Re: [PATCH 0/2] rust: kernel: add cfg_select! and use it for config-based selection

[Nika Krasnova <nika@nikableh.moe>]

On 2026-06-29 21:07:34 +0200, Miguel Ojeda wrote:
> The tracking issue says the latest change was in Rust 1.91.0. Did it
> not work there? Were there changes after that?

Checked 1.91.0 directly: core::cfg_select is there behind
#![feature(cfg_select)] and behaves the same as 1.94.1 -- single-brace
expression position included. So 1.91.0 is the cutoff (RUSTC_VERSION >=
109100), nothing changed between 1.91.0 and 1.94.1.

> [...] If I understand you correctly, you are saying that even with
> the conversion here, we wouldn't even need the expression case, no?

That is correct. Everything in this series is item or statement 
position;
nothing uses the expression-operand form.

> [...] if we want to go with this one in all versions, then we should
> make sure we only allow that subset that cleanly maps to the future.

Agreed. I'll restrict the fallback to exactly that subset
(item/statement, single braces) by dropping the expression-operand form.
Everyone hits the fallback until the MSRV bump, so a non-portable use
fails uniformly instead of compiling locally and breaking in linux-next.
The accepted subset is a strict subset of core::cfg_select, so the
eventual switch needs no call-site changes. (I'll also have the fallback
emit compile_error! on no-match, to match core.)

> Now, to be honest, the easiest is to just wait a year and then start
> using `core`'s directly... i.e. it is not like we are in a rush to
> start using it anyway...

Fair, I'm fine with dropping it. The counterargument is that the 
fallback
is small and self-contained, and the conversion removes a chunk of
easy-to-desync pairs of #[cfg] now rather than in a year. Happy to send 
a
v2 if you want it.

-- 
Nika Krasnova