introduce CoercePointeeWellformed for coherence checks at typeck stage

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -48,6 +48,10 @@ pub(super) fn check_trait<'tcx>(
     checker
         .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,
+    )?;
     Ok(())
 }
 
@@ -782,3 +786,27 @@ fn visit_implementation_of_pointer_like(checker: &Checker<'_>) -> Result<(), Err
         .with_note(why_disqualified)
         .emit())
 }
+
+fn visit_implementation_of_coerce_pointee_wellformed(
+    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 ty::Adt(def, _args) = self_ty.kind() else {
+        return Err(tcx.dcx().emit_err(errors::CoercePointeeNotConcreteType {
+            span: tcx.def_span(checker.impl_def_id),
+        }));
+    };
+    let did = def.did();
+    let span =
+        if let Some(local) = did.as_local() { tcx.source_span(local) } else { tcx.def_span(did) };
+    if !def.is_struct() {
+        return Err(tcx
+            .dcx()
+            .emit_err(errors::CoercePointeeNotStruct { span, kind: def.descr().into() }));
+    }
+    if !def.repr().transparent() {
+        return Err(tcx.dcx().emit_err(errors::CoercePointeeNotTransparent { span }));
+    }
+    Ok(())
+}