I'm finding F# to be even closer because it has HM type inference, discriminated unions (note they are not as layout-efficient as Rust's, it is better in F# 9 though), is fully expression oriented and overall a quite terse language (Rust isn't really terse but can be made so in some cases).

It's worse at dealing with ref structs so C# is better for some low-level tasks. But it has other niceties like IL-level function and lambda inlining.

I have been a huge (but lonely) proponent of C# and rust being the best two-language combo. But once (someday! Soon they say!) first-class discriminated union support lands, it’ll be the ideal recommendation for anyone that wants to get away from manually taking care of lifetimes.

You are correct, C# straddles that line better than any other language right now imo thanks to the APIs you linked. There was a good write up about this Rust vs C# Span comparison on HN a few weeks ago but the link escapes me

D would fit in as well, if the community wasn't so tiny.

C# was derided as "Microsoft's Java" from the beginning, even though it pulled ahead very early on with generics and never looked back. Microsoft was loathed by many for a long time. Go is popular because Google is (was?) loved/admired.