But it is expensive, isn't it? It's allocating an array even when logging is disabled. There's no overload that just takes a string or object or T, only a params array.

Interesting point, then it's really not obvious from the warning itself and the usage. Looking at the code you don't expect it to allocate an array, I know that's what params string[] do though.

Would it be possible then to have an overload that take a single value for common cases without the array allocation?
Or alternatively a dedicated error message explaining why it's expensive.

Otherwise, I will definitely update the code where it's used with guards, knowing that new information. I will interview some colleagues to see if I was alone with that missing piece of knowledge.

Looking at the code you don't expect it to allocate an array

That's what analyzers are best at, highlighting things you can't see in the code.

Would it be possible then to have an overload that take a single value for common cases without the array allocation?

Possibly, or a params ReadOnlySpan<object> overload; you'd still get boxing for value types, but reference types like string would avoid the array allocation when logging is disabled.

Or alternatively a dedicated error message explaining why it's expensive.

I'd be happy to see the analyzer updated with more details about what the cost is, e.g. array allocation, unknown method call, boxing of value types, string allocation (concatenation, interpolation, ...), etc.