templates: add self.trailers().contains_key(key)

as a simpler and more readable alternative to

    self.trailers().filter(|t| t.key() == "Change-Id")

cli/src/commit_templater.rs

@@ -74,6 +74,7 @@ use crate::diff_util::DiffStats;
 use crate::formatter::Formatter;
 use crate::revset_util;
 use crate::template_builder;
+use crate::template_builder::expect_plain_text_expression;
 use crate::template_builder::merge_fn_map;
 use crate::template_builder::BuildContext;
 use crate::template_builder::CoreTemplateBuildFnTable;
@@ -341,6 +342,14 @@ impl<'repo> TemplateLanguage<'repo> for CommitTemplateLanguage<'repo> {
             }
             CommitTemplatePropertyKind::TrailerList(property) => {
                 // TODO: migrate to table?
+                if function.name == "contains_key" {
+                    let [key_node] = function.expect_exact_arguments()?;
+                    let key_property =
+                        expect_plain_text_expression(self, diagnostics, build_ctx, key_node)?;
+                    let out_property = (property, key_property)
+                        .map(|(trailers, key)| trailers.iter().any(|t| t.key == key));
+                    Ok(Self::wrap_boolean(out_property))
+                } else {
                     template_builder::build_formattable_list_method(
                         self,
                         diagnostics,
@@ -353,6 +362,7 @@ impl<'repo> TemplateLanguage<'repo> for CommitTemplateLanguage<'repo> {
                 }
             }
         }
+    }
 }
 
 // If we need to add multiple languages that support Commit types, this can be

cli/tests/test_commit_template.rs

@@ -1709,4 +1709,22 @@ fn test_log_format_trailers() {
         "-r@",
     ]);
     insta::assert_snapshot!(output, @"Test User <test.user@example.com> I6a6a69649a45c67d3e96a7e5007c110ede34dec5[EOF]");
+
+    let output = work_dir.run_jj([
+        "log",
+        "--no-graph",
+        "-T",
+        r#"self.trailers().contains_key("Signed-off-by")"#,
+        "-r@",
+    ]);
+    insta::assert_snapshot!(output, @"true[EOF]");
+
+    let output = work_dir.run_jj([
+        "log",
+        "--no-graph",
+        "-T",
+        r#"self.trailers().contains_key("foo")"#,
+        "-r@",
+    ]);
+    insta::assert_snapshot!(output, @"false[EOF]");
 }

docs/config.md

@@ -240,14 +240,15 @@ is ignored.
 
 ### Commit trailers
 
-Trailers may be automatically added to the commit description with the
-`commit_trailers` template.
+You can configure automatic addition of trailers to commit descriptions using
+the `commit_trailers` template. Trailers defined in this template will only be
+added if they are not already present in the description.
 
 ```toml
 [templates]
 commit_trailers = '''
-"Signed-off-by: " ++ committer ++ "\n"
-'''
+format_signed_off_by_trailer(self)
+++ if(!trailers.contains_key("Change-Id"), format_gerrit_change_id_trailer(self))'''
 ```
 
 Some ready-to-use trailer templates are available for frequently used trailers:
@@ -256,6 +257,11 @@ Some ready-to-use trailer templates are available for frequently used trailers:
 * `format_gerrit_change_id_trailer(commit)` creates a "Change-Id" trailer
   suitable to be used with Gerrit. It is based Jujutsu's change id.
 
+The `trailers.contains_key(key)` method can be used within the template to
+conditionally add a trailer if no other trailer with the same key exists.
+
+Existing trailers are also accessible via `commit.trailers()`.
+
 ### Diff colors and styles
 
 In color-words and git diffs, word-level hunks are rendered with underline. You

docs/templates.md

@@ -248,6 +248,13 @@ defined.
 * `.map(|item| expression) -> ListTemplate`: Apply template `expression`
   to each element. Example: `parents.map(|c| c.commit_id().short())`
 
+### List<Trailer> type
+
+The following methods are defined. See also the `List` type.
+
+* `.contains_key(key: Template) -> Boolean`: True if the commit description
+  contains at least one trailer with the key `key`.
+
 ### ListTemplate type
 
 The following methods are defined. See also the `List` type.