templater: add .escape_json method to string

CHANGELOG.md

@@ -36,6 +36,10 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   `ellipsis` parameter; passing this prepends or appends the ellipsis to the
   content if it is truncated to fit the maximum width.
 
+* Templates now support additional string method `.escape_json(x)` which serializes
+  the string in JSON format. It is useful for making machine-readable templates
+  by escaping problematic characters like `\n`.
+
 ### Fixed bugs
 
 * `jj status` now shows untracked files under untracked directories.

cli/src/template_builder.rs

@@ -895,6 +895,14 @@ fn builtin_string_methods<'a, L: TemplateLanguage<'a> + ?Sized>(
             Ok(L::wrap_string(out_property))
         },
     );
+    map.insert(
+        "escape_json",
+        |_language, _diagnostics, _build_ctx, self_property, function| {
+            function.expect_no_arguments()?;
+            let out_property = self_property.map(|s| serde_json::to_string(&s).unwrap());
+            Ok(L::wrap_string(out_property))
+        },
+    );
     map
 }
 
@@ -2659,6 +2667,9 @@ mod tests {
         // ranges with end > start are empty
         insta::assert_snapshot!(env.render_ok(r#""abcdef".substr(4, 2)"#), @"");
         insta::assert_snapshot!(env.render_ok(r#""abcdef".substr(-2, -4)"#), @"");
+
+        insta::assert_snapshot!(env.render_ok(r#""hello".escape_json()"#), @r#""hello""#);
+        insta::assert_snapshot!(env.render_ok(r#""he \n ll \n \" o".escape_json()"#), @r#""he \n ll \n \" o""#);
     }
 
     #[test]

docs/templates.md

@@ -318,6 +318,11 @@ defined.
 * `.substr(start: Integer, end: Integer) -> String`: Extract substring. The
   `start`/`end` indices should be specified in UTF-8 bytes. Negative values
   count from the end of the string.
+* `.escape_json() -> String`: Serializes the string in JSON format. This
+  function is useful for making machine-readable templates. For example, you
+  can use it in a template like `"{ \"foo\": " ++ self.foo().escape_json() ++ "}"` to
+  return a JSON/JSONL.
+
 
 #### String literals