Optionally pass --config down to cargo when building the sysroot

[erickt]

This allows us to do things like point cargo at a vendored directory to support offline builds of the sysroot.

Note: this is blocked on RalfJung/rustc-build-sysroot#34 being landed and released in a build.

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[RalfJung]

It looks like rust-src since recently contains the vendored dependencies of std, so no optional config should be needed for this? See rust-lang/rust#156196.

[erickt]

Oh interesting! I’ll test it out with the latest rust-src and get back to you.

[erickt]

Right now it looks like miri isn't able to find the vendored sources automatically if I block network access:

% unshare -r -n -- rustup run nightly cargo run -- miri setup --verbose
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.04s
     Running `target/debug/cargo-miri miri setup --verbose`
Preparing a sysroot for Miri (target: x86_64-unknown-linux-gnu) in .../.cache/miri...
warning: spurious network error (3 tries remaining): [6] Could not resolve hostname (Could not resolve host: static.crates.io)
...

That's also the case if I explicitly point it at the rust-src:

env MIRI_LIB_SRC=$HOME/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library unshare -r -n -- rustup run nightly cargo run -- miri setup --verbose --frozen
warning: spurious network error (3 tries remaining): [6] Could not resolve hostname (Could not resolve host: static.crates.io)
...

However if I use this patch and my patch to rustc-build-sysroot it works:

% unshare -r -n -- rustup run nightly cargo run -- miri setup --verbose --frozen --config ~/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/.cargo/config.toml
   Compiling compiler_builtins v0.1.160 (.../.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins)
   Compiling core v0.0.0 (.../.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
...

Would you rather we hardcode cargo-miri or rustc-build-sysroot to always point the config at $MIRI_LIB_SRC/.cargo/config.toml? I'm not sure if that'll work with a normal rust checkout though, since it doesn't have a .cargo/config.toml.

[RalfJung]

We could make it so that if the .cargo/config.toml exists, we use it -- would that suffice for your usecase? That would avoid having to add new flags.

[erickt]

@RalfJung Yeah I think that would work. We may also be able to just set $CARGO_HOME to $MIRI_LIB_SRC? Otherwise we could explicitly pass down the config file with RalfJung/rustc-build-sysroot#34.

[RalfJung]

@rustbot author
based on the feedback above

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