Avoid redundant WTF-8 checks in PathBuf

Eliminate checks for WTF-8 boundaries in `PathBuf::set_extension` and
`add_extension`, where joining WTF-8 surrogate halves is impossible.
Don't convert the `str` to `OsStr`, because `OsString::push` specializes
to skip the joining when given strings.

Author: thaliaarchi

library/std/src/path.rs

@@ -1508,10 +1508,8 @@ impl PathBuf {
 
     fn _set_extension(&mut self, extension: &OsStr) -> bool {
         for &b in extension.as_encoded_bytes() {
-            if b < 128 {
-                if is_separator(b as char) {
-                    panic!("extension cannot contain path separators: {:?}", extension);
-                }
+            if b.is_ascii() && is_separator(b as char) {
+                panic!("extension cannot contain path separators: {extension:?}");
             }
         }
 
@@ -1526,11 +1524,13 @@ impl PathBuf {
         self.inner.truncate(end_file_stem.wrapping_sub(start));
 
         // add the new extension, if any
-        let new = extension;
+        let new = extension.as_encoded_bytes();
         if !new.is_empty() {
             self.inner.reserve_exact(new.len() + 1);
-            self.inner.push(OsStr::new("."));
-            self.inner.push(new);
+            self.inner.push(".");
+            // SAFETY: Since a UTF-8 string was just pushed, it is not possible
+            // for the buffer to end with a surrogate half.
+            unsafe { self.inner.extend_from_slice_unchecked(new) };
         }
 
         true
@@ -1587,7 +1587,7 @@ impl PathBuf {
             Some(f) => f.as_encoded_bytes(),
         };
 
-        let new = extension;
+        let new = extension.as_encoded_bytes();
         if !new.is_empty() {
             // truncate until right after the file name
             // this is necessary for trimming the trailing slash
@@ -1597,8 +1597,10 @@ impl PathBuf {
 
             // append the new extension
             self.inner.reserve_exact(new.len() + 1);
-            self.inner.push(OsStr::new("."));
-            self.inner.push(new);
+            self.inner.push(".");
+            // SAFETY: Since a UTF-8 string was just pushed, it is not possible
+            // for the buffer to end with a surrogate half.
+            unsafe { self.inner.extend_from_slice_unchecked(new) };
         }
 
         true