Use custom toolchain - without rustup

[otavio]

Hello,

I am trying to use a custom toolchain with cross but couldn't find a way to do so. I get:

% cross build --target i686-unknown-linux-gnu
Error:
   0: could not execute `rustup toolchain list`
   1: No such file or directory (os error 2)

Looking at the source, it seems something like this is used on rustc bisect, but I couldn't make it work. Any suggestion?

[Alexhuszagh]

Have you been using CROSS_CUSTOM_TOOLCHAIN=1? I believe this still might not work, but a minor modification should be able to fix this. The only use of rustup when using cargo-bisect-rustc or CROSS_CUSTOM_TOOLCHAIN=1 should be in lib.rs:

let installed_toolchains = rustup::installed_toolchains(msg_info)?;

if !installed_toolchains
    .into_iter()
    .any(|t| t == toolchain.to_string())
{
    rustup::install_toolchain(&toolchain, msg_info)?;
}

Toggling that line to only work if is_custom is not true should fix the issue. I think that's the last piece of code that relies on rustup with a custom toolchain. However, if possible, maybe add the toolchain to rustup itself? Rustup can handle custom toolchains itself, which can be very handy when building Rust from source:

$ rustup toolchain link --help
rustup-toolchain-link 
Create a custom toolchain by symlinking to a directory

USAGE:
    rustup toolchain link <toolchain> <path>

This way, you can still change Rust toolchains easily while also being able to use custom builds. This way, the linked toolchain would also be available in rustup toolchain list.

[Alexhuszagh]

There's actually another, separate issue. I'll see if I can provide a working solution when I get back. Basically, we need a rustup install for a lot of things, but if you use a custom toolchain, we likely also need to ensure it works without rustup. Currently, using the system Rust on Fedora 36, I get that cross expects the Rust toolchain to be found at /stable-x86_64-unknown-linux-gnu, which is clearly not where it is. rustc --print sysroot correctly reports that it is at /usr.

[otavio]

I found a similar thing. I sent #1083 as draft with current change. See:

cross build -v --target i686-unknown-linux-gnu
+ cargo metadata --format-version 1 --filter-platform i686-unknown-linux-gnu
+ rustc --print sysroot
+ /nix/store/c49n0m1hl7c1x1kpjqyhncnypbpspvzn-rust-mixed/bin/rustc
...
docker: Error response from daemon: error while creating mount source path '/nix/store/stable-x86_64-unknown-linux-gnu': mkdir /nix/store/stable-x86_64-unknown-linux-gnu: read-only file system.
+ rustup component list --toolchain c49n0m1hl7c1x1kpjqyhncnypbpspvzn-rust-mixed
Error:
   0: could not execute `rustup component list --toolchain c49n0m1hl7c1x1kpjqyhncnypbpspvzn-rust-mixed`
   1: No such file or directory (os error 2)

[Alexhuszagh]

We've actually got a lot of issues here, some of which might be less easy to fix. Also, some of them will not work out-of-the-box on other container engines very well like Podman outside of a Linux host (since it only makes the home directory available when making a machine).

This means that QualifiedToolchain::default for an install at /usr/bin/rustc will produce a toolchain name of usr and a sysroot of /stable-x86_64-unknown-linux-gnu, not exactly what's desired. This is because QualifiedToolchain::new does:

    pub fn new(channel: &str, date: &Option<String>, host: &ImagePlatform, sysroot: &Path) -> Self {
        let mut this = Self {
            channel: channel.to_owned(),
            date: date.clone(),
            host: host.clone(),
            is_custom: false,
            full: if let Some(date) = date {
                format!("{}-{}-{}", channel, date, host.target)
            } else {
                format!("{}-{}", channel, host.target)
            },
            sysroot: sysroot.to_owned(),
        };
        this.sysroot.set_file_name(&this.full);
        this
    }

This line, this.sysroot.set_file_name(&this.full);, sets the name to stable-x86_64-unknown-linux-gnu, which messes with the sysroot entirely. I should have a working solution soon.

[Emilgardis]

this has now been implemented in #1085

[otavio]

0.2.5 broke this again. #1085 has fixed it, but it fails again using the new release. Using the original commit I use:

export NIX_STORE=/nix/store
export CROSS_CUSTOM_TOOLCHAIN=1                                                                                                                                                                                                                                                   

However, using 0.2.5 it fails with:

% cross build -vv --target x86_64-unknown-linux-musl
+ cargo metadata --format-version 1 --filter-platform x86_64-unknown-linux-musl
+ rustc --print sysroot
+ rustup toolchain list
Error:
   0: could not execute `rustup toolchain list`
   1: No such file or directory (os error 2)

[Emilgardis]

#817 and #1085 have not been added to 0.2, so this is expected. cross 0.2 doesn't know about CROSS_CUSTOM_TOOLCHAIN

[Emilgardis]

ah! My bad, CROSS_CUSTOM_TOOLCHAIN was added in 0.2.2

#803

The fix was not backported