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Avoid unreachable integer underflow check in CStr::count_bytes() #163005

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Merged
rust-bors merged 1 commit into from
Sep 21, 2026
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Avoid unreachable integer underflow check in CStr::count_bytes() #163005

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3 changes: 2 additions & 1 deletion library/core/src/ffi/c_str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -513,7 +513,8 @@ impl CStr {
#[stable(feature = "cstr_count_bytes", since = "1.79.0")]
#[rustc_const_stable(feature = "const_cstr_from_ptr", since = "1.81.0")]
pub const fn count_bytes(&self) -> usize {
self.inner.len() - 1
// SAFETY: This length includes the nul-terminator, so it's at least one.
unsafe { self.inner.len().unchecked_sub(1) }

@Mark-Simulacrum Mark-Simulacrum Sep 20, 2026

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I think I'm OK with this. An alternative could be to try to_bytes().len(), which technically should/could avoid the panic (to_bytes_with_nul, which it calls, has an assert_unchecked(!empty)).

But the added indirection doesn't seem like it has a ton of value and probably hurts optimization (at least in terms of how long it takes to compile).

@clarfonthey clarfonthey Sep 20, 2026

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assert_unchecked does actually hurt a lot more than it helps, so, the unchecked sub should just help convey the length information properly, honestly.

}

/// Returns `true` if `self.to_bytes()` has a length of 0.
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