permit negative impls for non-auto traits

Author: nikomatsakis

src/libcore/clone.rs

@@ -227,4 +227,9 @@ mod impls {
             *self
         }
     }
+
+    // Shared references can be cloned, but mutable references *cannot*!
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[cfg(not(bootstrap))]
+    impl<T: ?Sized> !Clone for &mut T {}
 }

src/libcore/ops/deref.rs

@@ -81,6 +81,10 @@ impl<T: ?Sized> Deref for &T {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> !DerefMut for &T {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> Deref for &mut T {
     type Target = T;

src/librustc_error_codes/error_codes.rs

@@ -97,7 +97,6 @@ E0184: include_str!("./error_codes/E0184.md"),
 E0185: include_str!("./error_codes/E0185.md"),
 E0186: include_str!("./error_codes/E0186.md"),
 E0191: include_str!("./error_codes/E0191.md"),
-E0192: include_str!("./error_codes/E0192.md"),
 E0193: include_str!("./error_codes/E0193.md"),
 E0195: include_str!("./error_codes/E0195.md"),
 E0197: include_str!("./error_codes/E0197.md"),
@@ -424,6 +423,8 @@ E0745: include_str!("./error_codes/E0745.md"),
 E0746: include_str!("./error_codes/E0746.md"),
 E0747: include_str!("./error_codes/E0747.md"),
 E0748: include_str!("./error_codes/E0748.md"),
+E0749: include_str!("./error_codes/E0749.md"),
+E0750: include_str!("./error_codes/E0750.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
@@ -458,6 +459,7 @@ E0748: include_str!("./error_codes/E0748.md"),
 //  E0188, // can not cast an immutable reference to a mutable pointer
 //  E0189, // deprecated: can only cast a boxed pointer to a boxed object
 //  E0190, // deprecated: can only cast a &-pointer to an &-object
+//  E0192, // negative impl only applicable to auto traits
 //  E0194, // merged into E0403
 //  E0196, // cannot determine a type for this closure
     E0208,

src/librustc_error_codes/error_codes/E0749.md

@@ -0,0 +1,4 @@
+Negative impls are not allowed to have any items. Negative impls
+declare that a trait is **not** implemented (and never will be) and
+hence there is no need to specify the values for trait methods or
+other items.

src/librustc_error_codes/error_codes/E0750.md

@@ -0,0 +1,4 @@
+Negative impls cannot be default impls. A default impl supplies
+default values for the items within to be used by other impls, whereas
+a negative impl declares that there are no other impls. These don't
+make sense to combine.

src/librustc_typeck/check/mod.rs

@@ -1656,11 +1656,14 @@ fn check_opaque_for_inheriting_lifetimes(tcx: TyCtxt<'tcx>, def_id: DefId, span:
             _ => unreachable!(),
         };
 
-        tcx.sess.span_err(span, &format!(
+        tcx.sess.span_err(
+            span,
+            &format!(
             "`{}` return type cannot contain a projection or `Self` that references lifetimes from \
              a parent scope",
             if is_async { "async fn" } else { "impl Trait" },
-        ));
+        ),
+        );
     }
 }
 
@@ -1838,8 +1841,8 @@ fn maybe_check_static_with_link_section(tcx: TyCtxt<'_>, id: DefId, span: Span)
         Ok(ConstValue::ByRef { alloc, .. }) => {
             if alloc.relocations().len() != 0 {
                 let msg = "statics with a custom `#[link_section]` must be a \
-                       simple list of bytes on the wasm target with no \
-                       extra levels of indirection such as references";
+                           simple list of bytes on the wasm target with no \
+                           extra levels of indirection such as references";
                 tcx.sess.span_err(span, msg);
             }
         }
@@ -1968,6 +1971,24 @@ fn check_impl_items_against_trait<'tcx>(
         return;
     }
 
+    // Negative impls are not expected to have any items
+    match tcx.impl_polarity(impl_id) {
+        ty::ImplPolarity::Reservation | ty::ImplPolarity::Positive => {}
+        ty::ImplPolarity::Negative => {
+            if !impl_item_refs.is_empty() {
+                let first_item_span = tcx.hir().impl_item(impl_item_refs[0].id).span;
+                struct_span_err!(
+                    tcx.sess,
+                    first_item_span,
+                    E0749,
+                    "negative impls cannot have any items"
+                )
+                .emit();
+            }
+            return;
+        }
+    }
+
     // Locate trait definition and items
     let trait_def = tcx.trait_def(impl_trait_ref.def_id);
 
@@ -2007,7 +2028,7 @@ fn check_impl_items_against_trait<'tcx>(
                             impl_item.span,
                             E0323,
                             "item `{}` is an associated const, \
-                              which doesn't match its trait `{}`",
+                             which doesn't match its trait `{}`",
                             ty_impl_item.ident,
                             impl_trait_ref.print_only_trait_path()
                         );
@@ -3555,7 +3576,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let adjusted_ty = autoderef.unambiguous_final_ty(self);
         debug!(
             "try_index_step(expr={:?}, base_expr={:?}, adjusted_ty={:?}, \
-                               index_ty={:?})",
+             index_ty={:?})",
             expr, base_expr, adjusted_ty, index_ty
         );
 
@@ -4708,7 +4729,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 err.span_label(
                                     fn_span,
                                     "implicitly returns `()` as its body has no tail or `return` \
-                                 expression",
+                                     expression",
                                 );
                             }
                         },
@@ -5593,11 +5614,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             match self.at(&self.misc(span), self.param_env).sup(impl_ty, self_ty) {
                 Ok(ok) => self.register_infer_ok_obligations(ok),
                 Err(_) => {
-                    self.tcx.sess.delay_span_bug(span, &format!(
+                    self.tcx.sess.delay_span_bug(
+                        span,
+                        &format!(
                         "instantiate_value_path: (UFCS) {:?} was a subtype of {:?} but now is not?",
                         self_ty,
                         impl_ty,
-                    ));
+                    ),
+                    );
                 }
             }
         }
@@ -5783,7 +5807,7 @@ fn fatally_break_rust(sess: &Session) {
     handler.note_without_error("the compiler expectedly panicked. this is a feature.");
     handler.note_without_error(
         "we would appreciate a joke overview: \
-        https://github.com/rust-lang/rust/issues/43162#issuecomment-320764675",
+         https://github.com/rust-lang/rust/issues/43162#issuecomment-320764675",
     );
     handler.note_without_error(&format!(
         "rustc {} running on {}",

src/librustc_typeck/check/wfcheck.rs

@@ -112,15 +112,14 @@ pub fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: DefId) {
                 }
                 ty::ImplPolarity::Negative => {
                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
-                    if of_trait.is_some() && !is_auto {
+                    if let hir::Defaultness::Default { .. } = defaultness {
                         struct_span_err!(
                             tcx.sess,
                             item.span,
-                            E0192,
-                            "negative impls are only allowed for \
-                                   auto traits (e.g., `Send` and `Sync`)"
+                            E0750,
+                            "negative impls cannot be default impls"
                         )
-                        .emit()
+                        .emit();
                     }
                 }
                 ty::ImplPolarity::Reservation => {
@@ -886,13 +885,13 @@ fn check_opaque_types<'fcx, 'tcx>(
                                             .struct_span_err(
                                                 span,
                                                 "non-defining opaque type use \
-                                                    in defining scope",
+                                                 in defining scope",
                                             )
                                             .span_label(
                                                 param_span,
                                                 "cannot use static lifetime; use a bound lifetime \
-                                                instead or remove the lifetime parameter from the \
-                                                opaque type",
+                                                 instead or remove the lifetime parameter from the \
+                                                 opaque type",
                                             )
                                             .emit();
                                     } else {
@@ -907,13 +906,13 @@ fn check_opaque_types<'fcx, 'tcx>(
                                             .struct_span_err(
                                                 span,
                                                 "non-defining opaque type use \
-                                                in defining scope",
+                                                 in defining scope",
                                             )
                                             .span_note(
                                                 tcx.def_span(param.def_id),
                                                 &format!(
                                                     "used non-generic const {} for \
-                                                    generic parameter",
+                                                     generic parameter",
                                                     ty,
                                                 ),
                                             )
@@ -928,7 +927,7 @@ fn check_opaque_types<'fcx, 'tcx>(
                                     .struct_span_err(
                                         span,
                                         "non-defining opaque type use \
-                                            in defining scope",
+                                         in defining scope",
                                     )
                                     .span_note(spans, "lifetime used multiple times")
                                     .emit();
@@ -1014,7 +1013,7 @@ fn check_method_receiver<'fcx, 'tcx>(
                     span,
                     &format!(
                         "`{}` cannot be used as the type of `self` without \
-                            the `arbitrary_self_types` feature",
+                         the `arbitrary_self_types` feature",
                         receiver_ty,
                     ),
                 )

src/test/ui/error-codes/E0192.rs

@@ -8,8 +8,9 @@ default impl S {} //~ ERROR inherent impls cannot be `default`
 
 default unsafe impl Send for S {} //~ ERROR impls of auto traits cannot be default
 default impl !Send for Z {} //~ ERROR impls of auto traits cannot be default
+//~^ ERROR negative impls cannot be default impls
 
 trait Tr {}
-default impl !Tr for S {} //~ ERROR negative impls are only allowed for auto traits
+default impl !Tr for S {} //~ ERROR negative impls cannot be default impls
 
 fn main() {}