RFC: Safety Tags

[zjp-CN]

Summary

This RFC introduces a concise safety-comment convention for unsafe code in standard libraries:
tag every public unsafe function with #[safety::requires] and call with #[safety::checked].

Safety tags refine today’s safety-comment habits: a featherweight syntax that condenses every
requirement into a single, check-off reminder.

The following snippet compiles today if we enable enough nightly features, but we expect Clippy
and Rust-Analyzer to enforce tag checks and provide first-class IDE support.

#[safety::requires( // 💡 define safety tags on an unsafe function
    ValidPtr = "src must be [valid](https://doc.rust-lang.org/std/ptr/index.html#safety) for reads",
    Aligned = "src must be properly aligned, even if T has size 0",
    Initialized = "src must point to a properly initialized value of type T"
)]
pub unsafe fn read<T>(ptr: *const T) { }

fn main() {
    #[safety::checked( // 💡 discharge safety tags on an unsafe call
        ValidPtr, Aligned, Initialized = "optional reason"
    )]
    unsafe { read(&()) };
}

Rendered

[alice-i-cecile]

This flavor of thing would be nice to introduce more structure (and the possibility for automated tooling) into complex unsafe code bases like bevy_ecs, which I help maintain.

[danjl1100]

I really like the direction this is moving with requires and checked since the initial version.