Function rustc_query_impl::make_query::is_promotable_const_fn

pub fn is_promotable_const_fn<'tcx>(
    tcx: QueryCtxt<'tcx>, 
    key: is_promotable_const_fn<'tcx>
) -> QueryStackFrame

Returns true if calls to the function may be promoted.

This is either because the function is e.g., a tuple-struct or tuple-variant constructor, or because it has the #[rustc_promotable] attribute. The attribute should be removed in the future in favour of some form of check which figures out whether the function does not inspect the bits of any of its arguments (so is essentially just a constructor function).