Missing feature gate for the default of `'static`

[nikomatsakis]

We neglected to add a feature gate for the new default to 'static in statics and constants (added in #35915). Need to fix this pronto.

cc @llogiq -- you up for that? (Sorry to ask you twice, just decided I wanted a way to track this so I don't forget.)

[llogiq]

Sure. I guess I'll need to look into how feature gates are implemented and then conditionally use the old/new behavior.

[nikomatsakis]

I am up for mentoring this for whomever (hence the E-mentor tag). This was my description of how to add the change. I think what you need to do to fix this is as follows:

• add the feature gate (in libsyntax/feature_gate.rs)
• make a new rscope, I think, instead of re-using the elidable rscope (or else add a bool to the elidable one)
• this new rscope, in the method base_object_lifetime_default, should check the feature-gate (self.tcx().sess.features.borrow().static_in_const). If it is not set, it should issue an error to the given span suggesting the use of the feature gate. Either way it can return 'static, I guess.

[nikomatsakis]

ok @llogiq above comment may help :)

[nikomatsakis]

closed by #36332