Update from code review

Author: ChrisDenton

Diff for: library/std/src/sys/path/windows.rs

@@ -386,6 +386,7 @@ pub(crate) fn is_absolute_exact(path: &[u16]) -> bool {
     if result == 0 || result as usize != buffer_len - 1 {
         false
     } else {
+        // SAFETY: `GetFullPathNameW` initialized `result` bytes and does not exceed `nBufferLength - 1` (capacity).
         unsafe {
             new_path.set_len((result as usize) + 1);
         }

Diff for: library/std/src/sys/process/windows.rs

@@ -889,7 +889,7 @@ fn make_dirp(d: Option<&OsString>) -> io::Result<(*const u16, Vec<u16>)> {
                 let start = r"\\?\UN".len();
                 dir_str[start] = b'\\' as u16;
                 if path::is_absolute_exact(&dir_str[start..]) {
-                    unsafe { dir_str.as_ptr().add(start) }
+                    dir_str[start..].as_ptr()
                 } else {
                     // Revert the above change.
                     dir_str[start] = b'C' as u16;
@@ -899,7 +899,7 @@ fn make_dirp(d: Option<&OsString>) -> io::Result<(*const u16, Vec<u16>)> {
                 // Strip the leading `\\?\`
                 let start = r"\\?\".len();
                 if path::is_absolute_exact(&dir_str[start..]) {
-                    unsafe { dir_str.as_ptr().add(start) }
+                    dir_str[start..].as_ptr()
                 } else {
                     dir_str.as_ptr()
                 }