author/committer templates: add a username method

I haven't used a proper email address parser as I'm not really sure if it's
worth the extra dependency and effort -- thoughts?

CHANGELOG.md

@@ -142,6 +142,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * `jj new --insert-after` inserts the new commit between the target commit and
   its children.
 
+* `author`/`committer` templates now support `.username()`, which leaves out the
+domain information of `.email()`.
+
 ### Fixed bugs
 
 * When sharing the working copy with a Git repo, we used to forget to export

src/template_parser.rs

@@ -360,6 +360,14 @@ fn expect_arguments<const N: usize, const M: usize>(
     }
 }
 
+fn split_email(email: &str) -> (&str, Option<&str>) {
+    if let Some((username, rest)) = email.split_once('@') {
+        (username, Some(rest))
+    } else {
+        (email, None)
+    }
+}
+
 fn parse_method_chain<'a, I: 'a>(
     input_property: PropertyAndLabels<'a, I>,
     method_pairs: Pairs<Rule>,
@@ -556,6 +564,16 @@ fn parse_signature_method<'a, I: 'a>(
                 TemplatePropertyFn(|signature: &Signature| signature.email.clone()),
             ))
         }
+        "username" => {
+            expect_no_arguments(args_pair)?;
+            Property::String(chain_properties(
+                self_property,
+                TemplatePropertyFn(|signature: &Signature| {
+                    let (username, _) = split_email(&signature.email);
+                    username.to_owned()
+                }),
+            ))
+        }
         "timestamp" => {
             expect_no_arguments(args_pair)?;
             Property::Timestamp(chain_properties(

tests/test_templater.rs

@@ -257,6 +257,68 @@ fn test_templater_string_method() {
     insta::assert_snapshot!(render(r#""foo\nbar".first_line()"#), @"foo");
 }
 
+#[test]
+fn test_templater_signature() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+    let render = |template| get_template_output(&test_env, &repo_path, "@", template);
+
+    test_env.jj_cmd_success(&repo_path, &["new"]);
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <test.user@example.com>");
+    insta::assert_snapshot!(render(r#"author.name()"#), @"Test User");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user@example.com");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user");
+
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["--config-toml=user.name='Another Test User'", "new"],
+    );
+
+    insta::assert_snapshot!(render(r#"author"#), @"Another Test User <test.user@example.com>");
+    insta::assert_snapshot!(render(r#"author.name()"#), @"Another Test User");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user@example.com");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user");
+
+    test_env.jj_cmd_success(
+        &repo_path,
+        &[
+            "--config-toml=user.email='test.user@invalid@example.com'",
+            "new",
+        ],
+    );
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <test.user@invalid@example.com>");
+    insta::assert_snapshot!(render(r#"author.name()"#), @"Test User");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user@invalid@example.com");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user");
+
+    test_env.jj_cmd_success(&repo_path, &["--config-toml=user.email='test.user'", "new"]);
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <test.user>");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user");
+
+    test_env.jj_cmd_success(
+        &repo_path,
+        &[
+            "--config-toml=user.email='test.user+tag@example.com'",
+            "new",
+        ],
+    );
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <test.user+tag@example.com>");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"test.user+tag@example.com");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"test.user+tag");
+
+    test_env.jj_cmd_success(&repo_path, &["--config-toml=user.email='x@y'", "new"]);
+
+    insta::assert_snapshot!(render(r#"author"#), @"Test User <x@y>");
+    insta::assert_snapshot!(render(r#"author.email()"#), @"x@y");
+    insta::assert_snapshot!(render(r#"author.username()"#), @"x");
+}
+
 #[test]
 fn test_templater_label_function() {
     let test_env = TestEnvironment::default();