`fmt::Arguments::as_statically_known_str` optimizes poorly

I compiled this code with -Copt-level=3

#![feature(fmt_internals)]
#![expect(internal_features)]

#[unsafe(no_mangle)]
fn foo(a: std::fmt::Arguments<'_>) -> Option<&str> {
    a.as_statically_known_str()
}

Godbolt

I got the following assembly:

foo:
        mov     rdx, rsi
        shr     rdx
        xor     eax, eax
        ret

This function should just optimize to returning None, so the first two instructions seem extraneous.

I am unsure if this has any impact on code using the actual fmt public API.

Meta

Reproducible on godbolt with version:

rustc 1.95.0-nightly (7f99507f5 2026-02-19)
binary: rustc
commit-hash: 7f99507f57e6c4aa0dce3daf6a13cca8cd4dd312
commit-date: 2026-02-19
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0
Internal compiler ID: nightly

Reproduce the Rust 1.95.0-nightly example on Godbolt and inspect the generated assembly for fmt::Arguments::as_statically_known_str. Start by tracing that entry point through the compiler sources and its optimization behavior. Done means the unnecessary instructions are eliminated for this case, with relevant compiler tests covering the result.