Migrated from #32911 (discussion)

@jcouv: something to consider for a follow-up: I see that we're only adding the IsReadOnlyAttribute for source method symbols. I wonder if there are other method symbols we should consider (some synthesized ones in particular).

@RikkiGibson: Not certain. There's a few requirements for this to provide a benefit:

1. must be an instance method on a struct type
2. must be called on a ref readonly/in variable

Might be useful to enumerate any cases where (1) is true and then determine if (2) could be true for it.