Auto merge of #113210 - fee1-dead-contrib:effects-mvp, r=oli-obk

Effects/keyword generics MVP

This adds `feature(effects)`, which adds `const host: bool` to the generics of const functions, const traits and const impls. This will be used to replace the current logic around const traits.

r? `@oli-obk`

compiler/rustc_middle/src/ty/generics.rs

@@ -135,6 +135,9 @@ pub struct Generics {
 
     pub has_self: bool,
     pub has_late_bound_regions: Option<Span>,
+
+    // The index of the host effect when substituted. (i.e. might be index to parent substs)
+    pub host_effect_index: Option<usize>,
 }
 
 impl<'tcx> Generics {