templater: add config(name) function

This could be used in order to switch template outputs conditionally, or to
get the default push remote for example.

CHANGELOG.md

@@ -65,6 +65,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 * Add a new template alias `builtin_op_log_oneline` along with `format_operation_oneline` and `format_snapshot_operation_oneline`
 
+* New template function `config(name)` to access to configuration variable from
+  template.
+
 ### Fixed bugs
 
 * Fixed diff selection by external tools with `jj split`/`commit -i FILESETS`.

cli/src/commands/config/list.rs

@@ -15,6 +15,7 @@
 use clap_complete::ArgValueCandidates;
 use jj_lib::config::ConfigNamePathBuf;
 use jj_lib::config::ConfigSource;
+use jj_lib::settings::UserSettings;
 use tracing::instrument;
 
 use super::ConfigLevelArgs;
@@ -64,7 +65,7 @@ pub fn cmd_config_list(
     args: &ConfigListArgs,
 ) -> Result<(), CommandError> {
     let template = {
-        let language = config_template_language();
+        let language = config_template_language(command.settings());
         let text = match &args.template {
             Some(value) => value.to_owned(),
             None => command.settings().get_string("templates.config_list")?,
@@ -107,9 +108,11 @@ pub fn cmd_config_list(
 
 // AnnotatedValue will be cloned internally in the templater. If the cloning
 // cost matters, wrap it with Rc.
-fn config_template_language() -> GenericTemplateLanguage<'static, AnnotatedValue> {
+fn config_template_language(
+    settings: &UserSettings,
+) -> GenericTemplateLanguage<'static, AnnotatedValue> {
     type L = GenericTemplateLanguage<'static, AnnotatedValue>;
-    let mut language = L::new();
+    let mut language = L::new(settings);
     language.add_keyword("name", |self_property| {
         let out_property = self_property.map(|annotated| annotated.name.to_string());
         Ok(L::wrap_string(out_property))

cli/src/commit_templater.rs

@@ -49,6 +49,7 @@ use jj_lib::revset::RevsetDiagnostics;
 use jj_lib::revset::RevsetModifier;
 use jj_lib::revset::RevsetParseContext;
 use jj_lib::revset::UserRevsetExpression;
+use jj_lib::settings::UserSettings;
 use jj_lib::signing::SigStatus;
 use jj_lib::signing::SignError;
 use jj_lib::signing::SignResult;
@@ -151,6 +152,10 @@ impl<'repo> TemplateLanguage<'repo> for CommitTemplateLanguage<'repo> {
 
     template_builder::impl_core_wrap_property_fns!('repo, CommitTemplatePropertyKind::Core);
 
+    fn settings(&self) -> &UserSettings {
+        self.repo.base_repo().settings()
+    }
+
     fn build_function(
         &self,
         diagnostics: &mut TemplateDiagnostics,

cli/src/generic_templater.rs

@@ -15,6 +15,8 @@
 use std::cmp::Ordering;
 use std::collections::HashMap;
 
+use jj_lib::settings::UserSettings;
+
 use crate::template_builder;
 use crate::template_builder::BuildContext;
 use crate::template_builder::CoreTemplateBuildFnTable;
@@ -35,6 +37,7 @@ use crate::templater::TemplateProperty;
 /// types. It's cloned several times internally. Keyword functions need to be
 /// registered to extract properties from the self object.
 pub struct GenericTemplateLanguage<'a, C> {
+    settings: UserSettings,
     build_fn_table: GenericTemplateBuildFnTable<'a, C>,
 }
 
@@ -42,15 +45,18 @@ impl<'a, C> GenericTemplateLanguage<'a, C> {
     /// Sets up environment with no keywords.
     ///
     /// New keyword functions can be registered by `add_keyword()`.
-    // It's not "Default" in a way that the core methods table is NOT empty.
+    pub fn new(settings: &UserSettings) -> Self {
-    #[allow(clippy::new_without_default)]
+        Self::with_keywords(HashMap::new(), settings)
-    pub fn new() -> Self {
-        Self::with_keywords(HashMap::new())
     }
 
     /// Sets up environment with the given `keywords` table.
-    pub fn with_keywords(keywords: GenericTemplateBuildKeywordFnMap<'a, C>) -> Self {
+    pub fn with_keywords(
+        keywords: GenericTemplateBuildKeywordFnMap<'a, C>,
+        settings: &UserSettings,
+    ) -> Self {
         GenericTemplateLanguage {
+            // Clone settings to keep lifetime simple. It's cheap.
+            settings: settings.clone(),
             build_fn_table: GenericTemplateBuildFnTable {
                 core: CoreTemplateBuildFnTable::builtin(),
                 keywords,
@@ -86,6 +92,10 @@ impl<'a, C: 'a> TemplateLanguage<'a> for GenericTemplateLanguage<'a, C> {
 
     template_builder::impl_core_wrap_property_fns!('a, GenericTemplatePropertyKind::Core);
 
+    fn settings(&self) -> &UserSettings {
+        &self.settings
+    }
+
     fn build_function(
         &self,
         diagnostics: &mut TemplateDiagnostics,

cli/src/operation_templater.rs

@@ -23,6 +23,7 @@ use jj_lib::object_id::ObjectId;
 use jj_lib::op_store::OperationId;
 use jj_lib::operation::Operation;
 use jj_lib::repo::RepoLoader;
+use jj_lib::settings::UserSettings;
 
 use crate::template_builder;
 use crate::template_builder::merge_fn_map;
@@ -89,6 +90,10 @@ impl TemplateLanguage<'static> for OperationTemplateLanguage {
 
     template_builder::impl_core_wrap_property_fns!('static, OperationTemplatePropertyKind::Core);
 
+    fn settings(&self) -> &UserSettings {
+        self.repo_loader.settings()
+    }
+
     fn build_function(
         &self,
         diagnostics: &mut TemplateDiagnostics,

cli/src/template_builder.rs

@@ -19,8 +19,10 @@ use std::io;
 use itertools::Itertools as _;
 use jj_lib::backend::Signature;
 use jj_lib::backend::Timestamp;
+use jj_lib::config::ConfigNamePathBuf;
 use jj_lib::config::ConfigValue;
 use jj_lib::dsl_util::AliasExpandError as _;
+use jj_lib::settings::UserSettings;
 use jj_lib::time_util::DatePattern;
 use serde::de::IntoDeserializer as _;
 use serde::Deserialize;
@@ -88,6 +90,8 @@ pub trait TemplateLanguage<'a> {
     fn wrap_template(template: Box<dyn Template + 'a>) -> Self::Property;
     fn wrap_list_template(template: Box<dyn ListTemplate + 'a>) -> Self::Property;
 
+    fn settings(&self) -> &UserSettings;
+
     /// Translates the given global `function` call to a property.
     ///
     /// This should be delegated to
@@ -1490,6 +1494,24 @@ fn builtin_functions<'a, L: TemplateLanguage<'a> + ?Sized>() -> TemplateBuildFun
         });
         Ok(L::wrap_template(Box::new(template)))
     });
+    map.insert("config", |language, _diagnostics, _build_ctx, function| {
+        // Dynamic lookup can be implemented if needed. The name is literal
+        // string for now so the error can be reported early.
+        let [name_node] = function.expect_exact_arguments()?;
+        let name: ConfigNamePathBuf =
+            template_parser::expect_string_literal_with(name_node, |name, span| {
+                name.parse().map_err(|err| {
+                    TemplateParseError::expression("Failed to parse config name", span)
+                        .with_source(err)
+                })
+            })?;
+        let value = language.settings().get_value(&name).map_err(|err| {
+            TemplateParseError::expression("Failed to get config value", function.name_span)
+                .with_source(err)
+        })?;
+        // .decorated("", "") to trim leading/trailing whitespace
+        Ok(L::wrap_config_value(Literal(value.decorated("", ""))))
+    });
     map
 }
 
@@ -1796,6 +1818,7 @@ mod tests {
     use std::iter;
 
     use jj_lib::backend::MillisSinceEpoch;
+    use jj_lib::config::StackedConfig;
 
     use super::*;
     use crate::formatter;
@@ -1814,8 +1837,13 @@ mod tests {
 
     impl TestTemplateEnv {
         fn new() -> Self {
+            Self::with_config(StackedConfig::with_defaults())
+        }
+
+        fn with_config(config: StackedConfig) -> Self {
+            let settings = UserSettings::from_config(config).unwrap();
             TestTemplateEnv {
-                language: L::new(),
+                language: L::new(&settings),
                 aliases_map: TemplateAliasesMap::new(),
                 color_rules: Vec::new(),
             }

cli/tests/test_templater.rs

@@ -481,6 +481,47 @@ fn test_templater_bad_alias_decl() {
     "###);
 }
 
+#[test]
+fn test_templater_config_function() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["git", "init", "repo"]);
+    let repo_path = test_env.env_root().join("repo");
+    let render = |template| get_template_output(&test_env, &repo_path, "@-", template);
+    let render_err = |template| test_env.jj_cmd_failure(&repo_path, &["log", "-T", template]);
+
+    insta::assert_snapshot!(
+        render("config('user.name')"),
+        @r#""Test User""#);
+    insta::assert_snapshot!(
+        render("config('user')"),
+        @r#"{ email = "test.user@example.com", name = "Test User" }"#);
+    insta::assert_snapshot!(render_err("config('invalid name')"), @r"
+    Error: Failed to parse template: Failed to parse config name
+    Caused by:
+    1:  --> 1:8
+      |
+    1 | config('invalid name')
+      |        ^------------^
+      |
+      = Failed to parse config name
+    2: TOML parse error at line 1, column 9
+      |
+    1 | invalid name
+      |         ^
+    ");
+    insta::assert_snapshot!(render_err("config('unknown')"), @r"
+    Error: Failed to parse template: Failed to get config value
+    Caused by:
+    1:  --> 1:1
+      |
+    1 | config('unknown')
+      | ^----^
+      |
+      = Failed to get config value
+    2: Value not found for unknown
+    ");
+}
+
 fn get_template_output(
     test_env: &TestEnvironment,
     repo_path: &Path,

docs/templates.md

@@ -76,6 +76,7 @@ The following functions are defined.
   Insert separator between **non-empty** contents.
 * `surround(prefix: Template, suffix: Template, content: Template) -> Template`:
   Surround **non-empty** content with texts such as parentheses.
+* `config(name: String) -> ConfigValue`: Look up configuration value by `name`.
 
 ## Types