Remove a useless feature gateing

With the planned lazy TAIT system, this will not really make sense anymore anyway.

compiler/rustc_typeck/src/check/check.rs

@@ -94,69 +94,8 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     let declared_ret_ty = fn_sig.output();
 
-    let feature = match tcx.hir().get(fn_id) {
-        // TAIT usage in function return position.
-        // Example:
-        //
-        // ```rust
-        // type Foo = impl Debug;
-        // fn bar() -> Foo { 42 }
-        // ```
-        Node::Item(hir::Item { kind: ItemKind::Fn(..), .. }) |
-        // TAIT usage in associated function return position.
-        //
-        // Example with a free type alias:
-        //
-        // ```rust
-        // type Foo = impl Debug;
-        // impl SomeTrait for SomeType {
-        //     fn bar() -> Foo { 42 }
-        // }
-        // ```
-        //
-        // Example with an associated TAIT:
-        //
-        // ```rust
-        // impl SomeTrait for SomeType {
-        //     type Foo = impl Debug;
-        //     fn bar() -> Self::Foo { 42 }
-        // }
-        // ```
-        Node::ImplItem(hir::ImplItem {
-            kind: hir::ImplItemKind::Fn(..), ..
-        }) => None,
-        // Forbid TAIT in trait declarations for now.
-        // Examples:
-        //
-        // ```rust
-        // type Foo = impl Debug;
-        // trait Bar {
-        //     fn bar() -> Foo;
-        // }
-        // trait Bop {
-        //     type Bop: PartialEq<Foo>;
-        // }
-        // ```
-        Node::TraitItem(hir::TraitItem {
-            kind: hir::TraitItemKind::Fn(..),
-            ..
-        }) |
-        // Forbid TAIT in closure return position for now.
-        // Example:
-        //
-        // ```rust
-        // type Foo = impl Debug;
-        // let x = |y| -> Foo { 42 + y };
-        // ```
-        Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(..), .. }) => Some(sym::type_alias_impl_trait),
-        node => bug!("Item being checked wasn't a function/closure: {:?}", node),
-    };
-    let revealed_ret_ty = fcx.instantiate_opaque_types_from_value(
-        fn_id,
-        declared_ret_ty,
-        decl.output.span(),
-        feature,
-    );
+    let revealed_ret_ty =
+        fcx.instantiate_opaque_types_from_value(fn_id, declared_ret_ty, decl.output.span());
     debug!("check_fn: declared_ret_ty: {}, revealed_ret_ty: {}", declared_ret_ty, revealed_ret_ty);
     fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(revealed_ret_ty)));
     fcx.ret_type_span = Some(decl.output.span());

compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs

@@ -29,12 +29,11 @@ use rustc_middle::ty::{
 };
 use rustc_session::lint;
 use rustc_session::lint::builtin::BARE_TRAIT_OBJECTS;
-use rustc_session::parse::feature_err;
 use rustc_span::edition::Edition;
+use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::{original_sp, DUMMY_SP};
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{self, BytePos, MultiSpan, Span};
-use rustc_span::{hygiene::DesugaringKind, Symbol};
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::opaque_types::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
@@ -368,7 +367,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         parent_id: hir::HirId,
         value: T,
         value_span: Span,
-        feature: Option<Symbol>,
     ) -> T {
         let parent_def_id = self.tcx.hir().local_def_id(parent_id);
         debug!(
@@ -388,19 +386,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let mut infcx = self.infcx.inner.borrow_mut();
 
         for (ty, decl) in opaque_type_map {
-            if let Some(feature) = feature {
-                if let hir::OpaqueTyOrigin::TyAlias = decl.origin {
-                    if !self.tcx.features().enabled(feature) {
-                        feature_err(
-                            &self.tcx.sess.parse_sess,
-                            feature,
-                            value_span,
-                            "type alias impl trait is not permitted here",
-                        )
-                        .emit();
-                    }
-                }
-            }
             let _ = infcx.opaque_types.insert(ty, decl);
             let _ = infcx.opaque_types_vars.insert(decl.concrete_ty, decl.opaque_type);
         }

src/test/ui/type-alias-impl-trait/issue-60371.rs

@@ -11,7 +11,6 @@ impl Bug for &() {
     //~^ ERROR the trait bound `(): Bug` is not satisfied
 
     const FUN: fn() -> Self::Item = || ();
-    //~^ ERROR type alias impl trait is not permitted here
 }
 
 fn main() {}

src/test/ui/type-alias-impl-trait/issue-60371.stderr

@@ -7,15 +7,6 @@ LL |     type Item = impl Bug;
    = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
    = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
 
-error[E0658]: type alias impl trait is not permitted here
-  --> $DIR/issue-60371.rs:13:40
-   |
-LL |     const FUN: fn() -> Self::Item = || ();
-   |                                        ^
-   |
-   = note: see issue #63063 <https://github.com/rust-lang/rust/issues/63063> for more information
-   = help: add `#![feature(type_alias_impl_trait)]` to the crate attributes to enable
-
 error[E0277]: the trait bound `(): Bug` is not satisfied
   --> $DIR/issue-60371.rs:10:17
    |
@@ -25,7 +16,7 @@ LL |     type Item = impl Bug;
    = help: the following implementations were found:
              <&() as Bug>
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0277, E0658.
 For more information about an error, try `rustc --explain E0277`.

src/test/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs

@@ -1,4 +1,5 @@
 // compile-flags: -Zsave-analysis
+// check-pass
 
 #![feature(type_alias_impl_trait, rustc_attrs)]
 
@@ -11,9 +12,7 @@ type T = impl Sized;
 
 fn take(_: fn() -> T) {}
 
-#[rustc_error]
 fn main() {
-    //~^ ERROR fatal error triggered by #[rustc_error]
     take(|| {});
     take(|| {});
 }