[llvm] [Windows][Support] Add helper to expand short 8.3 form paths (PR #178480)

[Ben Dunbobbin via llvm-commits]

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@@ -2485,6 +2486,145 @@ TEST_F(FileSystemTest, widenPath) {
 #endif
 
 #ifdef _WIN32
+/// Checks whether short 8.3 form names are enabled in the given UTF-8 path.
+static llvm::Expected<bool> areShortNamesEnabled(llvm::StringRef Path8) {
+  // Create a directory under Path8 with a name long enough that Windows will
+  // provide a short 8.3 form name, if short 8.3 form names are enabled.
+  SmallString<256> Dir(Path8);
+  path::append(Dir, "verylongdir");
+  if (std::error_code EC = fs::create_directories(Dir))
+    return llvm::errorCodeToError(EC);
+  scope_exit Close([&] { fs::remove_directories(Dir); });
+
+  SmallVector<wchar_t, MAX_PATH> Path16;
+  if (std::error_code EC = sys::windows::widenPath(Dir, Path16))
+    return llvm::errorCodeToError(EC);
+
+  WIN32_FIND_DATAW Data;
+  HANDLE H = ::FindFirstFileW(Path16.data(), &Data);
+  if (H == INVALID_HANDLE_VALUE)
+    return llvm::make_error<llvm::StringError>(
+        "FindFirstFileW returned invalid handle",
+        llvm::inconvertibleErrorCode());
+  ::FindClose(H);
+
+  return (Data.cAlternateFileName[0] != L'\0');
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bd1976bris wrote:

 > While looking again at the `FindFirstFileW` documentation, I did notice that the string must be null-terminated. I modified the code to ensure that.

Apologies for the noise. `widenPath` does guarantee null termination. I have reverted the code back to as it was when approved.

https://github.com/llvm/llvm-project/pull/178480