`String.extend(repeat_n(...))` could optimize better

I tried this code (godbolt):

pub fn f(s: &mut String, n: usize) {
    s.extend(std::iter::repeat_n(' ', n))
}

I expected to see this happen: optimizes to memset or similar

Instead, this happened: scalar loop with unoptimized capacity checks/reserve calls

The equivalent version using Vec optimizes as expected:

pub fn f(s: &mut String, n: usize) {
    unsafe { s.as_mut_vec() }.extend(std::iter::repeat_n(b' ', n));
}

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (3672a55b7 2025-08-13)
binary: rustc
commit-hash: 3672a55b7cfd0a12e7097197b6242872473ffaa7
commit-date: 2025-08-13
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Start with the linked Godbolt reproducer and inspect the implementations of String::extend and repeat_n while comparing their generated assembly with the Vec version. Determine why capacity checks and reserve calls remain in the String path; done means the reported case generates memset-like code without regressing relevant behavior.