It can compute the answer because it’s able to inline the function call made via the function pointer argument to 'find'. But if you don’t like that example for some reason, look at the Go example I linked above.
The Go example seems like we're giving the game away much the same as the C. Sure enough if I factor out the predicate, the compiler just... doesn't inline it any more. "Hopefully the optimiser will spot what I meant and optimise it". OK.
We got into this with the claim (never substantiated) that this works for the C library function qsort, which is notable because it does indeed work for the C++ and Rust standard library sorts which are done the way I described.
Monomorphization does not guarantee inlining of lambdas. You can perfectly well output a monomorphized version of the 'contains' method specialized to the type of a particular lambda argument and then call this lambda on each loop iteration in the generated code. (Rust's assembly output is pretty impenetrable compared to Go's, but I believe this is exactly what happens if you compile something like this without optimizations: https://godbolt.org/z/qaYbGMMzM)
It does not surprise me that Rust inlines more aggressively than Go, but that doesn't have anything much to do with differences between Rust and Go generics or type systems.
Adopting the type-level fiction that every lambda has a unique type does not fundamentally make inlining any easier for the compiler. It may well be that in Rust the monomorphization triggered by this feature of the type system leads to more inlining than would otherwise happen. But that is just an idiosyncrasy of Rust. Compilers can do whatever kind of inlining they like regardless of whether or not they think that every anonymous function has its own unique type.
Edit: One further thing that's worth noting here is that the Go compiler does do cross-module inlining. So (unlike in the case of a typical C compiler) it is not the case that the kind of inlining we've been talking about here can only occur if the function and the lambda exist in the same module or source file. https://utcc.utoronto.ca/~cks/space/blog/programming/GoInlin...
