Add a function to access application data directory

[thelink2012]

Currently there is only a way to easily store application specific data using the standard library, std::env::home_dir. That's not a quite correct way. I was thinking about the addition of a std::env::appdata_dir function?

The home directory is only suitable for storing application data on Linux (and perhaps other *nixes?), on Windows the correct place is %AppData% or %LocalAppData% and on OSX it seems to be ~/Library/Preferences/ or ~/Library/Application Support/.

That brought my attention when I noticed that Cargo stores it's data on %USERPROFILE%/.cargo/ (std::env::home_dir gives%USERPROFILE on Windows), note how directories starting with . aren't hidden on Windows and those pollute the user profile directory.

However I have a few concerns about this myself:

• Not sure whether std::env should have functions that aren't directly a enviroment variable (e.g. ~/Library/...) though it says and various other important directories on it's description.
• Storing application data might be more complicated than that (especially on OSes not mentioned here) and it'd thus be better to have this kind of feature as a separate user crate.

[Stebalien]

We'd probably need to prototype this in an external crate first. Doing this in a cross-platform manner will be very difficult.

Linux (the XDG spec actually) defines:

• $XDG_CACHE_HOME: cache
• $XDG_DATA_HOME: application "data"
• $XDG_CONFIG_HOME: application "config"
• $XDG_RUNTIME_DIR: session-specific temporary files (sockets etc...)

Windows defines:

• %AppData: Any cache, data, config that can be safely synced between computers.
• %LocalAppData: Any cache, data, config that can't be safely synced between computers.

I have no idea what OSX uses but to cover Windows and Linux alone, we'd need we'd probably want at least the following functions (with better names):

• cache_dir (always local)
• local_data_dir
• local_config_dir
• data_dir
• config_dir

Even then, this wouldn't support $XDG_RUNTIME_DIR because there is no equivalent under Windows.

(I'm not saying we shouldn't try to do this, just that this is a really hard (if not impossible) problem.)

[Valloric]

Agreed that this should first go into an external crate to percolate for a while. After we're pretty damn sure that all the paths make sense, then move it into std.

[ben0x539]

Maybe we can steal the glib design of these sort of functions since they presumably have some amount of portability nailed down.

[Stebalien]

@ben0x539 not really. Those methods don't distinguish between roaming versus local. Also, their user cache dir for windows is out of date; Microsoft now suggests using per-application cache directories.

[SamWhited]

Excluding the runtime dir, this Python module has a fairly complete list of where things should be on each platform and is useful for reference: https://github.com/ActiveState/appdirs/blob/master/appdirs.py

[corngood]

I was also looking for this, and came across https://docs.rs/app_dirs/. It's the best example of an existing crate that I've found so far.