Auto merge of #89139 - camsteffen:write-perf, r=Mark-Simulacrum

Use ZST for fmt unsafety

as suggested here - https://github.com/rust-lang/rust/pull/83302#issuecomment-923529151.

diff --cc compiler/rustc_span/src/symbol.rs
Simple merge

diff --cc src/tools/clippy/clippy_utils/src/higher.rs
Simple merge