rustc: Fixup verbatim UNC paths as well on Windows

src/librustc/util/fs.rs

@@ -11,14 +11,22 @@
 use std::path::{self, Path, PathBuf};
 use std::ffi::OsString;
 
-// Unfortunately, on windows, gcc cannot accept paths of the form `\\?\C:\...`
-// (a verbatim path). This form of path is generally pretty rare, but the
-// implementation of `fs::canonicalize` currently generates paths of this form,
-// meaning that we're going to be passing quite a few of these down to gcc.
+// Unfortunately, on windows, it looks like msvcrt.dll is silently translating
+// verbatim paths under the hood to non-verbatim paths! This manifests itself as
+// gcc looking like it cannot accept paths of the form `\\?\C:\...`, but the
+// real bug seems to lie in msvcrt.dll.
+//
+// Verbatim paths are generally pretty rare, but the implementation of
+// `fs::canonicalize` currently generates paths of this form, meaning that we're
+// going to be passing quite a few of these down to gcc, so we need to deal with
+// this case.
 //
 // For now we just strip the "verbatim prefix" of `\\?\` from the path. This
 // will probably lose information in some cases, but there's not a whole lot
-// more we can do with a buggy gcc...
+// more we can do with a buggy msvcrt...
+//
+// For some more information, see this comment:
+//   https://github.com/rust-lang/rust/issues/25505#issuecomment-102876737
 pub fn fix_windows_verbatim_for_gcc(p: &Path) -> PathBuf {
     if !cfg!(windows) {
         return p.to_path_buf()
@@ -28,11 +36,20 @@ pub fn fix_windows_verbatim_for_gcc(p: &Path) -> PathBuf {
         Some(path::Component::Prefix(p)) => p,
         _ => return p.to_path_buf(),
     };
-    let disk = match prefix.kind() {
-        path::Prefix::VerbatimDisk(disk) => disk,
-        _ => return p.to_path_buf(),
-    };
-    let mut base = OsString::from(format!("{}:", disk as char));
-    base.push(components.as_path());
-    PathBuf::from(base)
+    match prefix.kind() {
+        path::Prefix::VerbatimDisk(disk) => {
+            let mut base = OsString::from(format!("{}:", disk as char));
+            base.push(components.as_path());
+            PathBuf::from(base)
+        }
+        path::Prefix::VerbatimUNC(server, share) => {
+            let mut base = OsString::from(r"\\");
+            base.push(server);
+            base.push(r"\");
+            base.push(share);
+            base.push(components.as_path());
+            PathBuf::from(base)
+        }
+        _ => p.to_path_buf(),
+    }
 }