Candidate selection. See the rustc dev guide for more information on how this works.
- Candidate assembly.
- A cache value for the provisional cache: contains the depth-first number (DFN) and result.
- The “provisional evaluation cache” is used to store intermediate cache results when solving auto traits. Auto traits are unusual in that they can support cycles. So, for example, a “proof tree” like this would be ok:
- When does the builtin impl for
- The result of trait evaluation. The order is important here as the evaluation of a list is the maximum of the evaluations.
- Indicates that trait evaluation caused overflow and in which pass.
- The selection process begins by considering all impls, where clauses, and so forth that might resolve an obligation. Sometimes we’ll be able to say definitively that (e.g.) an impl does not apply to the obligation: perhaps it is defined for
usizebut the obligation is for
i32. In that case, we drop the impl out of the list. But the other cases are considered candidates.
- Replace all regions inside the coroutine interior with late bound regions. Note that each region slot in the types gets a new fresh late bound region, which means that none of the regions inside relate to any other, even if typeck had previously found constraints that would cause them to be related.