feat(mdbook): allow default configuration

extensions/mdbook/private/mdbook.bzl

@@ -23,6 +23,14 @@ def _map_inputs(file):
 def _mdbook_impl(ctx):
     output = ctx.actions.declare_directory(ctx.label.name)
 
+    book = ctx.file.book
+    if book == None:
+        book = ctx.actions.declare_file("{}.book.toml".format(ctx.label.name))
+        ctx.actions.write(
+            output = book,
+            content = "",
+        )
+
     toolchain = ctx.toolchains["@rules_rust_mdbook//:toolchain_type"]
 
     plugin_paths = depset([
@@ -33,7 +41,7 @@ def _mdbook_impl(ctx):
     path_sep = ";" if is_windows else ":"
     plugin_path = path_sep.join(plugin_paths.to_list())
 
-    inputs = depset([ctx.file.book] + ctx.files.srcs)
+    inputs = depset([book] + ctx.files.srcs)
 
     inputs_map_args = ctx.actions.args()
     inputs_map_args.use_param_file("%s", use_always = True)
@@ -45,7 +53,8 @@ def _mdbook_impl(ctx):
     args.add(output.path)
     args.add(toolchain.mdbook)
     args.add("build")
-    args.add("${{pwd}}/{}".format(ctx.file.book.dirname))
+    book_dir = "/".join(_src_dest_path(book).split("/")[:-1])
+    args.add("${{pwd}}/{}".format(book_dir))
 
     ctx.actions.run(
         mnemonic = "MdBookBuild",
@@ -64,7 +73,7 @@ def _mdbook_impl(ctx):
         ),
         MdBookInfo(
             srcs = depset(ctx.files.srcs),
-            config = ctx.file.book,
+            config = book,
             plugins = depset(ctx.files.plugins),
         ),
     ]
@@ -74,9 +83,11 @@ mdbook = rule(
     doc = "Rules to create book from markdown files using `mdBook`.",
     attrs = {
         "book": attr.label(
-            doc = "The `book.toml` file.",
+            doc = (
+                "The optional `book.toml` file. An empty default configuration is " +
+                "used when omitted."
+            ),
             allow_single_file = ["book.toml"],
-            mandatory = True,
         ),
         "plugins": attr.label_list(
             doc = (
@@ -121,7 +132,8 @@ def _mdbook_server_impl(ctx):
     workspace_name = ctx.workspace_name
 
     args.add("--mdbook={}".format(_rlocationpath(toolchain.mdbook, workspace_name)))
-    args.add("--config={}".format(_src_dest_path(book_info.config)))
+    config_dest = _src_dest_path(book_info.config)
+    args.add("--config={}".format(config_dest))
     args.add("--hostname={}".format(ctx.attr.hostname))
     args.add("--port={}".format(ctx.attr.port))
 

extensions/mdbook/test/default_config/BUILD.bazel

@@ -0,0 +1,12 @@
+load("@bazel_skylib//rules:build_test.bzl", "build_test")
+load("//:defs.bzl", "mdbook")
+
+mdbook(
+    name = "default_config",
+    srcs = glob(["src/**/*.md"]),
+)
+
+build_test(
+    name = "default_config_test",
+    targets = [":default_config"],
+)

extensions/mdbook/test/default_config/src/SUMMARY.md

@@ -0,0 +1,3 @@
+# Summary
+
+- [Chapter 1](./chapter_1.md)

extensions/mdbook/test/default_config/src/chapter_1.md

@@ -0,0 +1,3 @@
+# Chapter 1
+
+This book uses mdBook's default configuration.