Arc is invariant in its type parameter

[bkoropoff]

I would expect it to be covariant. This seems to be an unintentional side-effect of using *mut internally.

Code

Playground

use std::sync::Arc;

fn works<'r>(x: Box<&'static str>) -> Box<&'r str> {
    x
}

fn also_works<'r,'w>(x: &'w &'static str) -> &'w &'r str {
    x
}

fn breaks<'r>(x: Arc<&'static str>) -> Arc<&'r str> {
    x
}

fn main() {}

[bluss]

@gankro's Raw Pointer Cleanup seems to already touch upon this.

[apasel422]

I'm working on implementing Shared per @gankro's Raw Pointer Cleanup pre-RFC, but I'll point out that by naively changing ArcInner to use *const T instead of *mut T, we get RFC 1214 warnings in libstd/thread/mod:283:

../rust/src/libstd/thread/mod.rs:283:27: 283:32 warning: the parameter type `T` may not live long enough [E0310]
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~
../rust/src/libstd/thread/mod.rs:283:27: 283:32 help: run `rustc --explain E0310` to see a detailed explanation
../rust/src/libstd/thread/mod.rs:283:27: 283:32 help: consider adding an explicit lifetime bound `T: 'static`...
../rust/src/libstd/thread/mod.rs:283:27: 283:32 note: this warning results from recent bug fixes and clarifications; it will become a HARD ERROR in the next release. See RFC 1214 for details.
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~
../rust/src/libstd/thread/mod.rs:283:27: 283:32 note: ...so that the reference type `&alloc::arc::Arc<core::cell::UnsafeCell<core::option::Option<core::result::Result<T, Box<core::any::Any + Send>>>>>` does not outlive the data it points at
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~
../rust/src/libstd/thread/mod.rs:283:27: 283:32 warning: the parameter type `T` may not live long enough [E0310]
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~
../rust/src/libstd/thread/mod.rs:283:27: 283:32 help: run `rustc --explain E0310` to see a detailed explanation
../rust/src/libstd/thread/mod.rs:283:27: 283:32 help: consider adding an explicit lifetime bound `T: 'static`...
../rust/src/libstd/thread/mod.rs:283:27: 283:32 note: this warning results from recent bug fixes and clarifications; it will become a HARD ERROR in the next release. See RFC 1214 for details.
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~
../rust/src/libstd/thread/mod.rs:283:27: 283:32 note: ...so that the reference type `&core::cell::UnsafeCell<core::option::Option<core::result::Result<T, Box<core::any::Any + Send>>>>` does not outlive the data it points at
../rust/src/libstd/thread/mod.rs:283             *their_packet.get() = Some(try_result.map(|()| {
                                                               ^~~~~

These may or may not be spurious.

[arielb1]

@apasel422

That method has been giving me trouble, so probably spurious.

[apasel422]

@arielb1 Should I file that as a separate RFC 1214 bug?

[apasel422]

For what it's worth, Rc is also invariant.