Warn on readonly SpinLock fields

[JC3]

Brief description:

Would like to see Roslyn warn if you've got a System.Threading.SpinLock in a readonly field, as the lock will not function correctly in this case, and the exceptions that are thrown can be difficult to interpret (for example).

I think this'd be a good one to catch.

Languages applicable:

I'm only familiar with C#.

Code example that the analyzer should report:

readonly SpinLock someLock;
// presence of initializer at declaration point does not matter.

Where the suggested fix is to remove the readonly attribute.

[Youssef1313]

Looks like a candidate for CAxxxx about runtime API usage (I don't personally think it's a warning wave candidate).

[Youssef1313]

This is already a candidate. See dotnet/runtime#33773.

[JC3]

Looks like a candidate for CAxxxx about runtime API usage (I don't personally think it's a warning wave candidate).

That makes way more sense than what I said.

[sharwell]

Duplicate of dotnet/runtime#33773