Rust will actually optimize a chain of string additions to a chain of `push_str`s. This isn't hardcoded, just how addition is implemented. fn add(mut self,

Rust will actually optimize a chain of string additions to a chain of `push_str`s. This isn't hardcoded, just how addition is implemented. fn add(mut self, other: &str) -> String { self.push_str(other); self } https://github.com/rust-lang/rust/blob/master/src/liballoc/string.rs#L1749 https://github.com/rust-lang/rust/blob/master/src/liballoc/s... Of course, upfront sizing is still advantageous, but you avoid the pathological n² case.