Proposed refactoring to implement core::io

Nobody has claimed this yet.

Label: T-libs

There have of course been several efforts to refactor std::io to make it usable without std as core::io or similar.

Some links to prior art from a few hours of searching:

From what I can see all efforts are currently stalled, but I'd like to make progress on using std::io in a no_std environment. My use case is a crate I'm writing that uses std::io::{Read, Write}, and I want it to build with std and when no_std.

As mentioned in https://internals.rust-lang.org/t/refactoring-std-for-ultimate-portability/4301, there are circular dependencies between std::io and the rest of std.

I have an idea that I've not seen mentioned: use conditional compilation in the std::io module to enable use without std. Steps required in more detail:

Move rust/src/libstd/io/*.rs to a new directory rust/src/libio_template
Add a new rust/src/libstd/io/mod.rs with just an include!("../../libio_template/mod.rs").
Add new crate rust/src/libio_nostd, which will also include! libio_template.
Add cfg feature "libio_for_std" to both libstd and libio_nostd, with it enabled and disabled by default respectively.
Patch libio_template to compile in libstd and libio_nostd.
All done. no_std consumers can use libio_nostd, libstd API should be identical, future std::io patches must be correct both in libstd and libio_nostd.

The ideal solution, as proposed in https://internals.rust-lang.org/t/refactoring-std-for-ultimate-portability/4301, is probably a careful refactoring to decouple std, but that seems like a lot of work, which has already stalled once. A templated approach allows quick progress and can be eradicated progressively as more parts of std::io are refactored out of std.

I'm willing to work on a prototype branch or more formal RFC if there is interest from the libs team.

Start by reading the current rust/src/libstd/io/*.rs sources and the linked prior art on core::io and std portability. Explore the proposed rust/src/libio_template and rust/src/libio_nostd layout, then determine whether a prototype or formal RFC can support std and no_std while keeping the std::io API identical.

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