Bug: Using foreach, it's possible to accidentally mutate a ref readonly without any unsafe code

[hamarb123]

Version Used:

• Sharplab.io
• VS 17.4.0 with .NET 7.0.100 SDK

Steps to Reproduce:

1. Implement IEnumerable<T> explicitly on a struct, and do not mark the implementations as readonly
2. Cause mutation in your GetEnumerable implementation
3. Run a loop over a ref readonly or in parameter of your type

Minimal repro:
https://sharplab.io/#v2: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

Expected Behavior:

Since the explicit implementations are not marked as readonly, the compiler must assume they can mutate, and therefore should create a defensive copy before calling GetEnumerable.

Actual Behavior:

The compiler doesn't recognise that it needs to create a defensive copy, and thus at the point of foreach, it can mutate the readonly reference.