Document rustfmt on nightly-rustc

The recursion_limit attribute avoids the following error:

```
error[E0275]: overflow evaluating the requirement `std::ptr::Unique<rustc_ast::Pat>: std::marker::Send`
  |
  = help: consider adding a `#![recursion_limit="256"]` attribute to your crate (`rustfmt_nightly`)
```

src/bootstrap/builder.rs

@@ -465,7 +465,7 @@ macro_rules! describe {
                 doc::Std,
                 doc::Rustc,
                 doc::Rustdoc,
-                // doc::Rustfmt,
+                doc::Rustfmt,
                 doc::ErrorIndex,
                 doc::Nomicon,
                 doc::Reference,

src/bootstrap/doc.rs

@@ -594,7 +594,7 @@ fn run(self, builder: &Builder<'_>) {
 }
 
 macro_rules! tool_doc {
-    ($tool: ident, $should_run: literal, $path: literal, [$($krate: literal),+ $(,)?]) => {
+    ($tool: ident, $should_run: literal, $path: literal, [$($krate: literal),+ $(,)?] $(, binary=$bin:expr)?) => {
         #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
         pub struct $tool {
             stage: u32,
@@ -639,9 +639,6 @@ fn run(self, builder: &Builder<'_>) {
                 // Build rustc docs so that we generate relative links.
                 builder.ensure(Rustc { stage, target });
 
-                // Build the tool.
-                builder.ensure(tool::$tool { compiler });
-
                 // Symlink compiler docs to the output directory of rustdoc documentation.
                 let out_dir = builder.stage_out(compiler, Mode::ToolRustc).join(target.triple).join("doc");
                 t!(fs::create_dir_all(&out_dir));
@@ -666,7 +663,9 @@ fn run(self, builder: &Builder<'_>) {
                     cargo.arg("-p").arg($krate);
                 )+
 
-                cargo.rustdocflag("--document-private-items");
+                $(if !$bin {
+                    cargo.rustdocflag("--document-private-items");
+                })?
                 cargo.rustdocflag("--enable-index-page");
                 cargo.rustdocflag("--show-type-layout");
                 cargo.rustdocflag("-Zunstable-options");
@@ -677,6 +676,13 @@ fn run(self, builder: &Builder<'_>) {
 }
 
 tool_doc!(Rustdoc, "rustdoc-tool", "src/tools/rustdoc", ["rustdoc", "rustdoc-json-types"]);
+tool_doc!(
+    Rustfmt,
+    "rustfmt-nightly",
+    "src/tools/rustfmt",
+    ["rustfmt-nightly", "rustfmt-config_proc_macro"],
+    binary = true
+);
 
 #[derive(Ord, PartialOrd, Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct ErrorIndex {

src/tools/rustfmt/src/lib.rs

@@ -1,6 +1,7 @@
 #![feature(rustc_private)]
 #![deny(rust_2018_idioms)]
 #![warn(unreachable_pub)]
+#![recursion_limit = "256"]
 
 #[macro_use]
 extern crate derive_new;