Error when only is not applied to a sizedness trait

Author: CrooseGit

compiler/rustc_ast_lowering/src/lib.rs

@@ -2219,6 +2219,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         if let ast::BoundPolarity::Maybe(_) = modifiers.polarity {
             self.validate_relaxed_bound(trait_ref, *span, rbp);
+        } else if let ast::BoundPolarity::Only(_) = modifiers.polarity {
+            self.validate_only_bound(trait_ref, *span);
         }
 
         hir::PolyTraitRef {
@@ -2229,6 +2231,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    fn validate_only_bound(&self, trait_ref: hir::TraitRef<'_>, span: Span) {
+        if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res
+            && !(self.tcx.is_lang_item(def_id, hir::LangItem::MetaSized)
+                || self.tcx.is_lang_item(def_id, hir::LangItem::Sized)
+                || self.tcx.is_lang_item(def_id, hir::LangItem::PointeeSized))
+        {
+            self.dcx()
+                .struct_span_err(span, "`only` may only be applied to sizedness traits")
+                .emit();
+        }
+    }
+
     fn validate_relaxed_bound(
         &self,
         trait_ref: hir::TraitRef<'_>,

tests/ui/trait-bounds/only-bound.rs

@@ -28,4 +28,26 @@ trait OnlyPointeeSized: only PointeeSized {
     }
 }
 
+trait NonSizedOnly1: only Debug {}
+//~^ ERROR `only` may only be applied to sizedness traits
+
+trait Trait {}
+
+trait NonSizedOnly2: only Trait {}
+//~^ ERROR `only` may only be applied to sizedness traits
+
+trait OnlyOnly: only OnlyPointeeSized {}
+//~^ ERROR `only` may only be applied to sizedness traits
+
+trait DoubleOnly: only PointeeSized + only MetaSized {}
+// Redundant, but not illegal.
+
+trait InheritedOnly: OnlyPointeeSized {
+    fn foo(&self) {
+        size_of_val(self);
+        // This works fine, `only` is not transitive, this trait still has the
+        // `MetaSized` default supertrait.
+    }
+}
+
 fn main() {}

tests/ui/trait-bounds/only-bound.stderr

@@ -1,3 +1,21 @@
+error: `only` may only be applied to sizedness traits
+  --> $DIR/only-bound.rs:31:22
+   |
+LL | trait NonSizedOnly1: only Debug {}
+   |                      ^^^^^^^^^^
+
+error: `only` may only be applied to sizedness traits
+  --> $DIR/only-bound.rs:36:22
+   |
+LL | trait NonSizedOnly2: only Trait {}
+   |                      ^^^^^^^^^^
+
+error: `only` may only be applied to sizedness traits
+  --> $DIR/only-bound.rs:39:17
+   |
+LL | trait OnlyOnly: only OnlyPointeeSized {}
+   |                 ^^^^^^^^^^^^^^^^^^^^^
+
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/only-bound.rs:11:30
    |
@@ -59,6 +77,6 @@ LL |         size_of_val(&self);
 LL |         size_of_val(&mut self);
    |                     ++++
 
-error: aborting due to 4 previous errors
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0277`.