We rely on an undocumented property of GetUserProfileDirectoryW

[RalfJung]

The docs say that the size is written on failure, but in practice it is also written on success and we rely on that. See #141244 and rust-lang/miri#4332.

We shouldn't use system APIs outside their documented contract, so we should either adjust our implementation or get Microsoft to update the docs to promise this behavior we have been relying on.

Cc @ChrisDenton

[workingjubilee]

see MicrosoftDocs/sdk-api#1810

[ChrisDenton]

Wait, I recognise that name.

I'd completely forgot about writing that.

[cgettys-microsoft]

see MicrosoftDocs/sdk-api#1810
This has been merged. I think this issue can be closed based on that being merged - Ralf's PR seems a reasonable enough way to close it though.