How can we patch the core crate in Cargo.toml?

1

I am trying to patch the core crate, i.e. substitute it with my modified core crate when building via cargo build.

Having the following in Cargo.toml won't work

[patch.crates-io]
core = { path = 'my_core_path' }

Nor will the legacy [replace] do the magic.

I have a temporary workaround. Since I am using a manually built Rust compiler toolchain, I can always modify the source code of core crate located in rust_install_path/lib/rustlib/src/rust/library/core, and then ask the compiler to build the core crate locally with

[unstable]
build-std = ["core"]

However, I would like to know if the core crate can be uniformly patched just like any other crate.

2

All crates that form the standard library are precompiled and don't participate in the cargo dependency graph at all. In fact except when using build-std, cargo is not even aware of the existence of these crates. Rustc transparently loads them from the sysroot. Using build-std (or some other way of locally building the standard library like the unmaintained xargo) is the only way you can patch them. You can use the __CARGO_TESTS_ONLY_SRC_ROOT env var with build-std to set the location where cargo will look for the standard library sources. That way you don't have to modify the sources in the sysroot.

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