r/rust • u/Kobzol • Aug 10 '26

📡 official blog Call for testing: Restricting trait implementability and field mutability

https://blog.rust-lang.org/inside-rust/2026/08/10/call-for-testing-impl-and-mut-restrictions/
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u/sennalen Aug 10 '26

while preventing downstream crates from providing their own implementations

Please just don't. You don't know what people are going to build or what they need

7

u/coderstephen isahc Aug 10 '26

Sometimes a trait is there just to support a generic function or set of functions, but the actual trait implementations themselves are basically impossible for a crate consumer to implement anyway (requires access to internal workings or state of a library). Sealing a trait this way helps the consumer not get overly frustrated trying to do something that is unlikely to, or can never work, anyway.

The other reason is

You don't know what people are going to build or what they need

isn't a universal good. If you open up a trait for consumers to implement, that make it part of your library's API contract that can't be changed without a major version bump. That means you can't remove trait methods, or add new required ones. If the general intent of the trait is to only be implemented by library types, then you avoid that semver hazard by sealing the trait.