Make Bytes::from_static a const fn?

[andrewtj]

I think it'd be useful to make Bytes::from_static a const fn so that Bytes and types built on Bytes can be used in consts and statics. This would require a minimum Rust version of 1.39 though. (Opened a PR to discuss this instead of an issue because it's a two line change.)

[andrewtj]

Oops, should have run the test suite. It's not as small a change as I thought:

error[E0723]: can only call other `const fn` within a `const fn`, but `const loom::sync::atomic::ptr::AtomicPtr::<()>::new` is not stable as `const fn`
   --> tests/../src/bytes.rs:121:19
    |
121 |             data: AtomicPtr::new(ptr::null_mut()),
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
    = help: add `#![feature(const_fn)]` to the crate attributes to enable

error[E0716]: temporary value dropped while borrowed
   --> tests/../src/bytes.rs:122:22
    |
122 |             vtable: &STATIC_VTABLE,
    |                     -^^^^^^^^^^^^^
    |                     ||
    |                     |creates a temporary which is freed while still in use
    |                     requires that borrow lasts for `'static`
123 |         }
124 |     }
    |     - temporary value is freed at the end of this statement

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0716, E0723.
For more information about an error, try `rustc --explain E0716`.
error: could not compile `bytes`.
warning: build failed, waiting for other jobs to finish...
error: build failed

[carllerche]

Maybe there is a way to scope out loom?

[seanmonstar]

If we were to switch the loom support to use cfg(loom), it'd be easier to fix this.

[carllerche]

@seanmonstar that is what we are moving to in Tokio due to edge cases. We probably will want to do that here as well.

[seanmonstar]

Rebasing on master could allow you to only make this a const fn when cfg(not(all(loom, test))).

[andrewtj]

Thanks. I've made that change and also bumped Rust versions.

[carllerche]

LGTM 👍