Test if reserved filenames are allowed in Windows

crates/cargo-test-support/Cargo.toml

@@ -25,5 +25,8 @@ termcolor = "1.1.2"
 toml_edit = { version = "0.14.3", features = ["serde", "easy", "perf"] }
 url = "2.2.2"
 
+[target.'cfg(windows)'.dependencies]
+winapi = "0.3"
+
 [features]
 deny-warnings = []

crates/cargo-test-support/src/paths.rs

@@ -287,3 +287,61 @@ pub fn sysroot() -> String {
     let sysroot = String::from_utf8(output.stdout).unwrap();
     sysroot.trim().to_string()
 }
+
+/// Returns true if names such as aux.* are allowed.
+///
+/// Traditionally, Windows did not allow a set of file names (see `is_windows_reserved`
+/// for a list). More recent versions of Windows have relaxed this restriction. This test
+/// determines whether we are running in a mode that allows Windows reserved names.
+#[cfg(windows)]
+pub fn windows_reserved_names_are_allowed() -> bool {
+    use cargo_util::is_ci;
+
+    // Ensure tests still run in CI until we need to migrate.
+    if is_ci() {
+        return false;
+    }
+
+    use std::ffi::OsStr;
+    use std::os::windows::ffi::OsStrExt;
+    use std::ptr;
+    use winapi::um::fileapi::GetFullPathNameW;
+
+    let test_file_name: Vec<_> = OsStr::new("aux.rs").encode_wide().collect();
+
+    let buffer_length =
+        unsafe { GetFullPathNameW(test_file_name.as_ptr(), 0, ptr::null_mut(), ptr::null_mut()) };
+
+    if buffer_length == 0 {
+        // This means the call failed, so we'll conservatively assume reserved names are not allowed.
+        return false;
+    }
+
+    let mut buffer = vec![0u16; buffer_length as usize];
+
+    let result = unsafe {
+        GetFullPathNameW(
+            test_file_name.as_ptr(),
+            buffer_length,
+            buffer.as_mut_ptr(),
+            ptr::null_mut(),
+        )
+    };
+
+    if result == 0 {
+        // Once again, conservatively assume reserved names are not allowed if the
+        // GetFullPathNameW call failed.
+        return false;
+    }
+
+    // Under the old rules, a file name like aux.rs would get converted into \\.\aux, so
+    // we detect this case by checking if the string starts with \\.\
+    //
+    // Otherwise, the filename will be something like C:\Users\Foo\Documents\aux.rs
+    let prefix: Vec<_> = OsStr::new("\\\\.\\").encode_wide().collect();
+    if buffer.starts_with(&prefix) {
+        false
+    } else {
+        true
+    }
+}

tests/testsuite/package.rs

@@ -2009,6 +2009,12 @@ src/lib.rs
 #[cargo_test]
 #[cfg(windows)]
 fn reserved_windows_name() {
+    // If we are running on a version of Windows that allows these reserved filenames,
+    // skip this test.
+    if paths::windows_reserved_names_are_allowed() {
+        return;
+    }
+
     Package::new("bar", "1.0.0")
         .file("src/lib.rs", "pub mod aux;")
         .file("src/aux.rs", "")