disable proc-macro re-export inlining

Proc-macros don't emit their attributes or source spans across crates.
This means that rustdoc can't actually see the docs of a proc-macro if
it wasn't defined in the active crate, and attempting to inline it
creates an empty page with no docs or source link. In lieu of attempting
to fix that immediately, this commit forces proc-macro re-exports to
never inline, which at least creates usable links to complete
documentation.

src/librustdoc/clean/inline.rs

@@ -105,14 +105,15 @@ pub fn try_inline(cx: &DocContext, def: Def, name: ast::Name, visited: &mut FxHa
             record_extern_fqn(cx, did, clean::TypeKind::Const);
             clean::ConstantItem(build_const(cx, did))
         }
-        Def::Macro(did, mac_kind) => {
-            match mac_kind {
-                MacroKind::Bang => record_extern_fqn(cx, did, clean::TypeKind::Macro),
-                MacroKind::Attr => record_extern_fqn(cx, did, clean::TypeKind::Attr),
-                MacroKind::Derive => record_extern_fqn(cx, did, clean::TypeKind::Derive),
-                MacroKind::ProcMacroStub => return None,
+        // FIXME: proc-macros don't propagate attributes or spans across crates, so they look empty
+        Def::Macro(did, MacroKind::Bang) => {
+            let mac = build_macro(cx, did, name);
+            if let clean::MacroItem(..) = mac {
+                record_extern_fqn(cx, did, clean::TypeKind::Macro);
+                mac
+            } else {
+                return None;
             }
-            build_macro(cx, did, name)
         }
         _ => return None,
     };