Note why rust-call abi requires sized obl

Author: compiler-errors

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2663,9 +2663,15 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::LetElse
             | ObligationCauseCode::BinOp { .. }
             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
-            | ObligationCauseCode::RustCall
             | ObligationCauseCode::DropImpl
             | ObligationCauseCode::ConstParam(_) => {}
+            ObligationCauseCode::RustCall => {
+                if let Some(pred) = predicate.to_opt_poly_trait_pred()
+                    && Some(pred.def_id()) == self.tcx.lang_items().sized_trait()
+                {
+                    err.note("argument required to be sized due to `extern \"rust-call\"` ABI");
+                }
+            }
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }

tests/ui/unsized-locals/rust-call.stderr

@@ -6,6 +6,7 @@ LL |     f(*slice);
    |
    = help: within `([u8],)`, the trait `Sized` is not implemented for `[u8]`
    = note: required because it appears within the type `([u8],)`
+   = note: argument required to be sized due to `extern "rust-call"` ABI
 
 error: aborting due to previous error