cli: migrate --config-toml processing, make UserSettings immutable

It's unclear whether parse_args() or its caller should update LayeredConfigs.
--config-toml is processed by callee to apply --color early. -R/--repository
will be processed by caller since it will instantiate WorkspaceLoader.
Maybe --config-toml can be removed from EarlyArgs, and handle_early_args()
just updates ui state based on --color argument?

lib/src/settings.rs

@@ -61,25 +61,6 @@ impl UserSettings {
         }
     }
 
-    pub fn incorporate_toml_strings(
-        &mut self,
-        toml_strs: &[String],
-    ) -> Result<(), config::ConfigError> {
-        let mut config_builder = config::Config::builder().add_source(self.config.clone());
-        for s in toml_strs {
-            config_builder =
-                config_builder.add_source(config::File::from_str(s, config::FileFormat::Toml));
-        }
-        let new_config = config_builder.build()?;
-        let new_rng_seed = get_rng_seed_config(&new_config);
-        if new_rng_seed != get_rng_seed_config(&self.config) {
-            self.rng.reset(new_rng_seed);
-        }
-        self.timestamp = get_timestamp_config(&new_config, "debug.commit-timestamp");
-        self.config = new_config;
-        Ok(())
-    }
-
     pub fn with_repo(&self, repo_path: &Path) -> Result<RepoSettings, config::ConfigError> {
         let config = config::Config::builder()
             .add_source(self.config.clone())
@@ -204,11 +185,6 @@ impl JJRng {
         Self(Mutex::new(JJRng::internal_rng_from_seed(seed)))
     }
 
-    fn reset(&self, seed: Option<u64>) {
-        let mut rng = self.0.lock().unwrap();
-        *rng = JJRng::internal_rng_from_seed(seed)
-    }
-
     fn internal_rng_from_seed(seed: Option<u64>) -> ChaCha20Rng {
         match seed {
             Some(seed) => ChaCha20Rng::seed_from_u64(seed),

src/cli_util.rs

@@ -1607,7 +1607,7 @@ fn handle_early_args(
     ui: &mut Ui,
     app: &clap::Command,
     args: &[String],
-    settings: &mut UserSettings,
+    layered_configs: &mut LayeredConfigs,
 ) -> Result<(), CommandError> {
     // ignore_errors() bypasses errors like "--help" or missing subcommand
     let early_matches = app.clone().ignore_errors(true).get_matches_from(args);
@@ -1620,8 +1620,8 @@ fn handle_early_args(
         ui.set_pagination(crate::ui::PaginationChoice::No);
     }
     if !args.config_toml.is_empty() {
-        settings.incorporate_toml_strings(&args.config_toml)?;
+        layered_configs.parse_config_args(&args.config_toml)?;
-        ui.reset(settings.config());
+        ui.reset(&layered_configs.merge());
     }
     Ok(())
 }
@@ -1648,11 +1648,11 @@ pub fn parse_args(
     app: &clap::Command,
     tracing_subscription: &TracingSubscription,
     string_args: &[String],
-    settings: &mut UserSettings,
+    layered_configs: &mut LayeredConfigs,
 ) -> Result<(ArgMatches, Args), CommandError> {
     // TODO: read user configs from the repo pointed to by -R.
 
-    handle_early_args(ui, app, string_args, settings)?;
+    handle_early_args(ui, app, string_args, layered_configs)?;
     let matches = app.clone().try_get_matches_from(string_args)?;
 
     let args: Args = Args::from_arg_matches(&matches).unwrap();
@@ -1785,15 +1785,16 @@ impl CliRunner {
         let config = layered_configs.merge();
         ui.reset(&config);
         let string_args = expand_args(&self.app, std::env::args_os(), &config)?;
-        let mut settings = UserSettings::from_config(config);
         let (matches, args) = parse_args(
             ui,
             &self.app,
             &self.tracing_subscription,
             &string_args,
-            &mut settings,
+            &mut layered_configs,
         )?;
-        // TODO: maybe instantiate UserSettings here
+
+        let config = layered_configs.merge();
+        let settings = UserSettings::from_config(config);
         let command_helper = CommandHelper::new(
             self.app,
             cwd,

src/config.rs

@@ -36,13 +36,14 @@ pub enum ConfigError {
 /// 4. TODO: Repo config `.jj/repo/config.toml`
 /// 5. TODO: Workspace config `.jj/config.toml`
 /// 6. Override environment variables
-/// 7. TODO: Command-line arguments `--config-toml`
+/// 7. Command-line arguments `--config-toml`
 #[derive(Clone, Debug)]
 pub struct LayeredConfigs {
     default: config::Config,
     env_base: config::Config,
     user: Option<config::Config>,
     env_overrides: config::Config,
+    arg_overrides: Option<config::Config>,
 }
 
 impl LayeredConfigs {
@@ -53,6 +54,7 @@ impl LayeredConfigs {
             env_base: env_base(),
             user: None,
             env_overrides: env_overrides(),
+            arg_overrides: None,
         }
     }
 
@@ -63,6 +65,17 @@ impl LayeredConfigs {
         Ok(())
     }
 
+    pub fn parse_config_args(&mut self, toml_strs: &[String]) -> Result<(), ConfigError> {
+        let config = toml_strs
+            .iter()
+            .fold(config::Config::builder(), |builder, s| {
+                builder.add_source(config::File::from_str(s, config::FileFormat::Toml))
+            })
+            .build()?;
+        self.arg_overrides = Some(config);
+        Ok(())
+    }
+
     /// Creates new merged config.
     pub fn merge(&self) -> config::Config {
         let config_sources = [
@@ -70,6 +83,7 @@ impl LayeredConfigs {
             Some(&self.env_base),
             self.user.as_ref(),
             Some(&self.env_overrides),
+            self.arg_overrides.as_ref(),
         ];
         config_sources
             .into_iter()