Use source generators to auto-decorate the function?

in the absence of a dedicated AsyncMethodBuilder, could a source generator be used to rewrite the inner function to use your type?
https://devblogs.microsoft.com/dotnet/introducing-c-source-generators/

Start by reading the linked C# source generators article and reviewing how the issue's proposed rewrite relates to the absence of a dedicated AsyncMethodBuilder. Define whether source generation is feasible for the inner function and establish an explicit acceptance criterion before implementation.

Written by the indexing model from the issue text.