Rust derives currently add #[inline] to common trait implementations
Expanding a derived Debug implementation shows Rust currently emits #[inline]; the article shows the same for Default. That can help tiny generated methods, but the author points out it may not be ideal for large or deeply nested types such as error hierarchies, where aggressive inlining can trade call overhead for more generated code. The behavior is an implementation detail, not a language guarantee, so check the expansion for the compiler version you use rather than relying on it as a contract.
The linked Lobsters page showed no readable comments when checked.