RFC 2027: Non-object safe objects do not impl their traits

  - Remove the automatic 'impl Trait for dyn Trait'
    when Trait is not object safe

Author: bovinebuddha

src/librustc/traits/select.rs

@@ -2010,7 +2010,15 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                     }
 
                     if let Some(principal) = data.principal() {
-                        principal.with_self_ty(self.tcx(), self_ty)
+                        if !self.infcx.tcx.features().object_safe_for_dispatch {
+                            principal.with_self_ty(self.tcx(), self_ty)
+                        } else {
+                            if self.tcx().is_object_safe(principal.def_id()) {
+                                principal.with_self_ty(self.tcx(), self_ty)
+                            } else {
+                                return;
+                            }
+                        }
                     } else {
                         // Only auto-trait bounds exist.
                         return;

src/librustc_typeck/coherence/mod.rs

@@ -173,8 +173,11 @@ fn check_impl_overlap<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeI
 
         if let Some(principal_def_id) = data.principal_def_id() {
             if !tcx.is_object_safe(principal_def_id) {
-                // This is an error, but it will be reported by wfcheck.  Ignore it here.
+                // Without the 'object_safe_for_dispatch' feature this is an error
+                // which will be reported by wfcheck.  Ignore it here.
                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
+                // With the feature enabled, the trait is not implemented automatically,
+                // so this is valid.
             } else {
                 let mut supertrait_def_ids =
                     traits::supertrait_def_ids(tcx, principal_def_id);