Added feature gate.

src/librustc_resolve/diagnostics.rs

@@ -775,7 +775,8 @@ The `Self` keyword was used outside an impl, trait, or type definition.
 Erroneous code example:
 
 ```compile_fail,E0411
-<Self>::foo; // error: use of `Self` outside of an impl, trait, or type definition
+<Self>::foo; // error: use of `Self` outside of an impl, trait, or type
+             // definition
 ```
 
 The `Self` keyword represents the current type, which explains why it can only

src/librustc_resolve/lib.rs

@@ -2204,9 +2204,13 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
             ItemKind::Union(_, ref generics) => {
                 self.with_type_parameter_rib(HasTypeParameters(generics, ItemRibKind), |this| {
                     let item_def_id = this.definitions.local_def_id(item.id);
-                    this.with_self_rib(Def::SelfTy(None, Some(item_def_id)), |this| {
+                    if this.session.features_untracked().self_in_typedefs {
+                        this.with_self_rib(Def::SelfTy(None, Some(item_def_id)), |this| {
+                            visit::walk_item(this, item);
+                        });
+                    } else {
                         visit::walk_item(this, item);
-                    });
+                    }
                 });
             }
 
@@ -2977,7 +2981,12 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
             if is_self_type(path, ns) {
                 __diagnostic_used!(E0411);
                 err.code(DiagnosticId::Error("E0411".into()));
-                err.span_label(span, "`Self` is only available in traits, impls, and type definitions");
+                let available_in = if this.session.features_untracked().self_in_typedefs {
+                    "impls, traits, and type definitions"
+                } else {
+                    "traits and impls"
+                };
+                err.span_label(span, format!("`Self` is only available in {}", available_in));
                 return (err, Vec::new());
             }
             if is_self_value(path, ns) {

src/libsyntax/feature_gate.rs

@@ -138,7 +138,7 @@ declare_features! (
     (active, thread_local, "1.0.0", Some(29594), None),
     (active, trace_macros, "1.0.0", Some(29598), None),
 
-    // rustc internal, for now:
+    // rustc internal, for now
     (active, intrinsics, "1.0.0", None, None),
     (active, lang_items, "1.0.0", None, None),
     (active, format_args_nl, "1.29.0", None, None),
@@ -168,6 +168,7 @@ declare_features! (
     (active, optin_builtin_traits, "1.0.0", Some(13231), None),
 
     // Allows use of #[staged_api]
+    //
     // rustc internal
     (active, staged_api, "1.0.0", None, None),
 
@@ -245,24 +246,25 @@ declare_features! (
     // Allows associated type defaults
     (active, associated_type_defaults, "1.2.0", Some(29661), None),
 
-    // allow `repr(simd)`, and importing the various simd intrinsics
+    // Allows `repr(simd)`, and importing the various simd intrinsics
     (active, repr_simd, "1.4.0", Some(27731), None),
 
-    // allow `extern "platform-intrinsic" { ... }`
+    // Allows `extern "platform-intrinsic" { ... }`
     (active, platform_intrinsics, "1.4.0", Some(27731), None),
 
-    // allow `#[unwind(..)]`
+    // Allows `#[unwind(..)]`
     // rustc internal for rust runtime
     (active, unwind_attributes, "1.4.0", None, None),
 
-    // allow the use of `#[naked]` on functions.
+    // Allows the use of `#[naked]` on functions.
     (active, naked_functions, "1.9.0", Some(32408), None),
 
-    // allow `#[no_debug]`
+    // Allows `#[no_debug]`
     (active, no_debug, "1.5.0", Some(29721), None),
 
-    // allow `#[omit_gdb_pretty_printer_section]`
-    // rustc internal.
+    // Allows `#[omit_gdb_pretty_printer_section]`
+    //
+    // rustc internal
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
 
     // Allows cfg(target_vendor = "...").
@@ -292,10 +294,10 @@ declare_features! (
     // The `!` type. Does not imply exhaustive_patterns (below) any more.
     (active, never_type, "1.13.0", Some(35121), None),
 
-    // Allows exhaustive pattern matching on types that contain uninhabited types.
+    // Allows exhaustive pattern matching on types that contain uninhabited types
     (active, exhaustive_patterns, "1.13.0", Some(51085), None),
 
-    // Allows all literals in attribute lists and values of key-value pairs.
+    // Allows all literals in attribute lists and values of key-value pairs
     (active, attr_literals, "1.13.0", Some(34981), None),
 
     // Allows untagged unions `union U { ... }`
@@ -334,6 +336,7 @@ declare_features! (
     (active, sanitizer_runtime, "1.17.0", None, None),
 
     // Used to identify crates that contain the profiler runtime
+    //
     // rustc internal
     (active, profiler_runtime, "1.18.0", None, None),
 
@@ -391,7 +394,7 @@ declare_features! (
     // extern types
     (active, extern_types, "1.23.0", Some(43467), None),
 
-    // Allow trait methods with arbitrary self types
+    // Allows trait methods with arbitrary self types
     (active, arbitrary_self_types, "1.23.0", Some(44874), None),
 
     // `crate` in paths
@@ -400,7 +403,7 @@ declare_features! (
     // In-band lifetime bindings (e.g. `fn foo(x: &'a u8) -> &'a u8`)
     (active, in_band_lifetimes, "1.23.0", Some(44524), None),
 
-    // generic associated types (RFC 1598)
+    // Generic associated types (RFC 1598)
     (active, generic_associated_types, "1.23.0", Some(44265), None),
 
     // Resolve absolute paths as paths from other crates
@@ -475,7 +478,7 @@ declare_features! (
     // Scoped lints
     (active, tool_lints, "1.28.0", Some(44690), None),
 
-    // allow irrefutable patterns in if-let and while-let statements (RFC 2086)
+    // Allows irrefutable patterns in if-let and while-let statements (RFC 2086)
     (active, irrefutable_let_patterns, "1.27.0", Some(44495), None),
 
     // Allows use of the :literal macro fragment specifier (RFC 1576)
@@ -505,11 +508,14 @@ declare_features! (
     // impl Debug for Foo<'_>
     (active, impl_header_lifetime_elision, "1.30.0", Some(15872), Some(Edition::Edition2018)),
 
-    // Support for arbitrary delimited token streams in non-macro attributes.
+    // Support for arbitrary delimited token streams in non-macro attributes
     (active, unrestricted_attribute_tokens, "1.30.0", Some(44690), None),
 
-    // Allows `use x::y;` to resolve through `self::x`, not just `::x`.
+    // Allows `use x::y;` to resolve through `self::x`, not just `::x`
     (active, uniform_paths, "1.30.0", Some(53130), None),
+
+    // Allows `Self` in type definitions
+    (active, self_in_typedefs, "1.30.0", Some(49303), None),
 );
 
 declare_features! (

src/test/ui/feature-gates/feature-gate-self-in-typedefs.rs

@@ -0,0 +1,18 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+enum StackList<'a, T: 'a> {
+    Nil,
+    Cons(T, &'a Self)
+    //~^ ERROR cannot find type `Self` in this scope
+    //~| `Self` is only available in traits and impls
+}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-self-in-typedefs.stderr

@@ -0,0 +1,9 @@
+error[E0411]: cannot find type `Self` in this scope
+  --> $DIR/feature-gate-self-in-typedefs.rs:13:17
+   |
+LL |     Cons(T, &'a Self)
+   |                 ^^^^ `Self` is only available in traits and impls
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0411`.