std::ptr::Unique

help
1

The compiler (version 1.68.2) issues an error which (after a little sanitization) contains

 help: the trait `Sync` is not implemented for             `dyn for<'a> Fn(&'a [A]) -> Option<B>`
   = note: required for                   `std::ptr::Unique<dyn for<'a> Fn(&'a [A]) -> Option<B>>` to implement `Sync`
   = note: required because it appears within the type `Box<dyn for<'a> Fn(&'a [A]) -> Option<B>>`

This is the first time that I consciously see std::ptr::Unique. A search suggests that it is something that was unstable and was removed (at least under that name) 4 years ago.

What do I need to know about std::ptr::Unique in order to write Rust using modern tooling in 2023?

2

It still exists as an implementation detail. You can ignore the middle line.

Basically, what it's saying is that Box<dyn for<'a> Fn(&'a [A]) -> Option<B>> is not Sync because dyn for<'a> Fn(&'a [A]) -> Option<B> is not Sync.

3

I also stumbled upon Unique at some point and it also confused me. Maybe it would be better to not expose it in error messages if it's a (perma-unstable?) implementation detail?

1 Like
4

As far as the compiler is concerned, it's no different from mentioning that Vec<my_crate::Foo> is !Sync because my_crate::Foo is !Sync. The std::ptr::Unique type is just another private item which is contributing to the error.

5

Well, the compiler does know std is special, so it could theoretically choose to not show specifically private std items in the type trace.

6

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