Crash when using \\?\ style path on Windows

[philip-peterson]

Problem
Running a command, e.g. cargo metadata or cargo check with the --manifest-path set to equal an absolute Windows long-style path causes an error:

The filename, directory name, or volume label syntax is incorrect. (os error 123)

Steps

1. Create a project anywhere and note its path. Let's pretend it's C:\Path
2. Run a cargo command with mainfest-path set to that path as a long absolute path: cargo metadata --manifest-path \\?\C:\Path\Cargo.toml
3. Note the failure.

Notes

Output of cargo version: cargo 1.47.0-nightly (ab32ee8 2020-08-10)

May be related to #7986, #7420, #7643

[Eh2406]

For some related discussion see #6198. As I understand is that the operations on \\?\ as supposed to be pretty limited. Some apis are strict to only to the "allowed" things, some try to do something useful even if it is not technically "allowed". This leads to massively inconsistent apis everywhere including in STD. Bugs can be filed for each inconsistency, but each will be a long discussion from the competing goals. So fixing all the oddities even just in Cargo is going to be a herculean task. If you are volunteering, more power to you!

This case may only have a handful of places where Cargo interacts with that path. So getting --manifest-path to work may be doable.