split up `#[rustc_deny_explicit_impl]` attribute

This commit splits the `#[rustc_deny_explicit_impl(implement_via_object = ...)]` attribute
into two attributes `#[rustc_deny_explicit_impl]` and `#[rustc_do_not_implement_via_object]`.

This allows us to have special traits that can have user-defined impls but do not have the
automatic trait impl for trait objects (`impl Trait for dyn Trait`).

2 files changed, 5 insertions, 44 deletions

diff --git a/compiler/rustc_hir_analysis/src/coherence/mod.rs b/compiler/rustc_hir_analysis/src/coherence/mod.rs
index 3aad4bafeb5..1be4aa2f63a 100644
--- a/compiler/rustc_hir_analysis/src/coherence/mod.rs
+++ b/compiler/rustc_hir_analysis/src/coherence/mod.rs
@@ -206,7 +206,9 @@ fn check_object_overlap<'tcx>(
                 // so this is valid.
             } else {
                 let mut supertrait_def_ids = tcx.supertrait_def_ids(component_def_id);
-                if supertrait_def_ids.any(|d| d == trait_def_id) {
+                if supertrait_def_ids
+                    .any(|d| d == trait_def_id && tcx.trait_def(d).implement_via_object)
+                {
                     let span = tcx.def_span(impl_def_id);
                     return Err(struct_span_code_err!(
                         tcx.dcx(),
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index 5e662bb96bc..ada70117b62 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -1261,49 +1261,8 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
             no_dups.then_some(list)
         });
 
-    let mut deny_explicit_impl = false;
-    let mut implement_via_object = true;
-    if let Some(attr) = tcx.get_attr(def_id, sym::rustc_deny_explicit_impl) {
-        deny_explicit_impl = true;
-        let mut seen_attr = false;
-        for meta in attr.meta_item_list().iter().flatten() {
-            if let Some(meta) = meta.meta_item()
-                && meta.name_or_empty() == sym::implement_via_object
-                && let Some(lit) = meta.name_value_literal()
-            {
-                if seen_attr {
-                    tcx.dcx().span_err(meta.span, "duplicated `implement_via_object` meta item");
-                }
-                seen_attr = true;
-
-                match lit.symbol {
-                    kw::True => {
-                        implement_via_object = true;
-                    }
-                    kw::False => {
-                        implement_via_object = false;
-                    }
-                    _ => {
-                        tcx.dcx().span_err(
-                            meta.span,
-                            format!(
-                                "unknown literal passed to `implement_via_object` attribute: {}",
-                                lit.symbol
-                            ),
-                        );
-                    }
-                }
-            } else {
-                tcx.dcx().span_err(
-                    meta.span(),
-                    format!("unknown meta item passed to `rustc_deny_explicit_impl` {meta:?}"),
-                );
-            }
-        }
-        if !seen_attr {
-            tcx.dcx().span_err(attr.span, "missing `implement_via_object` meta item");
-        }
-    }
+    let deny_explicit_impl = tcx.has_attr(def_id, sym::rustc_deny_explicit_impl);
+    let implement_via_object = !tcx.has_attr(def_id, sym::rustc_do_not_implement_via_object);
 
     ty::TraitDef {
         def_id: def_id.to_def_id(),