Add support for watching additional directories

guide/src/format/configuration/general.md

@@ -87,6 +87,7 @@ This controls the build process of your book.
 build-dir = "book"                # the directory where the output is placed
 create-missing = true             # whether or not to create missing pages
 use-default-preprocessors = true  # use the default preprocessors
+extra-watch-dirs = []             # directories to watch for triggering builds
 ```
 
 - **build-dir:** The directory to put the rendered book in. By default this is
@@ -108,3 +109,6 @@ use-default-preprocessors = true  # use the default preprocessors
     default preprocessors from running.
   - Adding `[preprocessor.links]`, for example, will ensure, regardless of
     `use-default-preprocessors` that `links` it will run.
+- **extra-watch-dirs**: A list of paths to directories that will be watched in
+  the `watch` and `serve` commands. Changes to files under these directories will
+  trigger rebuilds. Useful if your book depends on files outside its `src` directory.

src/cmd/watch.rs

@@ -146,6 +146,17 @@ where
     // Add the book.toml file to the watcher if it exists
     let _ = watcher.watch(book.root.join("book.toml"), NonRecursive);
 
+    for dir in &book.config.build.extra_watch_dirs {
+        let path = dir.canonicalize().unwrap();
+        if let Err(e) = watcher.watch(&path, Recursive) {
+            error!(
+                "Error while watching extra directory {:?}:\n    {:?}",
+                path, e
+            );
+            std::process::exit(1);
+        }
+    }
+
     info!("Listening for changes...");
 
     loop {
@@ -166,7 +177,11 @@ where
             })
             .collect::<Vec<_>>();
 
-        let paths = remove_ignored_files(&book.root, &paths[..]);
+        // If we are watching files outside the current repository (via extra-watch-dirs), then they are definitionally
+        // ignored by gitignore. So we handle this case by including such files into the watched paths list.
+        let any_external_paths = paths.iter().filter(|p| !p.starts_with(&book.root)).cloned();
+        let mut paths = remove_ignored_files(&book.root, &paths[..]);
+        paths.extend(any_external_paths);
 
         if !paths.is_empty() {
             closure(paths, &book.root);

src/config.rs

@@ -437,6 +437,8 @@ pub struct BuildConfig {
     /// Should the default preprocessors always be used when they are
     /// compatible with the renderer?
     pub use_default_preprocessors: bool,
+    /// Extra directories to trigger rebuild when watching/serving
+    pub extra_watch_dirs: Vec<PathBuf>,
 }
 
 impl Default for BuildConfig {
@@ -445,6 +447,7 @@ impl Default for BuildConfig {
             build_dir: PathBuf::from("book"),
             create_missing: true,
             use_default_preprocessors: true,
+            extra_watch_dirs: Vec::new(),
         }
     }
 }
@@ -770,6 +773,7 @@ mod tests {
             build_dir: PathBuf::from("outputs"),
             create_missing: false,
             use_default_preprocessors: true,
+            extra_watch_dirs: Vec::new(),
         };
         let rust_should_be = RustConfig { edition: None };
         let playground_should_be = Playground {
@@ -980,6 +984,7 @@ mod tests {
             build_dir: PathBuf::from("my-book"),
             create_missing: true,
             use_default_preprocessors: true,
+            extra_watch_dirs: Vec::new(),
         };
 
         let html_should_be = HtmlConfig {

tests/init.rs

@@ -95,7 +95,7 @@ fn run_mdbook_init_with_custom_book_and_src_locations() {
     let contents = fs::read_to_string(temp.path().join("book.toml")).unwrap();
     assert_eq!(
         contents,
-        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nuse-default-preprocessors = true\n"
+        "[book]\nauthors = []\nlanguage = \"en\"\nmultilingual = false\nsrc = \"in\"\n\n[build]\nbuild-dir = \"out\"\ncreate-missing = true\nextra-watch-dirs = []\nuse-default-preprocessors = true\n"
     );
 }