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(),
+    }
 }

src/config.rs

@@ -1,8 +1,10 @@
 use std::{
+    borrow::Cow,
     net::{IpAddr, Ipv4Addr},
-    path::Path,
+    path::{Path, PathBuf},
 };
 
+use once_cell::sync::Lazy;
 use ruma::{OwnedServerName, RoomVersionId};
 use serde::Deserialize;
 
@@ -14,6 +16,10 @@ mod proxy;
 use env_filter_clone::EnvFilterClone;
 use proxy::ProxyConfig;
 
+/// The default configuration file path
+pub(crate) static DEFAULT_PATH: Lazy<PathBuf> =
+    Lazy::new(|| [env!("CARGO_PKG_NAME"), "config.toml"].iter().collect());
+
 #[allow(clippy::struct_excessive_bools)]
 #[derive(Debug, Deserialize)]
 pub(crate) struct Config {
@@ -163,13 +169,29 @@ pub(crate) fn default_default_room_version() -> RoomVersionId {
     RoomVersionId::V10
 }
 
-/// Load the configuration from the given path
-pub(crate) async fn load<P>(path: P) -> Result<Config, error::Config>
+/// Search default locations for a configuration file
+///
+/// If one isn't found, the list of tried paths is returned.
+fn search() -> Result<PathBuf, error::ConfigSearch> {
+    use error::ConfigSearch as Error;
+
+    xdg::BaseDirectories::new()?
+        .find_config_file(&*DEFAULT_PATH)
+        .ok_or(Error::NotFound)
+}
+
+/// Load the configuration from the given path or XDG Base Directories
+pub(crate) async fn load<P>(path: Option<P>) -> Result<Config, error::Config>
 where
     P: AsRef<Path>,
 {
     use error::Config as Error;
 
+    let path = match path.as_ref().map(AsRef::as_ref) {
+        Some(x) => Cow::Borrowed(x),
+        None => Cow::Owned(search()?),
+    };
+
     let path = path.as_ref();
 
     toml::from_str(

src/error.rs

@@ -75,9 +75,25 @@ pub(crate) enum Observability {
 #[allow(missing_docs)]
 #[derive(Error, Debug)]
 pub(crate) enum Config {
+    #[error("failed to find configuration file")]
+    Search(#[from] ConfigSearch),
+
     #[error("failed to read configuration file {1:?}")]
     Read(#[source] std::io::Error, PathBuf),
 
     #[error("failed to parse configuration file {1:?}")]
     Parse(#[source] toml::de::Error, PathBuf),
 }
+
+/// Errors that can occur while searching for a config file
+// Missing docs are allowed here since that kind of information should be
+// encoded in the error messages themselves anyway.
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub(crate) enum ConfigSearch {
+    #[error("XDG Base Directory error")]
+    Xdg(#[from] xdg::BaseDirectoriesError),
+
+    #[error("no relevant configuration files found in XDG Base Directories")]
+    NotFound,
+}

src/main.rs

@@ -106,7 +106,7 @@ async fn try_main() -> Result<(), error::Main> {
 
     let args = args::parse();
 
-    let config = config::load(&args.config).await?;
+    let config = config::load(args.config.as_ref()).await?;
 
     let _guard = observability::init(&config);