Adding diagnostic item markers for multiple fs functions and structs

[asder8215]

A couple months ago there were a good number of TOCTOU/other filesystem lint issues created by @estebank in the clippy repo such as:

• Path::metdata after Path::exists
• opening multiple files under a directory without using open_at
• File deletion followed by file creation
• File path comparison without canonicalizing
• File creation followed by setting permissions

(There are more TOCTOU/filesystem bug lints that could be made aside from the list above, e.g. with symlinks).

I was particularly interested in working on the last issue on file creation followed by setting permissions. However, I don't think I could start working on it without diagnostic items on filesystem functions like fs::set_permissions or fs::create_dir_all. I decided to put diagnostic item attributes on all the filesystem functions and a couple of the structs because they may be useful in creating clippy lints against TOCTOU bugs or other relevant filesystem operation bugs.

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[clarfonthey]

Sorry for taking a bit to get to this, but this looks fine to me. I assume that the cfg_attr(not(test), …) is because in those cases, std is doubled (compiler std + currently-tested `std) and it messes things up?

[asder8215]

I assume that the cfg_attr(not(test), …) is because in those cases, std is doubled (compiler std + currently-tested `std) and it messes things up?

I'm not sure why the cfg_attr(not(test), ...), I just took this off how it's done for other filesystem functions/structs (e.g. create_dir is marked with #[cfg_attr(not(test), rustc_diagnostic_item = "fs_create_dir")]). I figured that was the pattern we should follow with adding diagnostic items to fs functions/structs.