require config path via cli, remove env var

nix/modules/default/default.nix

@@ -85,17 +85,12 @@ in
     systemd.services.grapevine = {
       description = "Grapevine (Matrix homeserver)";
       wantedBy = [ "multi-user.target" ];
-      environment = lib.mkMerge [
-        {
-          GRAPEVINE_CONFIG = configFile;
-        }
-        cfg.extraEnvironment
-      ];
+      environment = cfg.extraEnvironment;
 
       # Keep sorted
       serviceConfig = {
         DynamicUser = true;
-        ExecStart = "${lib.getExe cfg.package}";
+        ExecStart = "${lib.getExe cfg.package} --config ${configFile}";
         LockPersonality = true;
         MemoryDenyWriteExecute = true;
         PrivateDevices = true;

src/args.rs

@@ -1,11 +1,17 @@
 //! Integration with `clap`
 
+use std::path::PathBuf;
+
 use clap::Parser;
 
 /// Command line arguments
 #[derive(Parser)]
 #[clap(about, version = crate::version())]
-pub(crate) struct Args;
+pub(crate) struct Args {
+    /// Path to the configuration file
+    #[clap(long, short)]
+    pub(crate) config: PathBuf,
+}
 
 /// Parse command line arguments into structured data
 pub(crate) fn parse() -> Args {

src/error.rs

@@ -38,13 +38,6 @@ impl fmt::Display for DisplayWithSources<'_> {
 #[allow(missing_docs)]
 #[derive(Error, Debug)]
 pub(crate) enum Main {
-    #[error(
-        "the `{0}` environment variable must either be set to a configuration \
-         file path or set to an empty string to force configuration through \
-         environment variables"
-    )]
-    ConfigPathUnset(&'static str),
-
     #[error("invalid configuration")]
     ConfigInvalid(#[from] figment::Error),
 

src/main.rs

@@ -108,17 +108,11 @@ async fn main() -> ExitCode {
 async fn try_main() -> Result<(), error::Main> {
     use error::Main as Error;
 
-    args::parse();
+    let args = args::parse();
 
     // Initialize config
     let raw_config = Figment::new()
-        .merge(
-            Toml::file({
-                let name = "GRAPEVINE_CONFIG";
-                Env::var(name).ok_or(Error::ConfigPathUnset(name))?
-            })
-            .nested(),
-        )
+        .merge(Toml::file(&args.config).nested())
         .merge(Env::prefixed("GRAPEVINE_").global());
 
     let config = raw_config.extract::<Config>()?;