Auto merge of #1471 - RalfJung:win, r=RalfJung

on Windows, strip the '\\?\' prefix from the canonical path

This should help with #74146 (comment)

Author: bors

cargo-miri/bin.rs

@@ -258,8 +258,20 @@ fn setup(subcommand: MiriCommand) {
     // Determine where the rust sources are located.  `XARGO_RUST_SRC` env var trumps everything.
     let rust_src = match std::env::var_os("XARGO_RUST_SRC") {
         Some(val) => {
-            let val = PathBuf::from(val);
-            val.canonicalize().unwrap_or(val)
+            let path = PathBuf::from(val);
+            let path = path.canonicalize().unwrap_or(path);
+
+            // On Windows, this produces a path starting with `\\?\`, which xargo cannot deal with.
+            // Strip that prefix; the resulting path should still be valid.
+            #[cfg(windows)]
+            let path = {
+                let str = path.into_os_string().into_string()
+                    .expect("non-unicode paths are currently not supported");
+                let str = str.strip_prefix(r"\\?\").map(String::from).unwrap_or(str);
+                PathBuf::from(str)
+            };
+
+            path
         }
         None => {
             // Check for `rust-src` rustup component.