cli: introduce struct that stores unmerged configs

Thinking of config precedence, we'll probably need to store "base" (default +
env_base + user) config and "override" (env_overrides + --config-toml) config
separately to support repo config. Suppose env_overrides() is a temporary
value, it's better to override any file-derived values with $JJ_ env.

src/cli_util.rs

@@ -52,6 +52,7 @@ use jujutsu_lib::{dag_walk, file_util, git, revset};
 use thiserror::Error;
 use tracing_subscriber::prelude::*;
 
+use crate::config::LayeredConfigs;
 use crate::formatter::Formatter;
 use crate::merge_tools::{ConflictResolveError, DiffEditError};
 use crate::templater::TemplateFormatter;
@@ -1780,9 +1781,12 @@ impl CliRunner {
 
     pub fn run(self, ui: &mut Ui) -> Result<(), CommandError> {
         let cwd = env::current_dir().unwrap(); // TODO: maybe map_err to CommandError?
-        let mut settings = crate::config::read_config()?;
-        ui.reset(settings.config());
-        let string_args = expand_args(&self.app, std::env::args_os(), settings.config())?;
+        let mut layered_configs = LayeredConfigs::from_environment();
+        layered_configs.read_user_config()?;
+        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,

src/config.rs

@@ -17,7 +17,6 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::{env, fmt};
 
-use jujutsu_lib::settings::UserSettings;
 use thiserror::Error;
 
 #[derive(Error, Debug)]
@@ -28,6 +27,61 @@ pub enum ConfigError {
     AmbiguousSource(PathBuf, PathBuf),
 }
 
+/// Set of configs which can be merged as needed.
+///
+/// Sources from the lowest precedence:
+/// 1. Default
+/// 2. Base environment variables
+/// 3. User config `~/.jjconfig.toml` or `$JJ_CONFIG`
+/// 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`
+#[derive(Clone, Debug)]
+pub struct LayeredConfigs {
+    default: config::Config,
+    env_base: config::Config,
+    user: Option<config::Config>,
+    env_overrides: config::Config,
+}
+
+impl LayeredConfigs {
+    /// Initializes configs with infallible sources.
+    pub fn from_environment() -> Self {
+        LayeredConfigs {
+            default: default_config(),
+            env_base: env_base(),
+            user: None,
+            env_overrides: env_overrides(),
+        }
+    }
+
+    pub fn read_user_config(&mut self) -> Result<(), ConfigError> {
+        self.user = config_path()?
+            .map(|path| read_config_path(&path))
+            .transpose()?;
+        Ok(())
+    }
+
+    /// Creates new merged config.
+    pub fn merge(&self) -> config::Config {
+        let config_sources = [
+            Some(&self.default),
+            Some(&self.env_base),
+            self.user.as_ref(),
+            Some(&self.env_overrides),
+        ];
+        config_sources
+            .into_iter()
+            .flatten()
+            .fold(config::Config::builder(), |builder, source| {
+                builder.add_source(source.clone())
+            })
+            .build()
+            .expect("loaded configs should be merged without error")
+    }
+}
+
 fn config_path() -> Result<Option<PathBuf>, ConfigError> {
     if let Ok(config_path) = env::var("JJ_CONFIG") {
         // TODO: We should probably support colon-separated (std::env::split_paths)
@@ -154,17 +208,6 @@ fn read_config_path(config_path: &Path) -> Result<config::Config, config::Config
         .build()
 }
 
-pub fn read_config() -> Result<UserSettings, ConfigError> {
-    let mut config_builder = config::Config::builder()
-        .add_source(default_config())
-        .add_source(env_base());
-    if let Some(path) = config_path()? {
-        config_builder = config_builder.add_source(read_config_path(&path)?);
-    }
-    let config = config_builder.add_source(env_overrides()).build().unwrap();
-    Ok(UserSettings::from_config(config))
-}
-
 /// Command name and arguments specified by config.
 #[derive(Clone, Debug, Eq, Hash, PartialEq, serde::Deserialize)]
 #[serde(untagged)]