Remove the directories dependency (#4904)

eframe now has its own logic to find the storage_dir to persist the app
when the persistence feature is enabled, instead of using the
directories crate. The directory should be the same as before (verified
with a unit test).

* Closes <https://github.com/emilk/egui/issues/4884>
* [x] I have followed the instructions in the PR template

crates/eframe/src/epi.rs

@@ -351,8 +351,8 @@ pub struct NativeOptions {
     /// persisted (only if the "persistence" feature is enabled).
     pub persist_window: bool,
 
-    /// The folder where `eframe` will store the app state. If not set, eframe will get the paths
-    /// from [directories].
+    /// The folder where `eframe` will store the app state. If not set, eframe will use a default
+    /// data storage path for each target system.
     pub persistence_path: Option<std::path::PathBuf>,
 
     /// Controls whether to apply dithering to minimize banding artifacts.

crates/eframe/src/native/file_storage.rs

@@ -10,13 +10,80 @@ use std::{
 /// [`egui::ViewportBuilder::app_id`] of [`crate::NativeOptions::viewport`]
 /// or the title argument to [`crate::run_native`].
 ///
-/// On native the path is picked using [`directories::ProjectDirs::data_dir`](https://docs.rs/directories/5.0.1/directories/struct.ProjectDirs.html#method.data_dir) which is:
+/// On native, the path is:
 /// * Linux:   `/home/UserName/.local/share/APP_ID`
 /// * macOS:   `/Users/UserName/Library/Application Support/APP_ID`
-/// * Windows: `C:\Users\UserName\AppData\Roaming\APP_ID`
+/// * Windows: `C:\Users\UserName\AppData\Roaming\APP_ID\data`
 pub fn storage_dir(app_id: &str) -> Option<PathBuf> {
-    directories::ProjectDirs::from("", "", app_id)
-        .map(|proj_dirs| proj_dirs.data_dir().to_path_buf())
+    use egui::os::OperatingSystem as OS;
+    use std::env::var_os;
+    match OS::from_target_os() {
+        OS::Nix => var_os("XDG_DATA_HOME")
+            .map(PathBuf::from)
+            .filter(|p| p.is_absolute())
+            .or_else(|| home::home_dir().map(|p| p.join(".local").join("share")))
+            .map(|p| {
+                p.join(
+                    app_id
+                        .to_lowercase()
+                        .replace(|c: char| c.is_ascii_whitespace(), ""),
+                )
+            }),
+        OS::Mac => home::home_dir().map(|p| {
+            p.join("Library")
+                .join("Application Support")
+                .join(app_id.replace(|c: char| c.is_ascii_whitespace(), "-"))
+        }),
+        OS::Windows => roaming_appdata().map(|p| p.join(app_id).join("data")),
+        OS::Unknown | OS::Android | OS::IOS => None,
+    }
+}
+
+// Adapted from
+// https://github.com/rust-lang/cargo/blob/6e11c77384989726bb4f412a0e23b59c27222c34/crates/home/src/windows.rs#L19-L37
+#[cfg(all(windows, not(target_vendor = "uwp")))]
+#[allow(unsafe_code)]
+fn roaming_appdata() -> Option<PathBuf> {
+    use std::ffi::OsString;
+    use std::os::windows::ffi::OsStringExt;
+    use std::ptr;
+    use std::slice;
+
+    use windows_sys::Win32::Foundation::S_OK;
+    use windows_sys::Win32::System::Com::CoTaskMemFree;
+    use windows_sys::Win32::UI::Shell::{
+        FOLDERID_RoamingAppData, SHGetKnownFolderPath, KF_FLAG_DONT_VERIFY,
+    };
+
+    extern "C" {
+        fn wcslen(buf: *const u16) -> usize;
+    }
+    unsafe {
+        let mut path = ptr::null_mut();
+        match SHGetKnownFolderPath(
+            &FOLDERID_RoamingAppData,
+            KF_FLAG_DONT_VERIFY as u32,
+            0,
+            &mut path,
+        ) {
+            S_OK => {
+                let path_slice = slice::from_raw_parts(path, wcslen(path));
+                let s = OsString::from_wide(&path_slice);
+                CoTaskMemFree(path.cast());
+                Some(PathBuf::from(s))
+            }
+            _ => {
+                // Free any allocated memory even on failure. A null ptr is a no-op for `CoTaskMemFree`.
+                CoTaskMemFree(path.cast());
+                None
+            }
+        }
+    }
+}
+
+#[cfg(any(not(windows), target_vendor = "uwp"))]
+fn roaming_appdata() -> Option<PathBuf> {
+    None
 }
 
 // ----------------------------------------------------------------------------
@@ -171,3 +238,23 @@ where
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn directories_storage_dir(app_id: &str) -> Option<PathBuf> {
+        directories::ProjectDirs::from("", "", app_id)
+            .map(|proj_dirs| proj_dirs.data_dir().to_path_buf())
+    }
+
+    #[test]
+    fn storage_path_matches_directories() {
+        use super::storage_dir;
+        for app_id in [
+            "MyApp", "My App", "my_app", "my-app", "My.App", "my/app", "my:app", r"my\app",
+        ] {
+            assert_eq!(directories_storage_dir(app_id), storage_dir(app_id));
+        }
+    }
+}