Proposal: Add LongPathAware support to cswinrt.exe

Summary

Make cswinrt.exe long-path aware so it can generate projections when output paths exceed the legacy Windows MAX_PATH limit. Also report file-write failures with a nonzero exit code and actionable diagnostic instead of silently succeeding.

Rationale

• With Windows LongPathsEnabled=1, cswinrt.exe can still fail to emit generated C# files because the executable does not declare <longPathAware>true</longPathAware> in its application manifest.

• The failure is currently silent: cswinrt.exe may return exit code 0 with no diagnostic even though the expected projection was not written. Consumers then encounter misleading downstream compiler errors.

• A deterministic reproduction uses a 245-character output directory, resulting in a 279-character generated projection path. Current behavior is exit code 0, empty stderr, and a missing projection.

• CsWinRT should opt into Windows long-path handling, preserve native filesystem paths during file I/O, and validate stream open/write results.

• Expected behavior: generate the projection successfully on a long-path-enabled system, or return a nonzero exit code with a clear output-file diagnostic if writing fails.

Important Notes

Standalone reproduction

Run this with the affected cswinrt.exe:

$exe = "C:\path\to\cswinrt.exe"
$out = Join-Path $env:TEMP "cswinrt-long-path-repro"
Remove-Item -LiteralPath $out -Recurse -Force -ErrorAction SilentlyContinue

while ($out.Length -lt 240) {
    $out = Join-Path $out "segment123456789"
}

[IO.Directory]::CreateDirectory($out) | Out-Null
$file = Join-Path $out "Windows.Foundation.Collections.cs"

$result = & $exe -input sdk -output $out `
    -include Windows.Foundation.Collections 2>&1
$exitCode = $LASTEXITCODE

"Output directory length: $($out.Length)"
"Projection path length: $($file.Length)"
"Exit code: $exitCode"
"Projection exists: $([IO.File]::Exists($file))"
"Output: $result"

Observed with the affected executable:

Projection path length: greater than 260
Exit code: 0
Projection exists: False
Output:

The projection is silently omitted despite the successful exit code and absence of diagnostics.

LongPathsEnabled=1 is already enabled on the affected system. Windows additionally requires the executable to opt in using:

<longPathAware>true</longPathAware>

The implementation should also:

• Preserve std::filesystem::path through file operations rather than converting it to a narrow string.
• Check file-stream open and write results.
• Return a nonzero exit code with the affected path when output generation fails.

Open Questions

• This proposal primarily covers generated output paths. Should metadata input paths be audited in a separate issue?

• Which Windows versions and CI environments should exercise the long-path test?

• Should CsWinRT detect and explain when the system-wide LongPathsEnabled policy is disabled?

Start with the cswinrt.exe output-generation entry point and run the provided PowerShell reproduction on a Windows system with long paths enabled. Trace the generated output path through file operations, then verify that long-path generation succeeds or that failures return a nonzero exit code with an actionable path diagnostic.