Don't build the library and standard library before documenting them

Rustdoc doesn't require the build artifacts to generate the docs, and
especially in the case of rustc, it greatly increases the time needed to
run the build.

Author: jyn514

src/bootstrap/doc.rs

@@ -446,7 +446,6 @@ impl Step for Std {
         t!(fs::create_dir_all(&out));
         let compiler = builder.compiler(stage, builder.config.build);
 
-        builder.ensure(compile::Std { compiler, target });
         let out_dir = builder.stage_out(compiler, Mode::Std).join(target.triple).join("doc");
 
         t!(fs::copy(builder.src.join("src/doc/rust.css"), out.join("rust.css")));
@@ -564,9 +563,10 @@ impl Step for Rustc {
         let out = builder.compiler_doc_out(target);
         t!(fs::create_dir_all(&out));
 
-        // Build rustc.
+        // Build the standard library, so that proc-macros can use it.
+        // (Normally, only the metadata would be necessary, but proc-macros are special since they run at compile-time.)
         let compiler = builder.compiler(stage, builder.config.build);
-        builder.ensure(compile::Rustc { compiler, target });
+        builder.ensure(compile::Std { compiler, target: builder.config.build });
 
         // This uses a shared directory so that librustdoc documentation gets
         // correctly built and merged with the rustc documentation. This is