• Add collation encoding to PlanCache::computeKey(), and a "indexability discriminator" for paths that are assigned string predicates.
• Add a new "collation" private member to PlanCacheEntry and CachedSolution.
• Add a new "collation" option to the index filter commands (planCacheClearFilters, planCacheSetFilter). Note that the implementation of these commands in existing deployments ignores unknown options, so users should not set this option until they've fully upgraded their cluster (if we bump the wire version, it will be possible to have the shell/mongos error if users do this, but it's likely not worth the effort).
• Add a new "collation" option to the plan cache shell helpers.

The "query shape" glossary entry in the manual will also need updating.