Make cenum_impl_drop_cast a hard error

This changes the `cenum_impl_drop_cast` lint to be a hard error. This
lint has been deny-by-default and warning in dependencies since
rust-lang#97652 about 2.5 years ago.

Closes rust-lang#73333

Author: ehuss

compiler/rustc_hir_typeck/src/cast.rs

@@ -831,7 +831,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
             // prim -> prim
             (Int(CEnum), Int(_)) => {
-                self.cenum_impl_drop_lint(fcx);
+                self.err_if_cenum_impl_drop(fcx);
                 Ok(CastKind::EnumCast)
             }
             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
@@ -1091,19 +1091,14 @@ impl<'a, 'tcx> CastCheck<'tcx> {
         }
     }
 
-    fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
+    fn err_if_cenum_impl_drop(&self, fcx: &FnCtxt<'a, 'tcx>) {
         if let ty::Adt(d, _) = self.expr_ty.kind()
             && d.has_dtor(fcx.tcx)
         {
             let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
             let cast_ty = fcx.resolve_vars_if_possible(self.cast_ty);
 
-            fcx.tcx.emit_node_span_lint(
-                lint::builtin::CENUM_IMPL_DROP_CAST,
-                self.expr.hir_id,
-                self.span,
-                errors::CastEnumDrop { expr_ty, cast_ty },
-            );
+            fcx.dcx().emit_err(errors::CastEnumDrop { span: self.span, expr_ty, cast_ty });
         }
     }
 

compiler/rustc_hir_typeck/src/errors.rs

@@ -677,9 +677,11 @@ pub(crate) struct CannotCastToBool<'tcx> {
     pub help: CannotCastToBoolHelp,
 }
 
-#[derive(LintDiagnostic)]
+#[derive(Diagnostic)]
 #[diag(hir_typeck_cast_enum_drop)]
 pub(crate) struct CastEnumDrop<'tcx> {
+    #[primary_span]
+    pub span: Span,
     pub expr_ty: Ty<'tcx>,
     pub cast_ty: Ty<'tcx>,
 }

compiler/rustc_lint/src/lib.rs

@@ -594,6 +594,11 @@ fn register_builtins(store: &mut LintStore) {
          <https://github.com/rust-lang/rust/pull/125380> for more information",
     );
     store.register_removed("unsupported_calling_conventions", "converted into hard error");
+    store.register_removed(
+        "cenum_impl_drop_cast",
+        "converted into hard error, \
+         see <https://github.com/rust-lang/rust/issues/73333> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint_defs/src/builtin.rs

@@ -27,7 +27,6 @@ declare_lint_pass! {
         BARE_TRAIT_OBJECTS,
         BINDINGS_WITH_VARIANT_NAME,
         BREAK_WITH_LABEL_AND_LOOP,
-        CENUM_IMPL_DROP_CAST,
         COHERENCE_LEAK_CHECK,
         CONFLICTING_REPR_HINTS,
         CONST_EVALUATABLE_UNCHECKED,
@@ -2612,58 +2611,6 @@ declare_lint! {
     @edition Edition2024 => Warn;
 }
 
-declare_lint! {
-    /// The `cenum_impl_drop_cast` lint detects an `as` cast of a field-less
-    /// `enum` that implements [`Drop`].
-    ///
-    /// [`Drop`]: https://doc.rust-lang.org/std/ops/trait.Drop.html
-    ///
-    /// ### Example
-    ///
-    /// ```rust,compile_fail
-    /// # #![allow(unused)]
-    /// enum E {
-    ///     A,
-    /// }
-    ///
-    /// impl Drop for E {
-    ///     fn drop(&mut self) {
-    ///         println!("Drop");
-    ///     }
-    /// }
-    ///
-    /// fn main() {
-    ///     let e = E::A;
-    ///     let i = e as u32;
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// Casting a field-less `enum` that does not implement [`Copy`] to an
-    /// integer moves the value without calling `drop`. This can result in
-    /// surprising behavior if it was expected that `drop` should be called.
-    /// Calling `drop` automatically would be inconsistent with other move
-    /// operations. Since neither behavior is clear or consistent, it was
-    /// decided that a cast of this nature will no longer be allowed.
-    ///
-    /// This is a [future-incompatible] lint to transition this to a hard error
-    /// in the future. See [issue #73333] for more details.
-    ///
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    /// [issue #73333]: https://github.com/rust-lang/rust/issues/73333
-    /// [`Copy`]: https://doc.rust-lang.org/std/marker/trait.Copy.html
-    pub CENUM_IMPL_DROP_CAST,
-    Deny,
-    "a C-like enum implementing Drop is cast",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
-        reference: "issue #73333 <https://github.com/rust-lang/rust/issues/73333>",
-    };
-}
-
 declare_lint! {
     /// The `fuzzy_provenance_casts` lint detects an `as` cast between an integer
     /// and a pointer.

tests/mir-opt/building/enum_cast.droppy.built.after.mir

@@ -41,27 +41,6 @@ fn far(far: Far) -> isize {
     far as isize
 }
 
-// EMIT_MIR enum_cast.droppy.built.after.mir
-enum Droppy {
-    A,
-    B,
-    C,
-}
-
-impl Drop for Droppy {
-    fn drop(&mut self) {}
-}
-
-fn droppy() {
-    {
-        let x = Droppy::C;
-        // remove this entire test once `cenum_impl_drop_cast` becomes a hard error
-        #[allow(cenum_impl_drop_cast)]
-        let y = x as usize;
-    }
-    let z = Droppy::B;
-}
-
 #[repr(i16)]
 enum SignedAroundZero {
     A = -2,

tests/mir-opt/building/enum_cast.rs

@@ -1,5 +1,3 @@
-#![deny(cenum_impl_drop_cast)]
-
 enum E {
     A = 0,
 }
@@ -14,5 +12,4 @@ fn main() {
     let e = E::A;
     let i = e as u32;
     //~^ ERROR cannot cast enum `E` into integer `u32` because it implements `Drop`
-    //~| WARN this was previously accepted
 }

tests/ui/cenum_impl_drop_cast.rs

@@ -1,31 +1,8 @@
 error: cannot cast enum `E` into integer `u32` because it implements `Drop`
-  --> $DIR/cenum_impl_drop_cast.rs:15:13
+  --> $DIR/cenum_impl_drop_cast.rs:13:13
    |
 LL |     let i = e as u32;
    |             ^^^^^^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #73333 <https://github.com/rust-lang/rust/issues/73333>
-note: the lint level is defined here
-  --> $DIR/cenum_impl_drop_cast.rs:1:9
-   |
-LL | #![deny(cenum_impl_drop_cast)]
-   |         ^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 
-Future incompatibility report: Future breakage diagnostic:
-error: cannot cast enum `E` into integer `u32` because it implements `Drop`
-  --> $DIR/cenum_impl_drop_cast.rs:15:13
-   |
-LL |     let i = e as u32;
-   |             ^^^^^^^^
-   |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #73333 <https://github.com/rust-lang/rust/issues/73333>
-note: the lint level is defined here
-  --> $DIR/cenum_impl_drop_cast.rs:1:9
-   |
-LL | #![deny(cenum_impl_drop_cast)]
-   |         ^^^^^^^^^^^^^^^^^^^^
-