[feature request] compile time check for experimental features

[jangko]

Currently there is no way to know if an experimental feature is available or not during compile time.
They are only listed in --advanced options.
This is important to write code for both experimental and non-experimental version in a project with possibility using different version of nim compiler supporting different set of experimental features.

[Araq]

Do you need to know whether it's available via .experimental or do you neeed to know whether it's enabled? Probably we need both...

[jangko]

The easiest is probably exposing Features via conditional symbols, then we can use when defined(..) syntax to check for it's availability.

Checking if it is enabled or not can be discussed further, as I'm not sure about the mechanism or whether it is really needed or not.

[timotheecour]

reopening, the are 2 issues, only 1 was fixed:

• whether the compiler supports the flag --experimental:foo (nimHasFoo)
• whether --experimental:foo is activated in some scope (we currently don't have a way to check for this)

it's easy to support global flag (eg --experimental:foo at cmdline), but what's needed is to tell whether --experimental:foo is activated in some scope, because {.experimental:foo.} can be used in a scope (or pushed in a scope)

this should be done via supporting compileOption("experimental", "foo")