Revert stabilization of `feature(never_type)`.

This commit is just covering the feature gate itself and the tests
that made direct use of `!` and thus need to opt back into the
feature.

A follow on commit brings back the other change that motivates the
revert: Namely, going back to the old rules for falling back to `()`.

src/libcore/clone.rs

@@ -179,7 +179,7 @@ mod impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Clone for ! {
         #[inline]
         fn clone(&self) -> Self {

src/libcore/cmp.rs

@@ -881,24 +881,24 @@ mod impls {
 
     ord_impl! { char usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialEq for ! {
         fn eq(&self, _: &!) -> bool {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Eq for ! {}
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl PartialOrd for ! {
         fn partial_cmp(&self, _: &!) -> Option<Ordering> {
             *self
         }
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Ord for ! {
         fn cmp(&self, _: &!) -> Ordering {
             *self

src/libcore/fmt/mod.rs

@@ -1780,14 +1780,14 @@ macro_rules! fmt_refs {
 
 fmt_refs! { Debug, Display, Octal, Binary, LowerHex, UpperHex, LowerExp, UpperExp }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Debug for ! {
     fn fmt(&self, _: &mut Formatter) -> Result {
         *self
     }
 }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Display for ! {
     fn fmt(&self, _: &mut Formatter) -> Result {
         *self

src/libcore/lib.rs

@@ -82,6 +82,7 @@
 #![feature(iterator_repeat_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(macro_at_most_once_rep)]
 #![feature(no_core)]

src/libcore/marker.rs

@@ -630,7 +630,7 @@ mod copy_impls {
         bool char
     }
 
-    #[stable(feature = "never_type", since = "1.26.0")]
+    #[unstable(feature = "never_type", issue = "35121")]
     impl Copy for ! {}
 
     #[stable(feature = "rust1", since = "1.0.0")]

src/librustc/lib.rs

@@ -52,6 +52,7 @@
 #![cfg_attr(windows, feature(libc))]
 #![feature(macro_lifetime_matcher)]
 #![feature(macro_vis_matcher)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(non_exhaustive)]
 #![feature(nonzero)]

src/librustc_mir/build/matches/simplify.rs

@@ -113,6 +113,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             PatternKind::Variant { adt_def, substs, variant_index, ref subpatterns } => {
                 let irrefutable = adt_def.variants.iter().enumerate().all(|(i, v)| {
                     i == variant_index || {
+                        self.hir.tcx().features().never_type &&
                         self.hir.tcx().features().exhaustive_patterns &&
                         self.hir.tcx().is_variant_uninhabited_from_all_modules(v, substs)
                     }

src/libstd/error.rs

@@ -233,7 +233,7 @@ impl<'a> From<Cow<'a, str>> for Box<Error> {
     }
 }
 
-#[stable(feature = "never_type", since = "1.26.0")]
+#[unstable(feature = "never_type", issue = "35121")]
 impl Error for ! {
     fn description(&self) -> &str { *self }
 }

src/libstd/lib.rs

@@ -275,6 +275,7 @@
 #![feature(macro_reexport)]
 #![feature(macro_vis_matcher)]
 #![feature(needs_panic_runtime)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(nonzero)]
 #![feature(num_bits_bytes)]

src/libsyntax/feature_gate.rs

@@ -272,6 +272,9 @@ declare_features! (
     // Allows cfg(target_has_atomic = "...").
     (active, cfg_target_has_atomic, "1.9.0", Some(32976), None),
 
+    // 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.
     (active, exhaustive_patterns, "1.13.0", None, None),
 
@@ -1635,6 +1638,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             ast::TyKind::BareFn(ref bare_fn_ty) => {
                 self.check_abi(bare_fn_ty.abi, ty.span);
             }
+            ast::TyKind::Never => {
+                gate_feature_post!(&self, never_type, ty.span,
+                                   "The `!` type is experimental");
+            }
             ast::TyKind::TraitObject(_, ast::TraitObjectSyntax::Dyn) => {
                 gate_feature_post!(&self, dyn_trait, ty.span,
                                    "`dyn Trait` syntax is unstable");

src/test/compile-fail/call-fn-never-arg-wrong-type.rs

@@ -10,6 +10,8 @@
 
 // Test that we can't pass other types for !
 
+#![feature(never_type)]
+
 fn foo(x: !) -> ! {
     x
 }

src/test/compile-fail/coerce-to-bang-cast.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
+
 fn foo(x: usize, y: !, z: usize) { }
 
 fn cast_a() {

src/test/compile-fail/coerce-to-bang.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
+
 fn foo(x: usize, y: !, z: usize) { }
 
 fn call_foo_a() {

src/test/compile-fail/inhabitedness-infinite-loop.rs

@@ -10,6 +10,7 @@
 
 // error-pattern:reached recursion limit
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 
 struct Foo<'a, T: 'a> {

src/test/compile-fail/loop-break-value.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
+
 fn main() {
     let val: ! = loop { break break; };
     //~^ ERROR mismatched types

src/test/compile-fail/match-privately-empty.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 
 mod private {

src/test/compile-fail/never-assign-dead-code.rs

@@ -10,6 +10,7 @@
 
 // Test that an assignment of type ! makes the rest of the block dead code.
 
+#![feature(never_type)]
 #![feature(rustc_attrs)]
 #![warn(unused)]
 

src/test/compile-fail/never-assign-wrong-type.rs

@@ -10,6 +10,7 @@
 
 // Test that we can't use another type in place of !
 
+#![feature(never_type)]
 #![deny(warnings)]
 
 fn main() {

src/test/compile-fail/uninhabited-irrefutable.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 
 mod foo {

src/test/compile-fail/uninhabited-patterns.rs

@@ -10,6 +10,7 @@
 
 #![feature(box_patterns)]
 #![feature(box_syntax)]
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(slice_patterns)]
 #![deny(unreachable_patterns)]

src/test/compile-fail/unreachable-loop-patterns.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![deny(unreachable_patterns)]
 

src/test/compile-fail/unreachable-try-pattern.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns, rustc_attrs)]
 #![warn(unreachable_code)]
 #![warn(unreachable_patterns)]

src/test/run-pass/diverging-fallback-control-flow.rs

@@ -14,6 +14,8 @@
 // These represent current behavior, but are pretty dubious.  I would
 // like to revisit these and potentially change them. --nmatsakis
 
+#![feature(never_type)]
+
 trait BadDefault {
     fn default() -> Self;
 }

src/test/run-pass/empty-types-in-patterns.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 #![feature(slice_patterns)]
 #![allow(unreachable_patterns)]

src/test/run-pass/impl-for-never.rs

@@ -10,6 +10,8 @@
 
 // Test that we can call static methods on ! both directly and when it appears in a generic
 
+#![feature(never_type)]
+
 trait StringifyType {
     fn stringify_type() -> &'static str;
 }

src/test/run-pass/issue-44402.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 #![feature(exhaustive_patterns)]
 
 // Regression test for inhabitedness check. The old

src/test/run-pass/loop-break-value.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
+
 #[allow(unused)]
 fn never_returns() {
     loop {

src/test/run-pass/mir_calls_to_shims.rs

@@ -11,6 +11,7 @@
 // ignore-wasm32-bare compiled with panic=abort by default
 
 #![feature(fn_traits)]
+#![feature(never_type)]
 
 use std::panic;
 

src/test/run-pass/never-result.rs

@@ -10,6 +10,8 @@
 
 // Test that we can extract a ! through pattern matching then use it as several different types.
 
+#![feature(never_type)]
+
 fn main() {
     let x: Result<u32, !> = Ok(123);
     match x {

src/test/run-pass/type-sizes.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(never_type)]
 
 use std::mem::size_of;
 

src/test/ui/feature-gate-exhaustive-patterns.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 fn foo() -> Result<u32, !> {
     Ok(123)
 }

src/test/ui/feature-gate-never_type.rs

@@ -0,0 +1,27 @@
+// Copyright 2012 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.
+
+// Test that ! errors when used in illegal positions with feature(never_type) disabled
+
+trait Foo {
+    type Wub;
+}
+
+type Ma = (u32, !, i32); //~ ERROR type is experimental
+type Meeshka = Vec<!>; //~ ERROR type is experimental
+type Mow = &fn(!) -> !; //~ ERROR type is experimental
+type Skwoz = &mut !; //~ ERROR type is experimental
+
+impl Foo for Meeshka {
+    type Wub = !; //~ ERROR type is experimental
+}
+
+fn main() {
+}

src/test/ui/feature-gate-never_type.stderr

@@ -0,0 +1,43 @@
+error[E0658]: The `!` type is experimental (see issue #35121)
+  --> $DIR/feature-gate-never_type.rs:17:17
+   |
+LL | type Ma = (u32, !, i32); //~ ERROR type is experimental
+   |                 ^
+   |
+   = help: add #![feature(never_type)] to the crate attributes to enable
+
+error[E0658]: The `!` type is experimental (see issue #35121)
+  --> $DIR/feature-gate-never_type.rs:18:20
+   |
+LL | type Meeshka = Vec<!>; //~ ERROR type is experimental
+   |                    ^
+   |
+   = help: add #![feature(never_type)] to the crate attributes to enable
+
+error[E0658]: The `!` type is experimental (see issue #35121)
+  --> $DIR/feature-gate-never_type.rs:19:16
+   |
+LL | type Mow = &fn(!) -> !; //~ ERROR type is experimental
+   |                ^
+   |
+   = help: add #![feature(never_type)] to the crate attributes to enable
+
+error[E0658]: The `!` type is experimental (see issue #35121)
+  --> $DIR/feature-gate-never_type.rs:20:19
+   |
+LL | type Skwoz = &mut !; //~ ERROR type is experimental
+   |                   ^
+   |
+   = help: add #![feature(never_type)] to the crate attributes to enable
+
+error[E0658]: The `!` type is experimental (see issue #35121)
+  --> $DIR/feature-gate-never_type.rs:23:16
+   |
+LL |     type Wub = !; //~ ERROR type is experimental
+   |                ^
+   |
+   = help: add #![feature(never_type)] to the crate attributes to enable
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/print_type_sizes/uninhabited.rs

@@ -11,6 +11,7 @@
 // compile-flags: -Z print-type-sizes
 // compile-pass
 
+#![feature(never_type)]
 #![feature(start)]
 
 #[start]

src/test/ui/reachable/expr_add.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 #![allow(unused_variables)]
 #![deny(unreachable_code)]
 

src/test/ui/reachable/expr_assign.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 #![allow(unused_variables)]
 #![allow(unused_assignments)]
 #![allow(dead_code)]

src/test/ui/reachable/expr_call.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 #![allow(unused_variables)]
 #![allow(unused_assignments)]
 #![allow(dead_code)]

src/test/ui/reachable/expr_cast.rs

@@ -12,7 +12,7 @@
 #![allow(unused_assignments)]
 #![allow(dead_code)]
 #![deny(unreachable_code)]
-#![feature(type_ascription)]
+#![feature(never_type, type_ascription)]
 
 fn a() {
     // the cast is unreachable:

src/test/ui/reachable/expr_method.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 #![allow(unused_variables)]
 #![allow(unused_assignments)]
 #![allow(dead_code)]

src/test/ui/reachable/expr_type.rs

@@ -12,7 +12,7 @@
 #![allow(unused_assignments)]
 #![allow(dead_code)]
 #![deny(unreachable_code)]
-#![feature(type_ascription)]
+#![feature(never_type, type_ascription)]
 
 fn a() {
     // the cast is unreachable:

src/test/ui/reachable/expr_unary.rs

@@ -7,7 +7,7 @@
 // <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.
-
+#![feature(never_type)]
 #![allow(unused_variables)]
 #![allow(unused_assignments)]
 #![allow(dead_code)]