Extensibility

Cross compiling

rustup currently only gets you 50% of the way towards cross compiling, but you still need a cross-compiling C toolchain, or at the very least a cross-linker for the target platform.

In the future, we'd like to extend this to make cross compiling as easy as humanly possible. However, there are a lot of potential targets, and it will be a long time before we have good support for a significant number of them baked into rustup, so I think it would be useful to think about making this part of rustup extensible.

The best way to achieve this would probably be to use crates.io: crates which follow a particular naming convention would be installable via rustup (which would itself call out to cargo, but we can be intelligent and make sure we always use a toolchain targeting the configured host platform to compile the crate) and each would provide some set of host/target pairs. (There may be multiple options even for a single host/target pair)

Command line extensibility

These would be custom subcommands, again installable via rustup through cargo, and would work basically the same as cargo subcommands but for rustup. These would be used for more variable features which don't fit into the model above.

The issue proposes extensible cross-compiling support through crates.io and installable rustup subcommands, but names no files, tests, or concrete implementation boundary. Start by reviewing rustup's existing command and toolchain architecture, then define a focused scope and acceptance criteria for one extension mechanism before attempting broader support.