perhaps more of a question than a proposal evaluation, but i saw in the pitch some discussion regarding the feasibility of using the new migration mode in the compiler to power 3rd party tooling to apply the automated migration changes. however, it's not clear to me how straightforward this would be to do given the current proposal.
from looking through the source a bit, it seems like non-SPM build systems would be able to easily invoke the new migration compiler commands, but the subsequent step of processing serialized diagnostics to extract, filter, and apply any relevant fix-its would perhaps have to be re-implemented separately from the implementation in SPM.
having the ability to apply fix-its in an automated manner seems like it could be a generically useful ability, independent of managing feature migrations. swiftc even has a (seemingly-now-defunct) flag '-fixit-all' that suggests it at one time did something like this.
i'm curious to hear if the authors have any thoughts about the possibility of exposing some of the more generic internal components built for the migration feature to support other use cases (or the same use case in other build environments).