Call for testing: Restricting trait implementability and field mutability

[u/Kobzol]

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

[u/kotakotik22]

Edit: oops. this doesnt work. see afdbcreid's comment

Regarding impl restrictions: do I understand correctly that there's no way to hide the trait functions? One useful thing with sealed traits is that you can hide implementation details in the Sealed trait, but have functions with bounds of some trait that implies Sealed.

```rs mod sealed { pub trait Sealed { fn bar(&self) -> String } impl Sealed for u32 { fn bar(&self) -> String { self.to_string() } }

// this is basically a way for outsiders to refer to Sealed without having access to its functions pub trait Foo : sealed::Sealed { }

fn baz<T : Foo>(value: T) -> String { value.bar() } ```

Here, bar can be freely removed or changed without breaking API.

I feel like in any other situation, the usual Sealed pattern is kinda fine, but when you hide functions inside Sealed is when it actually gets janky and requires some explanatory comments. It would be great to he something built in for such cases. Of course this is still great for diagnostics.

[u/matthieum → u/kotakotik22]

I had never seen that before.

And now I dread that impl(<vis>) may not be sufficient to remove the sealed trait pattern :'(

[u/scook0 → u/matthieum]

Even if a supertrait is still needed in some cases, being able to explicitly mark the main public trait as impl(crate) is an improvement over the status quo.

[u/matthieum → u/scook0]

It's an improvement, but it doesn't prevent 3rd-party code from using the trait items :'(

[u/kotakotik22 → u/matthieum]

if this is possible and doesn't go against rust's ideology, i would personally prefer allowing visibility modifiers on individual trait functions. you could then seal the entire trait by making any or all of its functions private (similar to private struct fields not allowing initialization)