Rust Trait Objects with Multiple Bounds: A Surprising Limitation
2025-03-27
This article delves into the reasons behind the limitations of multiple trait bounds in Rust trait objects. The author discovers a compilation error when attempting to use multiple trait constraints (e.g., `Mammal + Clone`) simultaneously within a trait object. The article explores the underlying mechanisms of dynamic dispatch in Rust and C++, comparing their vtable implementations. It examines using trait inheritance to circumvent this limitation and its inherent restrictions. Ultimately, the author suggests that allowing multiple trait bounds requires multiple vtable pointers, although this introduces some redundancy, it efficiently solves type conversion issues.