config: extract jj-lib's default values to embedded TOML file

It's nice that "jj config list --include-defaults" can show these default
values.

I just copied the jj-cli directory structure, but it's unlikely we'll add more
config/*.toml files.

cli/src/config.rs

@@ -424,7 +424,7 @@ fn config_files_for(
 ///
 /// This function sets up 1, 2, and 6.
 pub fn config_from_environment(default_layers: impl IntoIterator<Item = ConfigLayer>) -> RawConfig {
-    let mut config = StackedConfig::empty();
+    let mut config = StackedConfig::with_defaults();
     config.extend_layers(default_layers);
     config.add_layer(env_base_layer());
     config.add_layer(env_overrides_layer());

cli/src/merge_tools/mod.rs

@@ -362,7 +362,7 @@ mod tests {
     use super::*;
 
     fn config_from_string(text: &str) -> StackedConfig {
-        let mut config = StackedConfig::empty();
+        let mut config = StackedConfig::with_defaults();
         // Load defaults to test the default args lookup
         config.extend_layers(crate::config::default_config_layers());
         config.add_layer(ConfigLayer::parse(ConfigSource::User, text).unwrap());

lib/src/config.rs

@@ -27,6 +27,7 @@ use std::str::FromStr;
 use std::sync::Arc;
 
 use itertools::Itertools as _;
+use once_cell::sync::Lazy;
 use serde::de::IntoDeserializer as _;
 use serde::Deserialize;
 use thiserror::Error;
@@ -595,6 +596,14 @@ impl StackedConfig {
         StackedConfig { layers: vec![] }
     }
 
+    /// Creates a stack of configuration layers containing the default variables
+    /// referred to by `jj-lib`.
+    pub fn with_defaults() -> Self {
+        StackedConfig {
+            layers: DEFAULT_CONFIG_LAYERS.to_vec(),
+        }
+    }
+
     /// Loads config file from the specified `path`, inserts it at the position
     /// specified by `source`. The file should exist.
     pub fn load_file(
@@ -833,6 +842,11 @@ fn merge_items(lower_item: &mut ConfigItem, upper_item: &ConfigItem) {
     }
 }
 
+static DEFAULT_CONFIG_LAYERS: Lazy<[Arc<ConfigLayer>; 1]> = Lazy::new(|| {
+    let parse = |text: &str| Arc::new(ConfigLayer::parse(ConfigSource::Default, text).unwrap());
+    [parse(include_str!("config/misc.toml"))]
+});
+
 #[cfg(test)]
 mod tests {
     use assert_matches::assert_matches;

lib/src/config/README.md

@@ -0,0 +1,2 @@
+These files are compiled into the binary; they are not installed to the user's
+system.

lib/src/config/misc.toml

@@ -0,0 +1,16 @@
+[debug]
+# commit-timestamp = <now>
+# operation-timestamp = <now>
+# randomness-seed = <random>
+
+[git]
+abandon-unreachable-commits = true
+# auto-local-bookmark = false
+
+[operation]
+# hostname = <host>
+# username = <user>
+
+[user]
+email = ""
+name = ""

lib/src/git_backend.rs

@@ -2221,7 +2221,7 @@ mod tests {
     // UserSettings type. testutils returns jj_lib (2)'s UserSettings, whereas
     // our UserSettings type comes from jj_lib (1).
     fn user_settings() -> UserSettings {
-        let config = StackedConfig::empty();
+        let config = StackedConfig::with_defaults();
         UserSettings::from_config(config).unwrap()
     }
 }

lib/src/settings.rs

@@ -64,14 +64,13 @@ pub struct GitSettings {
 impl GitSettings {
     pub fn from_settings(settings: &UserSettings) -> Result<Self, ConfigGetError> {
         let auto_local_bookmark = {
+            // TODO: Drop support for git.auto-local-branch and move the default
+            // value to config/*.toml
             let opt1 = settings.get_bool("git.auto-local-bookmark").optional()?;
             let opt2 = settings.get_bool("git.auto-local-branch").optional()?;
             opt1.or(opt2).unwrap_or(false)
         };
-        let abandon_unreachable_commits = settings
-            .get_bool("git.abandon-unreachable-commits")
-            .optional()?
-            .unwrap_or(true);
+        let abandon_unreachable_commits = settings.get_bool("git.abandon-unreachable-commits")?;
         Ok(GitSettings {
             auto_local_bookmark,
             abandon_unreachable_commits,
@@ -146,8 +145,8 @@ fn to_timestamp(value: ConfigValue) -> Result<Timestamp, Box<dyn std::error::Err
 
 impl UserSettings {
     pub fn from_config(config: StackedConfig) -> Result<Self, ConfigGetError> {
-        let user_name = config.get("user.name").optional()?.unwrap_or_default();
-        let user_email = config.get("user.email").optional()?.unwrap_or_default();
+        let user_name = config.get("user.name")?;
+        let user_email = config.get("user.email")?;
         let commit_timestamp = config
             .get_value_with("debug.commit-timestamp", to_timestamp)
             .optional()?;

lib/tests/test_commit_builder.rs

@@ -166,7 +166,7 @@ fn test_rewrite(backend: TestRepoBackend) {
         ],
     );
 
-    let mut config = StackedConfig::empty();
+    let mut config = StackedConfig::with_defaults();
     config.add_layer(
         ConfigLayer::parse(
             ConfigSource::User,
@@ -221,7 +221,7 @@ fn test_rewrite(backend: TestRepoBackend) {
 #[test_case(TestRepoBackend::Local ; "local backend")]
 #[test_case(TestRepoBackend::Git ; "git backend")]
 fn test_rewrite_update_missing_user(backend: TestRepoBackend) {
-    let missing_user_settings = UserSettings::from_config(StackedConfig::empty()).unwrap();
+    let missing_user_settings = UserSettings::from_config(StackedConfig::with_defaults()).unwrap();
     let test_repo = TestRepo::init_with_backend(backend);
     let repo = &test_repo.repo;
 
@@ -240,7 +240,7 @@ fn test_rewrite_update_missing_user(backend: TestRepoBackend) {
     assert_eq!(initial_commit.committer().name, "");
     assert_eq!(initial_commit.committer().email, "");
 
-    let mut config = StackedConfig::empty();
+    let mut config = StackedConfig::with_defaults();
     config.add_layer(
         ConfigLayer::parse(
             ConfigSource::User,

lib/tests/test_init.rs

@@ -139,11 +139,9 @@ fn test_init_external_git() {
 
 #[test_case(TestRepoBackend::Local ; "local backend")]
 #[test_case(TestRepoBackend::Git ; "git backend")]
-fn test_init_no_config_set(backend: TestRepoBackend) {
-    // Test that we can create a repo without setting any config
-    // TODO: Perhaps, StackedConfig::empty() will be replaced with ::default()
-    // or something that includes the minimal configuration variables.
-    let settings = UserSettings::from_config(StackedConfig::empty()).unwrap();
+fn test_init_with_default_config(backend: TestRepoBackend) {
+    // Test that we can create a repo without setting any non-default config
+    let settings = UserSettings::from_config(StackedConfig::with_defaults()).unwrap();
     let test_workspace = TestWorkspace::init_with_backend(&settings, backend);
     let repo = &test_workspace.repo;
     let wc_commit_id = repo

lib/testutils/src/lib.rs

@@ -115,7 +115,7 @@ pub fn base_user_config() -> StackedConfig {
         operation.hostname = "host.example.com"
         debug.randomness-seed = 42
     "#;
-    let mut config = StackedConfig::empty();
+    let mut config = StackedConfig::with_defaults();
     config.add_layer(ConfigLayer::parse(ConfigSource::User, config_text).unwrap());
     config
 }