My expectations are betrayed, but I do appreciate that Rust at least stops me if that expectation matters. Really, whenever I concatenate something variable-le

My expectations are betrayed, but I do appreciate that Rust at least stops me if that expectation matters. Really, whenever I concatenate something variable-length like a string, I know there's potential allocation under the hood. There has to be. What I don't get is why that allocation has to be hidden as a mutation of a, rather than just be explicit as yeah, c is a new thing. I expect functions to return new things rather than mutate things via side effects. Isn't it better to let addition stay a function, rather than overload of + being slightly more convenient/efficient for building up strings?