rename the trait to validity and place a feature gate afront

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -49,8 +49,8 @@ pub(super) fn check_trait<'tcx>(
         .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn)?;
     checker.check(lang_items.pointer_like(), visit_implementation_of_pointer_like)?;
     checker.check(
-        lang_items.coerce_pointee_wellformed_trait(),
-        visit_implementation_of_coerce_pointee_wellformed,
+        lang_items.coerce_pointee_validated_trait(),
+        visit_implementation_of_coerce_pointee_validity,
     )?;
     Ok(())
 }
@@ -787,19 +787,21 @@ fn visit_implementation_of_pointer_like(checker: &Checker<'_>) -> Result<(), Err
         .emit())
 }
 
-fn visit_implementation_of_coerce_pointee_wellformed(
+fn visit_implementation_of_coerce_pointee_validity(
     checker: &Checker<'_>,
 ) -> Result<(), ErrorGuaranteed> {
     let tcx = checker.tcx;
     let self_ty = tcx.impl_trait_ref(checker.impl_def_id).unwrap().instantiate_identity().self_ty();
+    let span = tcx.def_span(checker.impl_def_id);
+    if !tcx.is_builtin_derived(checker.impl_def_id.into()) {
+        return Err(tcx.dcx().emit_err(errors::CoercePointeeNoUserValidityAssertion { span }));
+    }
     let ty::Adt(def, _args) = self_ty.kind() else {
-        return Err(tcx.dcx().emit_err(errors::CoercePointeeNotConcreteType {
-            span: tcx.def_span(checker.impl_def_id),
-        }));
+        return Err(tcx.dcx().emit_err(errors::CoercePointeeNotConcreteType { span }));
     };
     let did = def.did();
-    let span =
-        if let Some(local) = did.as_local() { tcx.source_span(local) } else { tcx.def_span(did) };
+    // Now get a more precise span of the `struct`.
+    let span = tcx.def_span(did);
     if !def.is_struct() {
         return Err(tcx
             .dcx()

compiler/rustc_hir_analysis/src/errors.rs

@@ -1195,6 +1195,13 @@ pub(crate) struct CoercePointeeNotConcreteType {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_coerce_pointee_no_user_validity_assertion, code = E0802)]
+pub(crate) struct CoercePointeeNoUserValidityAssertion {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_analysis_coerce_pointee_not_transparent, code = E0802)]
 pub(crate) struct CoercePointeeNotTransparent {