currently all the collect_features functions live in tidy. that doesn't work here, because we don't want to require building the compiler before building tidy.

mentoring instructions:

• change

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

  Lines 1121 to 1135 in 30721b0

  	fn run(self, builder: &Builder<'_>) {
  	let target = self.target;
  	builder.info(&format!("Generating unstable book md files ({target})"));
  	let out = builder.md_doc_out(target).join("unstable-book");
  	builder.create_dir(&out);
  	builder.remove_dir(&out);
  	let mut cmd = builder.tool_cmd(Tool::UnstableBookGen);
  	cmd.arg(builder.src.join("library"));
  	cmd.arg(builder.src.join("compiler"));
  	cmd.arg(builder.src.join("src"));
  	cmd.arg(out);
  	cmd.run(builder);
  	}

  to build a rustc for unstable-book-gen
• extend

  rust/src/tools/unstable-book-gen/src/main.rs

  Lines 45 to 46 in 30721b0

  	let unstable_lang_features = collect_unstable_feature_names(&lang_features);
  	let unstable_lib_features = collect_unstable_feature_names(&lib_features);

  to also run rustc -Zhelp and parse the output. then use that to generate stubs, by the same logic as the existing library and lang features.

@rustbot label E-medium E-help-wanted