From "life before main" to "common life in main"

language design
25

I only just got pinged so I hadn't seen this thread until now, but since my crates were mentioned: I feel strongly that the linkme form of the API is the right one for Rust, and that executing code at runtime before main is unnecessary and should not be added to Rust. The distributed slice elements in linkme are each static so they are mandated to be compile time constructible (link time technically, since you can have references to other statics, unlike in const).

Basically these comments got it right:

And yeah, it's gonna need compiler support, along the lines of:

The only extension I'd make to that comment is that on the platforms where we can't count on the linker to handle building the slices, rustc can solve this on its own by propagating the elements through rmeta (as if they were macros) through all the layers of the dependency graph until the point that the rustc invocation that's compiling main can get all the final slices all put together, prior to any linking.

The way this works is equivalent to if every crate implicitly got the following in its root module:

pub(rustc) mod distributed_slice_elements {
    // for each of my direct dependencies:
    pub(rustc) use $the_dependency::distributed_slice_elements::*;
    // (except deduplicated in the case of diamond dependencies)
}
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