follow xdg base dirs spec by default

src/args.rs

@@ -2,7 +2,7 @@
 
 use std::path::PathBuf;
 
-use clap::Parser;
+use clap::{CommandFactory as _, FromArgMatches as _, Parser};
 
 /// Command line arguments
 #[derive(Parser)]
@@ -10,10 +10,26 @@ use clap::Parser;
 pub(crate) struct Args {
     /// Path to the configuration file
     #[clap(long, short)]
-    pub(crate) config: PathBuf,
+    pub(crate) config: Option<PathBuf>,
 }
 
 /// Parse command line arguments into structured data
 pub(crate) fn parse() -> Args {
-    Args::parse()
+    let mut command = Args::command().mut_arg("config", |x| {
+        let help = "Set the path to the configuration file";
+        x.help(help).long_help(format!(
+            "{}\n\nIf this option is specified, the provided value is used \
+             as-is.\n\nIf this option is not specified, then the XDG Base \
+             Directory Specification is followed, searching for the path `{}` \
+             in the configuration directories.
+            ",
+            help,
+            crate::config::DEFAULT_PATH.display(),
+        ))
+    });
+
+    match Args::from_arg_matches(&command.get_matches_mut()) {
+        Ok(x) => x,
+        Err(e) => e.format(&mut command).exit(),
+    }
 }