Transitions to new language semantics

[Mathias LANG]

On Friday, 11 June 2021 at 07:36:47 UTC, Sönke Ludwig wrote:
> This is something that should have been discussed already, but 
> I can't remember whether that was actually the case, and it 
> always bothers me every time there is friction with new DIP 
> switches.
>
> [...]

I agree per-module UDAs would be nice.

We'd have to be careful about templates though: Currently the 
emission strategy is that if a template can be found in a root 
module, it will not be codegened (which is good because codegen 
is slow). With per-module UDAs we'd probably have to emit 
defensively when different attributes are used, and have to 
mangle the attributes into the template (to avoid them being 
folded despite being different).
This could become quite impactful if e.g. the standard library is 
shipped with a different default (try to use `-allinst` and 
you'll see a massive slowdown).

Another thing is that `-preview` switch are, for the most part, 
not finished, nor are they made compatible with libraries. I 
think every `-preview` switch should come with its "Enable by 
default" draft PR to see how much breaks on Buildkite, and those 
failures should mostly be fixed, so that users have a much better 
experience.