Module engine 

Structs

ObligationCtxt

Used if you want to have pleasant experience when dealing with obligations outside of hir or mir typeck.

Enums

FulfillmentEngine

A fulfillment engine, stored inline rather than boxed as a dyn TraitEngine because some of its holders (e.g. ObligationCtxt) are created very often (once per candidate probe during method resolution), so the heap allocation would be expensive.