = Path::new("foo.rs");
+    /// assert_eq!(path.with_added_extension("txt"), PathBuf::from("foo.rs.txt"));
+    ///
+    /// let path = Path::new("foo.tar.gz");
+    /// assert_eq!(path.with_added_extension(""), PathBuf::from("foo.tar.gz"));
+    /// assert_eq!(path.with_added_extension("xz"), PathBuf::from("foo.tar.gz.xz"));
+    /// assert_eq!(path.with_added_extension("").with_added_extension("txt"), PathBuf::from("foo.tar.gz.txt"));
+    /// ```
+    #[unstable(feature = "path_add_extension", issue = "127292")]
+    pub fn with_added_extension<S: AsRef<OsStr>>(&self, extension: S) -> PathBuf {
+        let mut new_path = self.to_path_buf();
+        new_path.add_extension(extension);
+        new_path
+    }
+
     /// Produces an iterator over the [`Component`]s of the path.
     ///
     /// When parsing the path, there is a small amount of normalization:
diff --git a/library/std/src/path/tests.rs b/library/std/src/path/tests.rs
index 92702b395dfe1..bb6126e494177 100644
--- a/library/std/src/path/tests.rs
+++ b/library/std/src/path/tests.rs
@@ -1401,6 +1401,37 @@ pub fn test_set_extension() {
     tfe!("/", "foo", "/", false);
 }
 
+#[test]
+pub fn test_add_extension() {
+    macro_rules! tfe (
+        ($path:expr, $ext:expr, $expected:expr, $output:expr) => ({
+            let mut p = PathBuf::from($path);
+            let output = p.add_extension($ext);
+            assert!(p.to_str() == Some($expected) && output == $output,
+                    "adding extension of {:?} to {:?}: Expected {:?}/{:?}, got {:?}/{:?}",
+                    $path, $ext, $expected, $output,
+                    p.to_str().unwrap(), output);
+        });
+    );
+
+    tfe!("foo", "txt", "foo.txt", true);
+    tfe!("foo.bar", "txt", "foo.bar.txt", true);
+    tfe!("foo.bar.baz", "txt", "foo.bar.baz.txt", true);
+    tfe!(".test", "txt", ".test.txt", true);
+    tfe!("foo.txt", "", "foo.txt", true);
+    tfe!("foo", "", "foo", true);
+    tfe!("", "foo", "", false);
+    tfe!(".", "foo", ".", false);
+    tfe!("foo/", "bar", "foo.bar", true);
+    tfe!("foo/.", "bar", "foo.bar", true);
+    tfe!("..", "foo", "..", false);
+    tfe!("foo/..", "bar", "foo/..", false);
+    tfe!("/", "foo", "/", false);
+
+    // edge cases
+    tfe!("/foo.ext////", "bar", "/foo.ext.bar", true);
+}
+
 #[test]
 pub fn test_with_extension() {
     macro_rules! twe (
@@ -1441,6 +1472,49 @@ pub fn test_with_extension() {
     twe!("ccc.bbb_bbb", "aaa_aaa_aaa", "ccc.aaa_aaa_aaa");
 }
 
+#[test]
+pub fn test_with_added_extension() {
+    macro_rules! twe (
+        ($input:expr, $extension:expr, $expected:expr) => ({
+            let input = Path::new($input);
+            let output = input.with_added_extension($extension);
+
+            assert!(
+                output.to_str() == Some($expected),
+                "calling Path::new({:?}).with_added_extension({:?}): Expected {:?}, got {:?}",
+                $input, $extension, $expected, output,
+            );
+        });
+    );
+
+    twe!("foo", "txt", "foo.txt");
+    twe!("foo.bar", "txt", "foo.bar.txt");
+    twe!("foo.bar.baz", "txt", "foo.bar.baz.txt");
+    twe!(".test", "txt", ".test.txt");
+    twe!("foo.txt", "", "foo.txt");
+    twe!("foo", "", "foo");
+    twe!("", "foo", "");
+    twe!(".", "foo", ".");
+    twe!("foo/", "bar", "foo.bar");
+    twe!("foo/.", "bar", "foo.bar");
+    twe!("..", "foo", "..");
+    twe!("foo/..", "bar", "foo/..");
+    twe!("/", "foo", "/");
+
+    // edge cases
+    twe!("/foo.ext////", "bar", "/foo.ext.bar");
+
+    // New extension is smaller than file name
+    twe!("aaa_aaa_aaa", "bbb_bbb", "aaa_aaa_aaa.bbb_bbb");
+    // New extension is greater than file name
+    twe!("bbb_bbb", "aaa_aaa_aaa", "bbb_bbb.aaa_aaa_aaa");
+
+    // New extension is smaller than previous extension
+    twe!("ccc.aaa_aaa_aaa", "bbb_bbb", "ccc.aaa_aaa_aaa.bbb_bbb");
+    // New extension is greater than previous extension
+    twe!("ccc.bbb_bbb", "aaa_aaa_aaa", "ccc.bbb_bbb.aaa_aaa_aaa");
+}
+
 #[test]
 fn test_eq_receivers() {
     use crate::borrow::Cow;