Rust unstable features needed #3

This issue is to collect currently unstable Rust features that we either need or would like to have for CPython, inspired by Rust-for-Linux/linux#2

Required

These unstable features are needed to properly support Rust in CPython. There aren't reasonable workarounds to these.

• -Zbuild-std https://doc.rust-lang.org/cargo/reference/unstable.html#build-std
  This is both for platform support and potentially reducing binary size. It also is a prerequisite for sanitizer support.

Nice to have

These unstable features may have workarounds, such as running on nightly in CI, but would be ideally stabilized.

• -Zsanitizer=address https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html
• -Zsanitizer=memory https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html
• -Zsanitizer=thread https://doc.rust-lang.org/beta/unstable-book/compiler-flags/sanitizer.html
• feature(arbitrary_self_types) https://doc.rust-lang.org/unstable-book/language-features/arbitrary-self-types.html
  For better abstractions over generic Python object APIs and specific types. PyO3 current uses type-specific traits to implement an ergonomic API over these types.
• macro_metavar_expr_concat https://doc.rust-lang.org/stable/unstable-book/language-features/macro-metavar-expr-concat.html
  This would make defining modules via PyModExport_<module name> in a declarative macro nicer. We could potentially use a proc macro instead.

To other org members: please feel free to edit this issue as seen fit.