Go has an experimental arena package [0], but the proposal is on hold and the code may be removed in the future.
C# does not support arenas. But it does provide the stackalloc keyword, whereas in Go you kind of need the compiler's blessing for avoiding heap allocations.
> Another problem is relatively high cost of FFI interop with C. It's gotten better, but Go still needs to switch stacks, etc. How is C#?
Async in C# is implemented as stackless coroutines. Calling into FFI is cheap.
> How does C# compilation speed compare?
In my experience, release builds are a bit slower than Go.
> Does the compiler optimize more aggressively than Go (which does very little optimization)? I've heard the C# AOT compiler is lacking, but it's not clear in what way.
Not much to say on this, but with each new .NET release, a core .NET team member posts a blog post about performance improvements in the new release. The most recent one: https://devblogs.microsoft.com/dotnet/performance-improvemen...
> Does C# have the equivalent of "go run"?
dotnet run
NuGet Gallery [1] is like a centralized DLL registry. Definitely not as good as Go.
> Can you use LINQ against databases like Postgres on Linux, without having to buy into a lot of Microsoft/.NET stuff?
Probably not. This is usually done with EF Core and the Postgres provider.