Add feature gate

src/librustdoc/clean/mod.rs

@@ -1611,7 +1611,7 @@ impl Clean<Type> for ast::Ty {
             TyTypeof(..) => {
                 panic!("Unimplemented type {:?}", self.node)
             },
-            TyMac(..) => {
+            TyMac(ref m) => {
                 cx.tcx().sess.span_bug(m.span, "unexpanded type macro found during cleaning")
             }
         }

src/libsyntax/ext/expand.rs

@@ -1555,6 +1555,7 @@ fn expand_and_rename_method(sig: ast::MethodSig, body: P<ast::Block>,
 pub fn expand_type(t: P<ast::Ty>, fld: &mut MacroExpander) -> P<ast::Ty> {
     let t = match t.node.clone() {
         ast::Ty_::TyMac(mac) => {
+            if fld.cx.ecfg.features.unwrap().type_macros {
                 let expanded_ty = match expand_mac_invoc(mac, t.span,
                                                          |r| r.make_ty(),
                                                          mark_ty,
@@ -1575,9 +1576,18 @@ pub fn expand_type(t: P<ast::Ty>, fld: &mut MacroExpander) -> P<ast::Ty> {
                     node: t.node,
                     span: t.span,
                     })
+            } else {
+                feature_gate::emit_feature_err(
+                    &fld.cx.parse_sess.span_diagnostic,
+                    "type_macros",
+                    t.span,
+                    "type macros are experimental (see tracking issue: 27336)");
+                t
+            }
         }
         _ => t
     };
+
     fold::noop_fold_ty(t, fld)
 }
 

src/libsyntax/feature_gate.rs

@@ -169,6 +169,8 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows associated type defaults
     ("associated_type_defaults", "1.2.0", Active),
+// Allows macros to appear in the type position.
+    ("type_macros", "1.3.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -228,8 +230,7 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
                      "no_std is experimental")),
     ("lang", Gated("lang_items",
                      "language items are subject to change")),
-    ("linkage", Gated("linkage",
-                      "the `linkage` attribute is experimental \
+    ("linkage", Gated("linkage", "the `linkage` attribute is experimental \
                        and not portable across platforms")),
     ("thread_local", Gated("thread_local",
                             "`#[thread_local]` is an experimental feature, and does not \
@@ -349,6 +350,7 @@ pub struct Features {
     pub const_fn: bool,
     pub static_recursion: bool,
     pub default_type_parameter_fallback: bool,
+    pub type_macros: bool,
 }
 
 impl Features {
@@ -375,6 +377,7 @@ impl Features {
             const_fn: false,
             static_recursion: false,
             default_type_parameter_fallback: false,
+            type_macros: false,
         }
     }
 }
@@ -878,6 +881,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         const_fn: cx.has_feature("const_fn"),
         static_recursion: cx.has_feature("static_recursion"),
         default_type_parameter_fallback: cx.has_feature("default_type_parameter_fallback"),
+        type_macros: cx.has_feature("type_macros"),
     }
 }