rustdoc: resolve intra-doc links to builtin macros

src/librustdoc/clean/mod.rs

@@ -974,6 +974,23 @@ impl Attributes {
 
         self.links.iter().filter_map(|&(ref s, did, ref fragment)| {
             match did {
+                Some(DefId { krate: CrateNum::BuiltinMacros, .. }) => {
+                    // builtin macros need to be handled separately, they'll always link to std
+                    // though
+                    let cache = cache();
+                    let url = match cache.extern_locations.get(krate) {
+                        Some(&(_, ref src, ExternalLocation::Local)) =>
+                            src.to_str().expect("invalid file path"),
+                        Some(&(_, _, ExternalLocation::Remote(ref s))) => s,
+                        Some(&(_, _, ExternalLocation::Unknown)) | None =>
+                            "https://doc.rust-lang.org/nightly",
+                    };
+                    Some((s.clone(),
+                          format!("{}{}std/macro.{}.html",
+                                  url,
+                                  if !url.ends_with('/') { "/" } else { "" },
+                                  s)))
+                }
                 Some(did) => {
                     if let Some((mut href, ..)) = href(did) {
                         if let Some(ref fragment) = *fragment {

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -1,6 +1,6 @@
 use errors::Applicability;
 use rustc::hir::def::{Res, DefKind, Namespace::{self, *}, PerNS};
-use rustc::hir::def_id::DefId;
+use rustc::hir::def_id::{DefId, CrateNum};
 use rustc::hir;
 use rustc::lint as lint;
 use rustc::ty;
@@ -388,11 +388,17 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
                 }
             };
 
-            if let Res::PrimTy(_) = res {
-                item.attrs.links.push((ori_link, None, fragment));
-            } else {
-                let id = register_res(cx, res);
-                item.attrs.links.push((ori_link, Some(id), fragment));
+            match res {
+                Res::PrimTy(_) => {
+                    item.attrs.links.push((ori_link, None, fragment));
+                }
+                Res::Def(DefKind::Macro(_), did) if did.krate == CrateNum::BuiltinMacros => {
+                    item.attrs.links.push((ori_link, Some(did), fragment));
+                }
+                res => {
+                    let id = register_res(cx, res);
+                    item.attrs.links.push((ori_link, Some(id), fragment));
+                }
             }
         }
 

src/test/rustdoc/intra-link-builtin-macros.rs

@@ -0,0 +1,3 @@
+// @has intra_link_builtin_macros/index.html
+// @has - '//a/@href' 'https://doc.rust-lang.org/nightly/std/macro.cfg.html'
+//! [cfg]