fs::copy on Windows has a 260 character filename limit

[jdm]

It relies on the CopyFileExW Windows API, which has path limits that yield surprising runtime behaviour. We could opt in to longer path limits (32k instead of 260) by prepending \? to the string, according to that same documentation.

[retep998]

CopyFileExW isn't special in this regard. All Rust filesystem code on Windows currently bumps into this limitation, and it's not just as simple as prepending \\?\.

[jdm]

One option might be to document this so it's a little bit less inexplicable to people who don't know the platform details.

[andrewbanchich]

@retep998 The official Microsoft docs suggest doing what @jdm mentioned. The only exception I'm aware of is if it's a UNC then you need to use \\?\UNC\.

What kind of issues would this not address?

[ChrisDenton]

Win32 has four types of absolute path:

Type	Win32 Example
DOS	C:\Windows
UNC	\\server\share\
Device	\\.\PIPE\name
Verbatim	\\?\C:\Windows \\?\UNC\server\share\ \\?\PIPE\name

It also has three types of relative path:

Type	Win32 Example
Path Relative	file.ext .\file.ext ..\file.ext
Root Relative	\file.ext
Drive Relative	D:file.ext

Rust's file system APIs would at least need to be internally consistent with how they handle all these paths.

A smarter std::path::Path could resolve this issue by having a function to make the path both absolute and verbatim. Or another option is to have an internal API function that automatically gets called before almost* every use of a Windows filesystem API which would do the same thing when necessary.

* I say "almost" because setting the Current Directory does not support verbatim paths. Only DOS and UNC paths are supported.

[ChrisDenton]

This should hopefully be fixed in nightly by #89174, which is currently scheduled for the 1.58.0 release.