Don’t suggest 'help: add #![feature(…)]' when it’s forbidden

[SimonSapin]

Steps to reproduce:

mkdir foo
cd foo
multirust override beta  # Or obtain Rust beta in some other way
echo 'extern crate core; fn main(){}' > a.rs
echo '#![feature(core)] extern crate core; fn main(){}' > b.rs

The output of rustc a.rs is:

a.rs:1:1: 1:19 error: use of unstable library feature 'core'
a.rs:1 extern crate core; fn main(){}
       ^~~~~~~~~~~~~~~~~~
a.rs:1:19: 1:19 help: add #![feature(core)] to the crate attributes to enable

b.rs does as suggested by the help message, but that fails too (as expected):

b.rs:1:1: 1:18 error: unstable feature
b.rs:1 #![feature(core)] extern crate core; fn main(){}
       ^~~~~~~~~~~~~~~~~
note: this feature may not be used in the beta release channel
error: aborting due to previous error

Proposed fix: in the beta and stable channels (i.e. whenever #![feature(…)] is forbidden), the help message that suggests adding #![feature(…)] should not be shown.

(Clean up the test case with multirust remove-override; cd ..; rm -r foo.)

CC @brson

[steveklabnik]

Gonna milestone this real quick, as it seems quite important.

[pnkfelix]

I might be able to whip up a quick fix, though I'm not 100% sure about testing it properly ... (I've never actually tried building something for a non-dev release channel before...)

[steveklabnik]

after chatting with @pnkfelix it seems like I have a patch, building it now

[steveklabnik]

gonna leave @pnkfelix to it

[conradkleinespel]

Hello good Rust folks 😄

Before the beta, I used std::slice::bytes:MutableByteVector which is in core. Is there any way to use this trait during the beta ? I'm not sure how I can get the code to compile.

Thanks for any tip / hints you can offer 👍

[SimonSapin]

Hi @conradkleinespel. Commenting on random unrelated issues is usually not the right thing to do. You should rather open a new issue, or ask on http://users.rust-lang.org/ . But to answer your question anyway, that trait only has one method:

    /// A trait for operations on mutable `[u8]`s.
    pub trait MutableByteVector {
        /// Sets all bytes of the receiver to the given value.
        fn set_memory(&mut self, value: u8);
    }

    impl MutableByteVector for [u8] {
        #[inline]
        fn set_memory(&mut self, value: u8) {
            unsafe { ptr::write_bytes(self.as_mut_ptr(), value, self.len()) };
        }
    }

… and std::ptr::write_bytes is stable and can be used on beta. So I’d recommend you copy/paste the trait into your code, or make a function.

[conradkleinespel]

@SimonSapin Thanks for taking the time to answer. I appreciate it.

I don't see how this is unrelated. This is related, since the code I'm talking about used to compile with #![feature(core)] and the compiler suggests to do that, during the beta period.

Thanks for your tip on copying the function 👍 I guess I'll just do that if nothing better is available and change this again when the trait is stable.

[SimonSapin]

This specific issue is about fixing one of the compiler’s error message, not about fixing code that triggers that error. It really helps everyone to keep discussions on topic.

[conradkleinespel]

Alright. I understand. I'll make sure to open a new issue or go on StackOverflow next time if appropriate. Thanks again 👍