1 files changed, 29 insertions, 11 deletions

diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 28cd3c4e4..5842c096f 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -1158,12 +1158,12 @@ impl FusedIterator for Ancestors<'_> {}
 /// Which method works best depends on what kind of situation you're in.
 #[cfg_attr(not(test), rustc_diagnostic_item = "PathBuf")]
 #[stable(feature = "rust1", since = "1.0.0")]
-// FIXME:
 // `PathBuf::as_mut_vec` current implementation relies
 // on `PathBuf` being layout-compatible with `Vec<u8>`.
-// When attribute privacy is implemented, `PathBuf` should be annotated as `#[repr(transparent)]`.
-// Anyway, `PathBuf` representation and layout are considered implementation detail, are
-// not documented and must not be relied upon.
+// However, `PathBuf` layout is considered an implementation detail and must not be relied upon. We
+// want `repr(transparent)` but we don't want it to show up in rustdoc, so we hide it under
+// `cfg(doc)`. This is an ad-hoc implementation of attribute privacy.
+#[cfg_attr(not(doc), repr(transparent))]
 pub struct PathBuf {
     inner: OsString,
 }
@@ -1983,12 +1983,12 @@ impl AsRef<OsStr> for PathBuf {
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Path")]
 #[stable(feature = "rust1", since = "1.0.0")]
-// FIXME:
 // `Path::new` current implementation relies
 // on `Path` being layout-compatible with `OsStr`.
-// When attribute privacy is implemented, `Path` should be annotated as `#[repr(transparent)]`.
-// Anyway, `Path` representation and layout are considered implementation detail, are
-// not documented and must not be relied upon.
+// However, `Path` layout is considered an implementation detail and must not be relied upon. We
+// want `repr(transparent)` but we don't want it to show up in rustdoc, so we hide it under
+// `cfg(doc)`. This is an ad-hoc implementation of attribute privacy.
+#[cfg_attr(not(doc), repr(transparent))]
 pub struct Path {
     inner: OsStr,
 }
@@ -2608,9 +2608,27 @@ impl Path {
     }
 
     fn _with_extension(&self, extension: &OsStr) -> PathBuf {
-        let mut buf = self.to_path_buf();
-        buf.set_extension(extension);
-        buf
+        let self_len = self.as_os_str().len();
+        let self_bytes = self.as_os_str().as_os_str_bytes();
+
+        let (new_capacity, slice_to_copy) = match self.extension() {
+            None => {
+                // Enough capacity for the extension and the dot
+                let capacity = self_len + extension.len() + 1;
+                let whole_path = self_bytes.iter();
+                (capacity, whole_path)
+            }
+            Some(previous_extension) => {
+                let capacity = self_len + extension.len() - previous_extension.len();
+                let path_till_dot = self_bytes[..self_len - previous_extension.len()].iter();
+                (capacity, path_till_dot)
+            }
+        };
+
+        let mut new_path = PathBuf::with_capacity(new_capacity);
+        new_path.as_mut_vec().extend(slice_to_copy);
+        new_path.set_extension(extension);
+        new_path
     }
 
     /// Produces an iterator over the [`Component`]s of the path.