Fixup Windows verbatim paths when used with the include! macro

[ChrisDenton]

On Windows, the following code can fail if the OUT_DIR environment variable is a verbatim path (i.e. begins with \\?\):

include!(concat!(env!("OUT_DIR"), "/src/repro.rs"));

This is because verbatim paths treat / literally, as if it were just another character in the file name.

The good news is that the standard library already has code to fix this. We can simply use components to normalize the path so it works as intended.

[rustbot]

r? @jieyouxu

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[rustbot]

Some changes occurred in run-make tests.

cc @jieyouxu

[ChrisDenton]

While I'd very much welcome a review from jieyouxu, this will need lang approval. I think it could just be considered a bug fix but it might also be considered a change in language.

cc @joshtriplett maybe?

[Nilstrieb]

Given that I see most projects already using / as the separator, calling it a bug fix seems fair. Still, I agree that lang should see it.

[jieyouxu]

Changes look reasonable from compiler side. Rolling a lang reviewer to decide if this behavior change is acceptable.

r? lang