config and cache now check XDG env vars first

`XDG_CACHE_HOME` and `XDG_CONFIG_HOME` are checked first and fall-back
to `~/.cache/` and `~/.config/`.

src/dirs.rs

@@ -1,7 +1,11 @@
-use dirs_rs;
+use dirs_rs::home_dir;
+use std::env;
+use std::ffi::OsString;
 use std::path::{Path, PathBuf};
 
-/// Wrapper for dirs that uses `~/.config/bat` for MacOS.
+/// Wrapper for dirs that treats MacOS more like Linux.
+/// First, env variables `XDG_CACHE_HOME` and `XDG_CONFIG_HOME` are checked and the fall back is
+/// `~/.cache/bat` and `~/.config/bat`.
 pub struct BatProjectDirs {
     cache_dir: PathBuf,
     config_dir: PathBuf,
@@ -10,32 +14,28 @@ pub struct BatProjectDirs {
 impl BatProjectDirs {
     fn new() -> Option<BatProjectDirs> {
         #[cfg(target_os = "macos")]
-        let cache_dir = match dirs_rs::home_dir() {
-            Some(mut d) => {
-                d.push(".cache/bat");
-                d
-            }
-            None => return None,
-        };
+        let cache_dir_op = env::var_os("XDG_CACHE_HOME")
+            .and_then(is_absolute_path)
+            .or_else(|| home_dir().map(|d| d.join(".cache")));
 
         #[cfg(not(target_os = "macos"))]
-        let cache_dir = match dirs_rs::cache_dir() {
-            Some(d) => d,
+        let cache_dir_op = dirs_rs::cache_dir();
+
+        let cache_dir = match cache_dir_op {
+            Some(d) => d.join("bat"),
             None => return None,
         };
 
         #[cfg(target_os = "macos")]
-        let config_dir = match dirs_rs::home_dir() {
-            Some(mut d) => {
-                d.push(".config/bat");
-                d
-            }
-            None => return None,
-        };
+        let config_dir_op = env::var_os("XDG_CONFIG_HOME")
+            .and_then(is_absolute_path)
+            .or_else(|| home_dir().map(|d| d.join(".config")));
 
         #[cfg(not(target_os = "macos"))]
-        let config_dir = match dirs_rs::config_dir() {
-            Some(d) => d,
+        let config_dir_op = dirs_rs::config_dir();
+
+        let config_dir = match config_dir_op {
+            Some(d) => d.join("bat"),
             None => return None,
         };
 
@@ -54,6 +54,16 @@ impl BatProjectDirs {
     }
 }
 
+// Returns path if it is an absolute path
+fn is_absolute_path(path: OsString) -> Option<PathBuf> {
+    let path = PathBuf::from(path);
+    if path.is_absolute() {
+        Some(path)
+    } else {
+        None
+    }
+}
+
 lazy_static! {
     pub static ref PROJECT_DIRS: BatProjectDirs =
         BatProjectDirs::new().expect("Could not get home directory");