Mark unsafe binder casts as unsafe

Author: compiler-errors

compiler/rustc_mir_build/messages.ftl

@@ -8,6 +8,7 @@ mir_build_bindings_with_variant_name =
     pattern binding `{$name}` is named the same as one of the variants of the type `{$ty_path}`
     .suggestion = to match on the variant, qualify the path
 
+
 mir_build_borrow = value is borrowed by `{$name}` here
 
 mir_build_borrow_of_layout_constrained_field_requires_unsafe =
@@ -370,6 +371,18 @@ mir_build_unreachable_pattern = unreachable pattern
     .unreachable_pattern_let_binding = there is a binding of the same name; if you meant to pattern match against the value of that binding, that is a feature of constants that is not available for `let` bindings
     .suggestion = remove the match arm
 
+mir_build_unsafe_binder_cast_requires_unsafe =
+    unsafe binder cast is unsafe and requires unsafe block
+    .label = unsafe binder cast
+    .note = casting to or from an `unsafe<...>` binder type is unsafe since it erases lifetime
+        information that may be required to uphold safety guarantees of a type
+
+mir_build_unsafe_binder_cast_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
+    unsafe binder cast is unsafe and requires unsafe block or unsafe fn
+    .label = unsafe binder cast
+    .note = casting to or from an `unsafe<...>` binder type is unsafe since it erases lifetime
+        information that may be required to uphold safety guarantees of a type
+
 mir_build_unsafe_field_requires_unsafe =
     use of unsafe field is unsafe and requires unsafe block
     .note = unsafe fields may carry library invariants

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -680,6 +680,9 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                     }
                 }
             }
+            ExprKind::PlaceUnsafeBinderCast { .. } | ExprKind::ValueUnsafeBinderCast { .. } => {
+                self.requires_unsafe(expr.span, UnsafeBinderCast);
+            }
             _ => {}
         }
         visit::walk_expr(self, expr);
@@ -728,6 +731,7 @@ enum UnsafeOpKind {
         /// (e.g., with `-C target-feature`).
         build_enabled: Vec<Symbol>,
     },
+    UnsafeBinderCast,
 }
 
 use UnsafeOpKind::*;
@@ -888,6 +892,15 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 },
             ),
+            UnsafeBinderCast => tcx.emit_node_span_lint(
+                UNSAFE_OP_IN_UNSAFE_FN,
+                hir_id,
+                span,
+                UnsafeOpInUnsafeFnUnsafeBinderCastRequiresUnsafe {
+                    span,
+                    unsafe_not_inherited_note,
+                },
+            ),
         }
     }
 
@@ -1096,6 +1109,15 @@ impl UnsafeOpKind {
                     function: tcx.def_path_str(*function),
                 });
             }
+            UnsafeBinderCast if unsafe_op_in_unsafe_fn_allowed => {
+                dcx.emit_err(UnsafeBinderCastRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+                    span,
+                    unsafe_not_inherited_note,
+                });
+            }
+            UnsafeBinderCast => {
+                dcx.emit_err(UnsafeBinderCastRequiresUnsafe { span, unsafe_not_inherited_note });
+            }
         }
     }
 }

compiler/rustc_mir_build/src/errors.rs

@@ -171,6 +171,18 @@ pub(crate) struct UnsafeOpInUnsafeFnBorrowOfLayoutConstrainedFieldRequiresUnsafe
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe,
+    code = E0133,
+)]
+pub(crate) struct UnsafeOpInUnsafeFnUnsafeBinderCastRequiresUnsafe {
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_unsafe_op_in_unsafe_fn_call_to_fn_with_requires_unsafe, code = E0133)]
 #[help]
@@ -505,6 +517,32 @@ pub(crate) struct CallToFunctionWithRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
 }
 
+#[derive(Diagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe,
+    code = E0133,
+)]
+pub(crate) struct UnsafeBinderCastRequiresUnsafe {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
+#[derive(Diagnostic)]
+#[diag(
+    mir_build_unsafe_binder_cast_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
+    code = E0133,
+)]
+pub(crate) struct UnsafeBinderCastRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
 #[derive(Subdiagnostic)]
 #[label(mir_build_unsafe_not_inherited)]
 pub(crate) struct UnsafeNotInheritedNote {

compiler/rustc_type_ir/src/fast_reject.rs

@@ -41,6 +41,7 @@ pub enum SimplifiedType<DefId> {
     Coroutine(DefId),
     CoroutineWitness(DefId),
     Function(usize),
+    UnsafeBinder,
     Placeholder,
     Error,
 }
@@ -138,7 +139,7 @@ pub fn simplify_type<I: Interner>(
         ty::FnPtr(sig_tys, _hdr) => {
             Some(SimplifiedType::Function(sig_tys.skip_binder().inputs().len()))
         }
-        ty::UnsafeBinder(binder) => simplify_type(cx, binder.skip_binder(), treat_params),
+        ty::UnsafeBinder(_) => Some(SimplifiedType::UnsafeBinder),
         ty::Placeholder(..) => Some(SimplifiedType::Placeholder),
         ty::Param(_) => match treat_params {
             TreatParams::AsRigid => Some(SimplifiedType::Placeholder),