Commits on Aug 19, 2023

1. effects: separate :noub effect bit from :consistent …

   The current `:consistent` effect bit carries dual meanings:
   1. "the return value is always consistent"
   2. "this method does not cause any undefined behavior".
   
   This design makes the effect bit unclear and hard to manage.
   Specifically, the current design prevents a post-inference analysis
   (as discussed in #50805) from safely refining
   `:consistent`-cy using post-optimization state IR. This is because it is
   impossible to tell whether the `:consistent`-cy has been tainted by the
   first or second meaning.
   
   To address this, this commit splits them into two distinct effect bits:
   `:consistent` for consistent return values and `:noub` for no undefined
   behavior.
   
   This commit also introduces an override mechanism for `:noub` as it is
   necessary for `@assume_effects` to concrete-evaluate the annotated
   methods. While this might sound risky and not in line with the existing
   designs of `:nonoverlayed` and `:noinbounds`, where their overrides are
   prohibited, but we already have an override mechanism in place for
   `:consistent`, which implicitly overrides `:noub`. Given this precedent,
   the override for `:noub` should probably be justified.

   

   aviatesk committed Aug 19, 2023
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