Windows: Remove the \\?\ prefix from paths (#259)

When a path is canonicalized on Windows, a `\\?\` prefix is used.
However, it appears that cmake can have problems with that. This simply
attempts to remove the prefix as we do still want to use absolute path
here due to #200.

Fixes #258

Author: ChrisDenton

src/lib.rs

@@ -1139,7 +1139,29 @@ fn uses_named_pipe_jobserver(makeflags: &OsStr) -> bool {
 /// Attempt to canonicalize; fall back to the original path if unsuccessful, in case `cmake` knows
 /// something we don't.
 fn try_canonicalize(path: &Path) -> PathBuf {
-    path.canonicalize().unwrap_or_else(|_| path.to_owned())
+    let path = path.canonicalize().unwrap_or_else(|_| path.to_owned());
+    // On Windows, attempt to remove the verbatim prefix from the canonicalized path.
+    // FIXME(ChrisDenton): once MSRV is >=1.79 use `std::path::absolute` instead of canonicalize.
+    // That will avoid the need for this hack.
+    #[cfg(windows)]
+    {
+        use std::os::windows::ffi::{OsStrExt, OsStringExt};
+        let mut wide: Vec<u16> = path.as_os_str().encode_wide().collect();
+        if wide.starts_with(&[b'\\' as u16, b'\\' as u16, b'?' as u16, b'\\' as u16]) {
+            if wide.get(5..7) == Some(&[b':' as u16, b'\\' as u16]) {
+                // Convert \\?\C:\ to C:\
+                wide.copy_within(4.., 0);
+                wide.truncate(wide.len() - 4);
+            } else if wide.get(4..8) == Some(&[b'U' as u16, b'N' as u16, b'C' as u16, b'\\' as u16])
+            {
+                // Convert \\?\UNC\ to \\
+                wide.copy_within(8.., 2);
+                wide.truncate(wide.len() - (8 - 2));
+            }
+            return OsString::from_wide(&wide).into();
+        }
+    }
+    path
 }
 
 #[cfg(test)]