Feature gate non-builtin attributes in inner attribute position

src/libsyntax/ext/expand.rs

@@ -1074,6 +1074,21 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
         self.collect(kind, InvocationKind::Attr { attr, traits, item })
     }
 
+    fn find_attr_invoc(&self, attrs: &mut Vec<ast::Attribute>) -> Option<ast::Attribute> {
+        let attr = attrs.iter()
+                        .position(|a| !attr::is_known(a) && !is_builtin_attr(a))
+                        .map(|i| attrs.remove(i));
+        if let Some(attr) = &attr {
+            if !self.cx.ecfg.enable_custom_inner_attributes() &&
+               attr.style == ast::AttrStyle::Inner && attr.path != "test" {
+                emit_feature_err(&self.cx.parse_sess, "custom_inner_attributes",
+                                 attr.span, GateIssue::Language,
+                                 "non-builtin inner attributes are unstable");
+            }
+        }
+        attr
+    }
+
     /// If `item` is an attr invocation, remove and return the macro attribute and derive traits.
     fn classify_item<T>(&mut self, mut item: T) -> (Option<ast::Attribute>, Vec<Path>, T)
         where T: HasAttrs,
@@ -1087,7 +1102,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                 return attrs;
             }
 
-            attr = find_attr_invoc(&mut attrs);
+            attr = self.find_attr_invoc(&mut attrs);
             traits = collect_derives(&mut self.cx, &mut attrs);
             attrs
         });
@@ -1108,7 +1123,7 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                 return attrs;
             }
 
-            attr = find_attr_invoc(&mut attrs);
+            attr = self.find_attr_invoc(&mut attrs);
             attrs
         });
 
@@ -1145,12 +1160,6 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
     }
 }
 
-pub fn find_attr_invoc(attrs: &mut Vec<ast::Attribute>) -> Option<ast::Attribute> {
-    attrs.iter()
-         .position(|a| !attr::is_known(a) && !is_builtin_attr(a))
-         .map(|i| attrs.remove(i))
-}
-
 impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
     fn fold_expr(&mut self, expr: P<ast::Expr>) -> P<ast::Expr> {
         let mut expr = self.cfg.configure_expr(expr).into_inner();
@@ -1582,6 +1591,12 @@ impl<'feat> ExpansionConfig<'feat> {
         fn proc_macro_expr = proc_macro_expr,
         fn proc_macro_non_items = proc_macro_non_items,
     }
+
+    fn enable_custom_inner_attributes(&self) -> bool {
+        self.features.map_or(false, |features| {
+            features.custom_inner_attributes || features.custom_attribute || features.rustc_attrs
+        })
+    }
 }
 
 // A Marker adds the given mark to the syntax context.

src/libsyntax/feature_gate.rs

@@ -518,6 +518,9 @@ declare_features! (
     // #![test_runner]
     // #[test_case]
     (active, custom_test_frameworks, "1.30.0", Some(50297), None),
+
+    // Non-builtin attributes in inner attribute position
+    (active, custom_inner_attributes, "1.30.0", Some(38356), None),
 );
 
 declare_features! (

src/test/compile-fail-fulldeps/proc-macro/attribute-with-error.rs

@@ -11,6 +11,8 @@
 // aux-build:attribute-with-error.rs
 // ignore-stage1
 
+#![feature(custom_inner_attributes)]
+
 extern crate attribute_with_error;
 
 use attribute_with_error::foo;

src/test/compile-fail-fulldeps/proc-macro/issue-41211.rs

@@ -13,6 +13,8 @@
 // FIXME: https://github.com/rust-lang/rust/issues/41430
 // This is a temporary regression test for the ICE reported in #41211
 
+#![feature(custom_inner_attributes)]
+
 #![emit_unchanged]
 //~^ ERROR attribute `emit_unchanged` is currently unknown to the compiler
 extern crate issue_41211;

src/test/compile-fail-fulldeps/proc-macro/proc-macro-gates.rs

@@ -22,7 +22,7 @@ extern crate proc_macro_gates as foo;
 use foo::*;
 
 fn _test_inner() {
-    #![a] // OK
+    #![a] //~ ERROR: non-builtin inner attributes are unstable
 }
 
 #[a] //~ ERROR: custom attributes cannot be applied to modules
@@ -30,6 +30,7 @@ mod _test2 {}
 
 mod _test2_inner {
     #![a] //~ ERROR: custom attributes cannot be applied to modules
+          //~| ERROR: non-builtin inner attributes are unstable
 }
 
 #[a = y] //~ ERROR: must only be followed by a delimiter token

src/test/ui/feature-gate/issue-43106-gating-of-bench.rs

@@ -20,6 +20,8 @@
 // See issue-12997-1.rs and issue-12997-2.rs to see how `#[bench]` is
 // handled in "weird places" when `--test` is passed.
 
+#![feature(custom_inner_attributes)]
+
 #![bench                   = "4100"]
 
 fn main() { }

src/test/ui/span/issue-36530.rs

@@ -8,7 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// gate-test-custom_inner_attributes
+
 #[foo] //~ ERROR is currently unknown to the compiler
 mod foo {
     #![foo] //~ ERROR is currently unknown to the compiler
+            //~| ERROR non-builtin inner attributes are unstable
 }

src/test/ui/span/issue-36530.stderr

@@ -1,19 +1,27 @@
 error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
-  --> $DIR/issue-36530.rs:11:3
+  --> $DIR/issue-36530.rs:13:3
    |
 LL | #[foo] //~ ERROR is currently unknown to the compiler
    |   ^^^
    |
    = help: add #![feature(custom_attribute)] to the crate attributes to enable
 
+error[E0658]: non-builtin inner attributes are unstable (see issue #38356)
+  --> $DIR/issue-36530.rs:15:5
+   |
+LL |     #![foo] //~ ERROR is currently unknown to the compiler
+   |     ^^^^^^^
+   |
+   = help: add #![feature(custom_inner_attributes)] to the crate attributes to enable
+
 error[E0658]: The attribute `foo` is currently unknown to the compiler and may have meaning added to it in the future (see issue #29642)
-  --> $DIR/issue-36530.rs:13:8
+  --> $DIR/issue-36530.rs:15:8
    |
 LL |     #![foo] //~ ERROR is currently unknown to the compiler
    |        ^^^
    |
    = help: add #![feature(custom_attribute)] to the crate attributes to enable
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0658`.