templater: add a local() method on Timestamps (#2900)

CHANGELOG.md

@@ -29,6 +29,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj git fetch` now accepts `-b` as a shorthand for `--branch`, making it more
   consistent with other commands that accept a branch
 
+* In the templating language, Timestamps now have a `.local()` method for
+  converting to the local timezone.
+
 ### Fixed bugs
 
 * On Windows, symlinks in the repo are now materialized as regular files in the

cli/src/template_builder.rs

@@ -571,6 +571,17 @@ fn build_timestamp_method<'a, L: TemplateLanguage<'a>>(
                 timestamp
             }))
         }
+        "local" => {
+            template_parser::expect_no_arguments(function)?;
+            let tz_offset = chrono::Local::now().offset().local_minus_utc() / 60;
+            language.wrap_timestamp(TemplateFunction::new(
+                self_property,
+                move |mut timestamp| {
+                    timestamp.tz_offset = tz_offset;
+                    timestamp
+                },
+            ))
+        }
         _ => return Err(TemplateParseError::no_such_method("Timestamp", function)),
     };
     Ok(property)

cli/tests/test_commit_template.rs

@@ -122,6 +122,27 @@ fn test_log_author_timestamp_utc() {
     "###);
 }
 
+#[cfg(unix)]
+#[test]
+fn test_log_author_timestamp_local() {
+    let mut test_env = TestEnvironment::default();
+    test_env.jj_cmd_ok(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    test_env.add_env_var("TZ", "UTC-05:30");
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "author.timestamp().local()"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @  2001-02-03 02:35:07.000 +05:30
+    ◉  1970-01-01 05:30:00.000 +05:30
+    "###);
+    test_env.add_env_var("TZ", "UTC+10:00");
+    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-T", "author.timestamp().local()"]);
+    insta::assert_snapshot!(stdout, @r###"
+    @  2001-02-02 11:05:07.000 -10:00
+    ◉  1969-12-31 14:00:00.000 -10:00
+    "###);
+}
+
 #[test]
 fn test_log_default() {
     let test_env = TestEnvironment::default();

docs/templates.md

@@ -187,6 +187,7 @@ The following methods are defined.
 * `.format(format: String) -> String`: Format with [the specified strftime-like
   format string](https://docs.rs/chrono/latest/chrono/format/strftime/).
 * `.utc() -> Timestamp`: Convert timestamp into UTC timezone.
+* `.local() -> Timestamp`: Convert timestamp into local timezone.
 
 ### TimestampRange type