Rollup merge of #110859 - compiler-errors:no-negative-drop-impls, r=oli-obk

Explicitly reject negative and reservation drop impls

Fixes #110858

It doesn't really make sense for a type to have a `!Drop` impl. Or at least, I don't want us to implicitly assign a meaning to it by the way the compiler *currently* handles it (incompletely), and rather I would like to see a PR (or an RFC...) assign a meaning to `!Drop` if we actually wanted one for it.

compiler/rustc_hir_analysis/src/check/dropck.rs

@@ -1,7 +1,6 @@
 // FIXME(@lcnr): Move this module out of `rustc_hir_analysis`.
 //
 // We don't do any drop checking during hir typeck.
-use crate::hir::def_id::{DefId, LocalDefId};
 use rustc_errors::{struct_span_err, ErrorGuaranteed};
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
@@ -9,6 +8,9 @@ use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::util::IgnoreRegions;
 use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
 
+use crate::errors;
+use crate::hir::def_id::{DefId, LocalDefId};
+
 /// This function confirms that the `Drop` implementation identified by
 /// `drop_impl_did` is not any more specialized than the type it is
 /// attached to (Issue #8142).
@@ -27,6 +29,19 @@ use rustc_middle::ty::{self, Predicate, Ty, TyCtxt};
 ///    cannot do `struct S<T>; impl<T:Clone> Drop for S<T> { ... }`).
 ///
 pub fn check_drop_impl(tcx: TyCtxt<'_>, drop_impl_did: DefId) -> Result<(), ErrorGuaranteed> {
+    match tcx.impl_polarity(drop_impl_did) {
+        ty::ImplPolarity::Positive => {}
+        ty::ImplPolarity::Negative => {
+            return Err(tcx.sess.emit_err(errors::DropImplPolarity::Negative {
+                span: tcx.def_span(drop_impl_did),
+            }));
+        }
+        ty::ImplPolarity::Reservation => {
+            return Err(tcx.sess.emit_err(errors::DropImplPolarity::Reservation {
+                span: tcx.def_span(drop_impl_did),
+            }));
+        }
+    }
     let dtor_self_type = tcx.type_of(drop_impl_did).subst_identity();
     let dtor_predicates = tcx.predicates_of(drop_impl_did);
     match dtor_self_type.kind() {

compiler/rustc_hir_analysis/src/errors.rs

@@ -823,3 +823,17 @@ pub(crate) struct MissingTildeConst {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+pub(crate) enum DropImplPolarity {
+    #[diag(hir_analysis_drop_impl_negative)]
+    Negative {
+        #[primary_span]
+        span: Span,
+    },
+    #[diag(hir_analysis_drop_impl_reservation)]
+    Reservation {
+        #[primary_span]
+        span: Span,
+    },
+}