Why are async state machines classes (and not structs) in Roslyn?

I didn’t have any foreknowledge of this, but since Roslyn is open-source these days, we can go hunting through the code for an explanation.

And here, on line 60 of the AsyncRewriter, we find:

// The CLR doesn't support adding fields to structs, so in order to enable EnC in an async method we need to generate a class.
var typeKind = compilationState.Compilation.Options.EnableEditAndContinue ? TypeKind.Class : TypeKind.Struct;

So, whilst there’s some appeal to using structs, the big win of allowing Edit and Continue to work within async methods was obviously chosen as the better option.