Use of undeclared unstable module when module is imported and a cfg attribute is used - import

Question

I have the below code:
#[cfg(all(feature = "unstable", unique))]
#[cfg(all(feature = "unstable", heap_api))]
#[cfg(all(feature = "unstable", alloc))]
use std::ptr::Unique;
use std::mem;
use alloc::heap;
pub struct Foo<T> {
arr: Unique<T>,
cap: usize,
probe_limit: usize,
}
However, when I try to compile it with cargo build --features "unstable" I get a compilation error. Note that I am using the nightly build of Rust and the unstable feature is set up correctly (otherwise I would get a different error).
error[E0412]: cannot find type `Unique` in this scope
--> src/hash/arr.rs:27:8
|
27 | arr: Unique<T>,
| ^^^^^^ not found in this scope
|
help: possible candidate is found in another module, you can import it into scope
| use std::ptr::Unique;
I'm not sure why Unique is not found. I am should be using it at the top of my file. use ::std::ptr::Unique does not work.

Answer 1

Let's deconstruct your cfg instruction:
#[cfg(all(feature = "unstable", unique))]
This is an outer attribute, meaning it is outside the item it will change, and will apply to the next item. With the cfg attribute, it means "if the feature inside the parenthesis is enabled, do the next block", and all is a AND between the features. So you have something like "if features unstable and unique are enabled, do the next statement.
Attributes! What you want is to have the inner attribute feature(unique) conditionally if you have set the feature unstable on the command line. A conditional attribute can be obtained with cfg_attr.
#![cfg_attr(feature = "unstable", feature(unique))]
This can be read as if feature unstable is enabled, then enable the inner feature(unique). Then you will be able to use std::ptr::Unique.
You should also add a #[cfg(feature = "unstable")] before the use and your struct, so that they won't be usable if the feature is not enabled.