Disallow (min) specialization imps with no items

Such implementations are usually mistakes and are not used in the
compiler or standard library (after this commit) so forbid them with
`min_specialization`.

compiler/rustc_data_structures/src/owned_slice.rs

@@ -109,9 +109,11 @@ impl Borrow<[u8]> for OwnedSlice {
 }
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Send`
+#[cfg(parallel_compiler)]
 unsafe impl Send for OwnedSlice {}
 
 // Safety: `OwnedSlice` is conceptually `(&'self.1 [u8], Box<dyn Send + Sync>)`, which is `Sync`
+#[cfg(parallel_compiler)]
 unsafe impl Sync for OwnedSlice {}
 
 #[cfg(test)]

compiler/rustc_hir_analysis/messages.ftl

@@ -275,6 +275,9 @@ hir_analysis_specialization_trait = implementing `rustc_specialization_trait` tr
 hir_analysis_closure_implicit_hrtb = implicit types in closure signatures are forbidden when `for<...>` is present
     .label = `for<...>` is here
 
+hir_analysis_empty_specialization = specialization impl does not specialize any associated items
+    .note = impl is a specialization of this impl
+
 hir_analysis_const_specialize = cannot specialize on const impl with non-const impl
 
 hir_analysis_static_specialize = cannot specialize on `'static` lifetime

compiler/rustc_hir_analysis/src/errors.rs

@@ -803,6 +803,15 @@ pub(crate) struct ClosureImplicitHrtb {
     pub for_sp: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_empty_specialization)]
+pub(crate) struct EmptySpecialization {
+    #[primary_span]
+    pub span: Span,
+    #[note]
+    pub base_impl_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_const_specialize)]
 pub(crate) struct ConstSpecialize {

compiler/rustc_hir_analysis/src/impl_wf_check/min_specialization.rs

@@ -100,12 +100,19 @@ fn parent_specialization_node(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId) -> Opti
         // Implementing a normal trait isn't a specialization.
         return None;
     }
+    if trait_def.is_marker {
+        // Overlapping marker implementations are not really specializations.
+        return None;
+    }
     Some(impl2_node)
 }
 
 /// Check that `impl1` is a sound specialization
 #[instrument(level = "debug", skip(tcx))]
 fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node) {
+    let span = tcx.def_span(impl1_def_id);
+    check_has_items(tcx, impl1_def_id, impl2_node, span);
+
     if let Some((impl1_substs, impl2_substs)) = get_impl_substs(tcx, impl1_def_id, impl2_node) {
         let impl2_def_id = impl2_node.def_id();
         debug!(?impl2_def_id, ?impl2_substs);
@@ -116,7 +123,6 @@ fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node
             unconstrained_parent_impl_substs(tcx, impl2_def_id, impl2_substs)
         };
 
-        let span = tcx.def_span(impl1_def_id);
         check_constness(tcx, impl1_def_id, impl2_node, span);
         check_static_lifetimes(tcx, &parent_substs, span);
         check_duplicate_params(tcx, impl1_substs, &parent_substs, span);
@@ -124,6 +130,13 @@ fn check_always_applicable(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node
     }
 }
 
+fn check_has_items(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) {
+    if let Node::Impl(impl2_id) = impl2_node && tcx.associated_item_def_ids(impl1_def_id).is_empty() {
+        let base_impl_span = tcx.def_span(impl2_id);
+        tcx.sess.emit_err(errors::EmptySpecialization { span, base_impl_span });
+    }
+}
+
 /// Check that the specializing impl `impl1` is at least as const as the base
 /// impl `impl2`
 fn check_constness(tcx: TyCtxt<'_>, impl1_def_id: LocalDefId, impl2_node: Node, span: Span) {

compiler/rustc_middle/src/mir/mod.rs

@@ -2728,8 +2728,6 @@ pub struct UserTypeProjection {
     pub projs: Vec<ProjectionKind>,
 }
 
-impl Copy for ProjectionKind {}
-
 impl UserTypeProjection {
     pub(crate) fn index(mut self) -> Self {
         self.projs.push(ProjectionElem::Index(()));

tests/ui/rfc-2632-const-trait-impl/specialization/const-default-bound-non-const-specialized-bound.rs

@@ -12,7 +12,9 @@ trait Specialize {}
 trait Foo {}
 
 #[const_trait]
-trait Bar {}
+trait Bar {
+    fn bar();
+}
 
 // bgr360: I was only able to exercise the code path that raises the
 // "missing ~const qualifier" error by making this base impl non-const, even
@@ -21,26 +23,36 @@ trait Bar {}
 impl<T> Bar for T
 where
     T: ~const Foo,
-{}
+{
+    default fn bar() {}
+}
 
 impl<T> Bar for T
 where
     T: Foo, //~ ERROR missing `~const` qualifier
     T: Specialize,
-{}
+{
+    fn bar() {}
+}
 
 #[const_trait]
-trait Baz {}
+trait Baz {
+    fn baz();
+}
 
 impl<T> const Baz for T
 where
     T: ~const Foo,
-{}
+{
+    default fn baz() {}
+}
 
 impl<T> const Baz for T //~ ERROR conflicting implementations of trait `Baz`
 where
     T: Foo,
     T: Specialize,
-{}
+{
+    fn baz() {}
+}
 
 fn main() {}

tests/ui/rfc-2632-const-trait-impl/specialization/const-default-bound-non-const-specialized-bound.stderr

@@ -1,11 +1,11 @@
 error: missing `~const` qualifier for specialization
-  --> $DIR/const-default-bound-non-const-specialized-bound.rs:28:8
+  --> $DIR/const-default-bound-non-const-specialized-bound.rs:32:8
    |
 LL |     T: Foo,
    |        ^^^
 
 error[E0119]: conflicting implementations of trait `Baz`
-  --> $DIR/const-default-bound-non-const-specialized-bound.rs:40:1
+  --> $DIR/const-default-bound-non-const-specialized-bound.rs:50:1
    |
 LL | impl<T> const Baz for T
    | ----------------------- first implementation here

tests/ui/rfc-2632-const-trait-impl/specialization/issue-95186-specialize-on-tilde-const.rs

@@ -11,27 +11,39 @@
 trait Specialize {}
 
 #[const_trait]
-trait Foo {}
+trait Foo {
+    fn foo();
+}
 
-impl<T> const Foo for T {}
+impl<T> const Foo for T {
+    default fn foo() {}
+}
 
 impl<T> const Foo for T
 where
     T: ~const Specialize,
-{}
+{
+    fn foo() {}
+}
 
 #[const_trait]
-trait Bar {}
+trait Bar {
+    fn bar() {}
+}
 
 impl<T> const Bar for T
 where
     T: ~const Foo,
-{}
+{
+    default fn bar() {}
+}
 
 impl<T> const Bar for T
 where
     T: ~const Foo,
     T: ~const Specialize,
-{}
+{
+    fn bar() {}
+}
 
 fn main() {}

tests/ui/rfc-2632-const-trait-impl/specialization/issue-95187-same-trait-bound-different-constness.rs

@@ -15,31 +15,43 @@ trait Specialize {}
 trait Foo {}
 
 #[const_trait]
-trait Bar {}
+trait Bar {
+    fn bar();
+}
 
 impl<T> Bar for T
 where
     T: Foo,
-{}
+{
+    default fn bar() {}
+}
 
 impl<T> const Bar for T
 where
     T: ~const Foo,
     T: Specialize,
-{}
+{
+    fn bar() {}
+}
 
 #[const_trait]
-trait Baz {}
+trait Baz {
+    fn baz();
+}
 
 impl<T> const Baz for T
 where
     T: Foo,
-{}
+{
+    default fn baz() {}
+}
 
 impl<T> const Baz for T
 where
     T: ~const Foo,
     T: Specialize,
-{}
+{
+    fn baz() {}
+}
 
 fn main() {}

tests/ui/specialization/min_specialization/specialize_nothing.rs

@@ -0,0 +1,14 @@
+#![feature(min_specialization)]
+
+trait Special {
+    fn be_special();
+}
+
+impl<T> Special for T {
+    fn be_special() {}
+}
+
+impl Special for usize {}
+//~^ ERROR specialization impl does not specialize any associated items
+
+fn main() {}

tests/ui/specialization/min_specialization/specialize_nothing.stderr

@@ -0,0 +1,14 @@
+error: specialization impl does not specialize any associated items
+  --> $DIR/specialize_nothing.rs:11:1
+   |
+LL | impl Special for usize {}
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: impl is a specialization of this impl
+  --> $DIR/specialize_nothing.rs:7:1
+   |
+LL | impl<T> Special for T {
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+