Auto merge of #103454 - camsteffen:remove-conservatively-uninhabited, r=oli-obk

Factor out `conservative_is_privately_uninhabited`

After #102660 there is no more need for `conservative_is_privately_uninhabited`.

r? `@oli-obk`

19 files changed, 99 insertions, 163 deletions

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 7d36a63943c..9b6c7d27c79 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -1564,10 +1564,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                 }
             }
             None => {
-                if !self
-                    .tcx()
-                    .conservative_is_privately_uninhabited(self.param_env.and(sig.output()))
-                {
+                if !sig.output().is_privately_uninhabited(self.tcx(), self.param_env) {
                     span_mirbug!(self, term, "call to converging function {:?} w/o dest", sig);
                 }
             }
diff --git a/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs b/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
index 122ad7009cb..a78b294181c 100644
--- a/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
+++ b/compiler/rustc_hir_typeck/src/generator_interior/drop_ranges/cfg_build.rs
@@ -217,7 +217,7 @@ impl<'a, 'tcx> DropRangeVisitor<'a, 'tcx> {
         let ty = self.tcx.erase_regions(ty);
         let m = self.tcx.parent_module(expr.hir_id).to_def_id();
         let param_env = self.tcx.param_env(m.expect_local());
-        if self.tcx.is_ty_uninhabited_from(m, ty, param_env) {
+        if !ty.is_inhabited_from(self.tcx, m, param_env) {
             // This function will not return. We model this fact as an infinite loop.
             self.drop_ranges.add_control_edge(self.expr_index + 1, self.expr_index + 1);
         }
diff --git a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
index 7bbfb70f2c3..bd3589c6c43 100644
--- a/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
+++ b/compiler/rustc_hir_typeck/src/generator_interior/mod.rs
@@ -542,10 +542,10 @@ fn check_must_not_suspend_ty<'tcx>(
     data: SuspendCheckData<'_, 'tcx>,
 ) -> bool {
     if ty.is_unit()
-    // FIXME: should this check `is_ty_uninhabited_from`. This query is not available in this stage
+    // FIXME: should this check `Ty::is_inhabited_from`. This query is not available in this stage
     // of typeck (before ReVar and RePlaceholder are removed), but may remove noise, like in
     // `must_use`
-    // || fcx.tcx.is_ty_uninhabited_from(fcx.tcx.parent_module(hir_id).to_def_id(), ty, fcx.param_env)
+    // || !ty.is_inhabited_from(fcx.tcx, fcx.tcx.parent_module(hir_id).to_def_id(), fcx.param_env)
     {
         return false;
     }
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index cdf279313a6..100a8788763 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -201,9 +201,9 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             plural_len: usize,
         ) -> bool {
             if ty.is_unit()
-                || cx.tcx.is_ty_uninhabited_from(
+                || !ty.is_inhabited_from(
+                    cx.tcx,
                     cx.tcx.parent_module(expr.hir_id).to_def_id(),
-                    ty,
                     cx.param_env,
                 )
             {
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 02772013437..21097b1fec6 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -2078,17 +2078,6 @@ rustc_queries! {
         desc { "normalizing opaque types in `{:?}`", key }
     }
 
-    /// Checks whether a type is definitely uninhabited. This is
-    /// conservative: for some types that are uninhabited we return `false`,
-    /// but we only return `true` for types that are definitely uninhabited.
-    /// `ty.conservative_is_privately_uninhabited` implies that any value of type `ty`
-    /// will be `Abi::Uninhabited`. (Note that uninhabited types may have nonzero
-    /// size, to account for partial initialisation. See #49298 for details.)
-    query conservative_is_privately_uninhabited(key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> bool {
-        desc { "conservatively checking if `{}` is privately uninhabited", key.value }
-        remap_env_constness
-    }
-
     query limits(key: ()) -> Limits {
         desc { "looking up limits" }
     }
diff --git a/compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs b/compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs
index b7aa455727d..33f72729798 100644
--- a/compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs
+++ b/compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs
@@ -41,6 +41,13 @@ impl<'tcx> InhabitedPredicate<'tcx> {
         self.apply_inner(tcx, param_env, &|_| Err(())).ok()
     }
 
+    /// Same as `apply`, but `NotInModule(_)` predicates yield `false`. That is,
+    /// privately uninhabited types are considered always uninhabited.
+    pub fn apply_ignore_module(self, tcx: TyCtxt<'tcx>, param_env: ParamEnv<'tcx>) -> bool {
+        let Ok(result) = self.apply_inner::<!>(tcx, param_env, &|_| Ok(true));
+        result
+    }
+
     fn apply_inner<E>(
         self,
         tcx: TyCtxt<'tcx>,
diff --git a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
index 786eced61d6..ace81bc4f83 100644
--- a/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
+++ b/compiler/rustc_middle/src/ty/inhabitedness/mod.rs
@@ -29,7 +29,7 @@
 //! }
 //! ```
 //! In this code, the type `Foo` will only be visibly uninhabited inside the
-//! modules `b`, `c` and `d`. Calling `uninhabited_predicate` on `Foo` will
+//! modules `b`, `c` and `d`. Calling `inhabited_predicate` on `Foo` will
 //! return `NotInModule(b) AND NotInModule(c)`.
 //!
 //! We need this information for pattern-matching on `Foo` or types that contain
@@ -57,57 +57,6 @@ pub(crate) fn provide(providers: &mut ty::query::Providers) {
         ty::query::Providers { inhabited_predicate_adt, inhabited_predicate_type, ..*providers };
 }
 
-impl<'tcx> TyCtxt<'tcx> {
-    /// Checks whether a type is visibly uninhabited from a particular module.
-    ///
-    /// # Example
-    /// ```
-    /// #![feature(never_type)]
-    /// # fn main() {}
-    /// enum Void {}
-    /// mod a {
-    ///     pub mod b {
-    ///         pub struct SecretlyUninhabited {
-    ///             _priv: !,
-    ///         }
-    ///     }
-    /// }
-    ///
-    /// mod c {
-    ///     use super::Void;
-    ///     pub struct AlsoSecretlyUninhabited {
-    ///         _priv: Void,
-    ///     }
-    ///     mod d {
-    ///     }
-    /// }
-    ///
-    /// struct Foo {
-    ///     x: a::b::SecretlyUninhabited,
-    ///     y: c::AlsoSecretlyUninhabited,
-    /// }
-    /// ```
-    /// In this code, the type `Foo` will only be visibly uninhabited inside the
-    /// modules b, c and d. This effects pattern-matching on `Foo` or types that
-    /// contain `Foo`.
-    ///
-    /// # Example
-    /// ```ignore (illustrative)
-    /// let foo_result: Result<T, Foo> = ... ;
-    /// let Ok(t) = foo_result;
-    /// ```
-    /// This code should only compile in modules where the uninhabitedness of Foo is
-    /// visible.
-    pub fn is_ty_uninhabited_from(
-        self,
-        module: DefId,
-        ty: Ty<'tcx>,
-        param_env: ty::ParamEnv<'tcx>,
-    ) -> bool {
-        !ty.inhabited_predicate(self).apply(self, param_env, module)
-    }
-}
-
 /// Returns an `InhabitedPredicate` that is generic over type parameters and
 /// requires calling [`InhabitedPredicate::subst`]
 fn inhabited_predicate_adt(tcx: TyCtxt<'_>, def_id: DefId) -> InhabitedPredicate<'_> {
@@ -171,6 +120,64 @@ impl<'tcx> Ty<'tcx> {
             _ => InhabitedPredicate::True,
         }
     }
+
+    /// Checks whether a type is visibly uninhabited from a particular module.
+    ///
+    /// # Example
+    /// ```
+    /// #![feature(never_type)]
+    /// # fn main() {}
+    /// enum Void {}
+    /// mod a {
+    ///     pub mod b {
+    ///         pub struct SecretlyUninhabited {
+    ///             _priv: !,
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// mod c {
+    ///     use super::Void;
+    ///     pub struct AlsoSecretlyUninhabited {
+    ///         _priv: Void,
+    ///     }
+    ///     mod d {
+    ///     }
+    /// }
+    ///
+    /// struct Foo {
+    ///     x: a::b::SecretlyUninhabited,
+    ///     y: c::AlsoSecretlyUninhabited,
+    /// }
+    /// ```
+    /// In this code, the type `Foo` will only be visibly uninhabited inside the
+    /// modules b, c and d. This effects pattern-matching on `Foo` or types that
+    /// contain `Foo`.
+    ///
+    /// # Example
+    /// ```ignore (illustrative)
+    /// let foo_result: Result<T, Foo> = ... ;
+    /// let Ok(t) = foo_result;
+    /// ```
+    /// This code should only compile in modules where the uninhabitedness of Foo is
+    /// visible.
+    pub fn is_inhabited_from(
+        self,
+        tcx: TyCtxt<'tcx>,
+        module: DefId,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        self.inhabited_predicate(tcx).apply(tcx, param_env, module)
+    }
+
+    /// Returns true if the type is uninhabited without regard to visibility
+    pub fn is_privately_uninhabited(
+        self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        !self.inhabited_predicate(tcx).apply_ignore_module(tcx, param_env)
+    }
 }
 
 /// N.B. this query should only be called through `Ty::inhabited_predicate`
diff --git a/compiler/rustc_mir_build/src/build/expr/into.rs b/compiler/rustc_mir_build/src/build/expr/into.rs
index 24ecd0a5399..efb6eba7e57 100644
--- a/compiler/rustc_mir_build/src/build/expr/into.rs
+++ b/compiler/rustc_mir_build/src/build/expr/into.rs
@@ -271,15 +271,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         // MIR checks and ultimately whether code is accepted or not. We can only
                         // omit the return edge if a return type is visibly uninhabited to a module
                         // that makes the call.
-                        target: if this.tcx.is_ty_uninhabited_from(
-                            this.parent_module,
-                            expr.ty,
-                            this.param_env,
-                        ) {
-                            None
-                        } else {
-                            Some(success)
-                        },
+                        target: expr
+                            .ty
+                            .is_inhabited_from(this.tcx, this.parent_module, this.param_env)
+                            .then_some(success),
                         from_hir_call,
                         fn_span,
                     },
diff --git a/compiler/rustc_mir_build/src/build/matches/simplify.rs b/compiler/rustc_mir_build/src/build/matches/simplify.rs
index 924d2f555b9..caf27eb39d7 100644
--- a/compiler/rustc_mir_build/src/build/matches/simplify.rs
+++ b/compiler/rustc_mir_build/src/build/matches/simplify.rs
@@ -264,10 +264,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let irrefutable = adt_def.variants().iter_enumerated().all(|(i, v)| {
                     i == variant_index || {
                         self.tcx.features().exhaustive_patterns
-                            && v.inhabited_predicate(self.tcx, adt_def)
+                            && !v
+                                .inhabited_predicate(self.tcx, adt_def)
                                 .subst(self.tcx, substs)
-                                .apply_any_module(self.tcx, self.param_env)
-                                != Some(true)
+                                .apply_ignore_module(self.tcx, self.param_env)
                     }
                 }) && (adt_def.did().is_local()
                     || !adt_def.is_variant_list_non_exhaustive());
diff --git a/compiler/rustc_mir_build/src/thir/pattern/check_match.rs b/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
index 93a3dd8962a..0df228a0d59 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/check_match.rs
@@ -818,7 +818,7 @@ fn non_exhaustive_match<'p, 'tcx>(
         }
     }
     if let ty::Ref(_, sub_ty, _) = scrut_ty.kind() {
-        if cx.tcx.is_ty_uninhabited_from(cx.module, *sub_ty, cx.param_env) {
+        if !sub_ty.is_inhabited_from(cx.tcx, cx.module, cx.param_env) {
             err.note("references are always considered inhabited");
         }
     }
diff --git a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
index 8dc9976eaea..3e370a05376 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
@@ -324,7 +324,7 @@ pub(crate) struct MatchCheckCtxt<'p, 'tcx> {
 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
     pub(super) fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
         if self.tcx.features().exhaustive_patterns {
-            self.tcx.is_ty_uninhabited_from(self.module, ty, self.param_env)
+            !ty.is_inhabited_from(self.tcx, self.module, self.param_env)
         } else {
             false
         }
diff --git a/compiler/rustc_mir_transform/src/generator.rs b/compiler/rustc_mir_transform/src/generator.rs
index c833de3a8a7..fcd63b6cfa1 100644
--- a/compiler/rustc_mir_transform/src/generator.rs
+++ b/compiler/rustc_mir_transform/src/generator.rs
@@ -1015,7 +1015,7 @@ fn insert_panic_block<'tcx>(
 
 fn can_return<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
     // Returning from a function with an uninhabited return type is undefined behavior.
-    if tcx.conservative_is_privately_uninhabited(param_env.and(body.return_ty())) {
+    if body.return_ty().is_privately_uninhabited(tcx, param_env) {
         return false;
     }
 
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index 0100860afb9..58e1fe937a6 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -1284,20 +1284,19 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     fn check_is_ty_uninhabited(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
         let ty = self.typeck_results.expr_ty(expr);
         let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
-        if self.ir.tcx.is_ty_uninhabited_from(m, ty, self.param_env) {
-            match self.ir.lnks[succ] {
-                LiveNodeKind::ExprNode(succ_span, succ_id) => {
-                    self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "expression");
-                }
-                LiveNodeKind::VarDefNode(succ_span, succ_id) => {
-                    self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "definition");
-                }
-                _ => {}
-            };
-            self.exit_ln
-        } else {
-            succ
+        if ty.is_inhabited_from(self.ir.tcx, m, self.param_env) {
+            return succ;
         }
+        match self.ir.lnks[succ] {
+            LiveNodeKind::ExprNode(succ_span, succ_id) => {
+                self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "expression");
+            }
+            LiveNodeKind::VarDefNode(succ_span, succ_id) => {
+                self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "definition");
+            }
+            _ => {}
+        };
+        self.exit_ln
     }
 
     fn warn_about_unreachable(
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index b59be0a0ea7..92e8542795f 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -442,8 +442,7 @@ fn layout_of_uncached<'tcx>(
             let element = cx.layout_of(element)?;
             let size = element.size.checked_mul(count, dl).ok_or(LayoutError::SizeOverflow(ty))?;
 
-            let abi = if count != 0 && tcx.conservative_is_privately_uninhabited(param_env.and(ty))
-            {
+            let abi = if count != 0 && ty.is_privately_uninhabited(tcx, param_env) {
                 Abi::Uninhabited
             } else {
                 Abi::Aggregate { sized: true }
diff --git a/compiler/rustc_ty_utils/src/layout_sanity_check.rs b/compiler/rustc_ty_utils/src/layout_sanity_check.rs
index 100926ad446..ee5e7bc2359 100644
--- a/compiler/rustc_ty_utils/src/layout_sanity_check.rs
+++ b/compiler/rustc_ty_utils/src/layout_sanity_check.rs
@@ -12,7 +12,7 @@ pub(super) fn sanity_check_layout<'tcx>(
     layout: &TyAndLayout<'tcx>,
 ) {
     // Type-level uninhabitedness should always imply ABI uninhabitedness.
-    if cx.tcx.conservative_is_privately_uninhabited(cx.param_env.and(layout.ty)) {
+    if layout.ty.is_privately_uninhabited(cx.tcx, cx.param_env) {
         assert!(layout.abi.is_uninhabited());
     }
 
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index 99d3bda6ebf..c60ade360df 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -416,62 +416,6 @@ fn asyncness(tcx: TyCtxt<'_>, def_id: DefId) -> hir::IsAsync {
     node.fn_sig().map_or(hir::IsAsync::NotAsync, |sig| sig.header.asyncness)
 }
 
-/// Don't call this directly: use ``tcx.conservative_is_privately_uninhabited`` instead.
-pub fn conservative_is_privately_uninhabited_raw<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    param_env_and: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
-) -> bool {
-    let (param_env, ty) = param_env_and.into_parts();
-    match ty.kind() {
-        ty::Never => {
-            debug!("ty::Never =>");
-            true
-        }
-        ty::Adt(def, _) if def.is_union() => {
-            debug!("ty::Adt(def, _) if def.is_union() =>");
-            // For now, `union`s are never considered uninhabited.
-            false
-        }
-        ty::Adt(def, substs) => {
-            debug!("ty::Adt(def, _) if def.is_not_union() =>");
-            // Any ADT is uninhabited if either:
-            // (a) It has no variants (i.e. an empty `enum`);
-            // (b) Each of its variants (a single one in the case of a `struct`) has at least
-            //     one uninhabited field.
-            def.variants().iter().all(|var| {
-                var.fields.iter().any(|field| {
-                    let ty = tcx.bound_type_of(field.did).subst(tcx, substs);
-                    tcx.conservative_is_privately_uninhabited(param_env.and(ty))
-                })
-            })
-        }
-        ty::Tuple(fields) => {
-            debug!("ty::Tuple(..) =>");
-            fields.iter().any(|ty| tcx.conservative_is_privately_uninhabited(param_env.and(ty)))
-        }
-        ty::Array(ty, len) => {
-            debug!("ty::Array(ty, len) =>");
-            match len.try_eval_usize(tcx, param_env) {
-                Some(0) | None => false,
-                // If the array is definitely non-empty, it's uninhabited if
-                // the type of its elements is uninhabited.
-                Some(1..) => tcx.conservative_is_privately_uninhabited(param_env.and(*ty)),
-            }
-        }
-        ty::Ref(..) => {
-            debug!("ty::Ref(..) =>");
-            // References to uninitialised memory is valid for any type, including
-            // uninhabited types, in unsafe code, so we treat all references as
-            // inhabited.
-            false
-        }
-        _ => {
-            debug!("_ =>");
-            false
-        }
-    }
-}
-
 pub fn provide(providers: &mut ty::query::Providers) {
     *providers = ty::query::Providers {
         asyncness,
@@ -481,7 +425,6 @@ pub fn provide(providers: &mut ty::query::Providers) {
         instance_def_size_estimate,
         issue33140_self_ty,
         impl_defaultness,
-        conservative_is_privately_uninhabited: conservative_is_privately_uninhabited_raw,
         ..*providers
     };
 }
diff --git a/src/test/ui/const-generics/conservative_is_privately_uninhabited_uses_correct_param_env-1.rs b/src/test/ui/const-generics/inhabited-assoc-ty-ice-1.rs
index c9e26c302bf..b385406b020 100644
--- a/src/test/ui/const-generics/conservative_is_privately_uninhabited_uses_correct_param_env-1.rs
+++ b/src/test/ui/const-generics/inhabited-assoc-ty-ice-1.rs
@@ -2,7 +2,7 @@
 #![feature(generic_const_exprs)]
 #![allow(incomplete_features)]
 
-// This tests that the `conservative_is_privately_uninhabited` fn doesn't cause
+// This tests that the inhabited check doesn't cause
 // ICEs by trying to evaluate `T::ASSOC` with an incorrect `ParamEnv`.
 
 trait Foo {
diff --git a/src/test/ui/const-generics/conservative_is_privately_uninhabited_uses_correct_param_env-2.rs b/src/test/ui/const-generics/inhabited-assoc-ty-ice-2.rs
index 3017920fc98..216d29c7cd4 100644
--- a/src/test/ui/const-generics/conservative_is_privately_uninhabited_uses_correct_param_env-2.rs
+++ b/src/test/ui/const-generics/inhabited-assoc-ty-ice-2.rs
@@ -2,7 +2,7 @@
 #![feature(generic_const_exprs)]
 #![allow(incomplete_features)]
 
-// This tests that the `conservative_is_privately_uninhabited` fn doesn't cause
+// This tests that the inhabited check doesn't cause
 // ICEs by trying to evaluate `T::ASSOC` with an incorrect `ParamEnv`.
 
 trait Foo {
diff --git a/src/test/ui/pattern/usefulness/uninhabited.rs b/src/test/ui/pattern/usefulness/uninhabited.rs
index 77cd0f4005e..5622808d4c7 100644
--- a/src/test/ui/pattern/usefulness/uninhabited.rs
+++ b/src/test/ui/pattern/usefulness/uninhabited.rs
@@ -2,7 +2,7 @@
 // aux-build:empty.rs
 //
 // This tests plays with matching and uninhabited types. This also serves as a test for the
-// `tcx.is_ty_uninhabited_from()` function.
+// `Ty::is_inhabited_from` function.
 #![feature(never_type)]
 #![feature(never_type_fallback)]
 #![feature(exhaustive_patterns)]