Tracking Issue for hint-min-opt-level

### Summary RFC: [#3924](https://github.com/rust-lang/rfcs/pull/3924) Implementation: Documentation: Allow Rust library to provide a simple hint about the minimum opt-level to build them with via the `hint.min-opt-level` setting in the manifest. ### Unresolved Issues * [ ] Should a profile with `opt-level = "s"` or `opt-level = "z"` override a dependency's `min-opt-level`? This RFC says it should, but sometimes users might not want that. We could also give more control over that. ### Future Extensions * There are many other possible optimization hints a library *might* wish to provide, and we could consider adding further hints for those in the future. * Have a `max-opt-level`, for crates that don't benefit from `opt-level = 3` to lower the optimiation level to 2. * The mechanism to override a dependency's opt-level using `profile.dev.package` forces a given opt-level whether the dependency asks for higher or lower. Users of overrides, particularly those for `"*"`, might want a mechanism for setting a minimum without overriding a higher optimization level. * Change the default optimization level for the dev profile to 1, rather than 0. opt-level 1 includes optimizations that can make compilation *faster*, such as by sending less code to the codegen backend (e.g. LLVM). * Provide a further mechanism for libraries whose performance depends heavily on their dependencies to optimize those dependencies. * Provide a mechanism for libraries to optimize code inlined or monomorphized by a user's crate. That would likely require compiler enhancements. * Surface this in a visible way for users to see that it has been applied to their dependencies. For instance, we could note it in `--timings`. ### About tracking issues Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however *not* meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.