The rustc-std-workspace-core crate

This crate is a shim and empty crate which simply depends on libcore and reexports all of its contents. The crate is the crux of empowering the standard library to depend on crates from crates.io

Crates on crates.io that the standard library depend on need to depend on the rustc-std-workspace-core crate from crates.io, which is empty. We use [patch] to override it to this crate in this repository. As a result, crates on crates.io will draw a dependency edge to libcore, the version defined in this repository. That should draw all the dependency edges to ensure Cargo builds crates successfully!

rustc-std-workspace-core also ensures compiler-builtins is in the crate graph. This crate is used by other crates in library/, other than std and alloc, so the compiler-builtins setup only needs to be configured in a single place. (Otherwise these crates would just need to depend on core and compiler-builtins separately.)

Note that crates on crates.io need to depend on this crate with the name core for everything to work correctly. To do that they can use:

core = { version = "1.0.0", optional = true, package = 'rustc-std-workspace-core' }

Through the use of the package key the crate is renamed to core, meaning it'll look like

--extern core=.../librustc_std_workspace_core-XXXXXXX.rlib

when Cargo invokes the compiler, satisfying the implicit extern crate core directive injected by the compiler.

The sources for the crates.io version can be found in src/rustc-std-workspace.