Fix rustbook submodule update location

I put this submodule update in the entirely wrong location. I put it in
the `RustcBook` step (for generating src/doc/rustc), when it really
should exist for all steps that use the `Rustbook` tool.

src/bootstrap/src/core/build_steps/doc.rs

@@ -1172,12 +1172,6 @@ impl Step for RustcBook {
     /// in the "md-doc" directory in the build output directory. Then
     /// "rustbook" is used to convert it to HTML.
     fn run(self, builder: &Builder<'_>) {
-        // These submodules are required to be checked out to build rustbook
-        // because they have Cargo dependencies that are needed.
-        #[allow(clippy::single_element_loop)] // This will change soon.
-        for path in ["src/doc/book"] {
-            builder.update_submodule(Path::new(path));
-        }
         let out_base = builder.md_doc_out(self.target).join("rustc");
         t!(fs::create_dir_all(&out_base));
         let out_listing = out_base.join("src/lints");

src/bootstrap/src/core/build_steps/tool.rs

@@ -241,6 +241,7 @@ macro_rules! bootstrap_tool {
         $(,is_external_tool = $external:expr)*
         $(,is_unstable_tool = $unstable:expr)*
         $(,allow_features = $allow_features:expr)?
+        $(,submodules = $submodules:expr)?
         ;
     )+) => {
         #[derive(PartialEq, Eq, Clone)]
@@ -287,6 +288,11 @@ macro_rules! bootstrap_tool {
             }
 
             fn run(self, builder: &Builder<'_>) -> PathBuf {
+                $(
+                    for submodule in $submodules {
+                        builder.update_submodule(Path::new(submodule));
+                    }
+                )*
                 builder.ensure(ToolBuild {
                     compiler: self.compiler,
                     target: self.target,
@@ -314,7 +320,7 @@ macro_rules! bootstrap_tool {
 }
 
 bootstrap_tool!(
-    Rustbook, "src/tools/rustbook", "rustbook";
+    Rustbook, "src/tools/rustbook", "rustbook", submodules = SUBMODULES_FOR_RUSTBOOK;
     UnstableBookGen, "src/tools/unstable-book-gen", "unstable-book-gen";
     Tidy, "src/tools/tidy", "tidy";
     Linkchecker, "src/tools/linkchecker", "linkchecker";
@@ -340,6 +346,10 @@ bootstrap_tool!(
     WasmComponentLd, "src/tools/wasm-component-ld", "wasm-component-ld", is_unstable_tool = true, allow_features = "min_specialization";
 );
 
+/// These are the submodules that are required for rustbook to work due to
+/// depending on mdbook plugins.
+pub static SUBMODULES_FOR_RUSTBOOK: &[&str] = &["src/doc/book"];
+
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub struct OptimizedDist {
     pub compiler: Compiler,