`size_of_val_raw` doesn't assume that `[u8]` has size at most `isize::MAX`

I tried this code:

#![feature(layout_for_ptr)]
use std::mem::size_of_val_raw;

#[unsafe(no_mangle)]
pub fn a(x: *const [u8]) -> bool {
    unsafe { size_of_val_raw(x) <= isize::MAX.cast_unsigned() }
}

#[unsafe(no_mangle)]
pub fn b(x: *const [u16]) -> bool {
    unsafe { size_of_val_raw(x) <= isize::MAX.cast_unsigned() }
}

I expected both functions to optimize to returning true, since the documentation for size_of_val_raw states that, for the operation to not be UB, the size of the slice must be at most isize::MAX.

Instead, only the b function optimizes to returning true.

Godbolt link

a:
        test    rsi, rsi
        setns   al
        ret

b:
        mov     al, 1
        ret

Meta

Reproducible on godbolt with version info:

rustc 1.99.0-nightly (0e29c21d9 2026-07-21)
binary: rustc
commit-hash: 0e29c21d93abc9bdc330182be1fbd33cfd5fee68
commit-date: 2026-07-21
host: x86_64-unknown-linux-gnu
release: 1.99.0-nightly
LLVM version: 22.1.8
Internal compiler ID: nightly

Start with the reduced functions a and b and compare their optimized output using the linked Godbolt reproduction. Read the documentation and implementation path for size_of_val_raw, focusing on how slice element types affect the isize::MAX validity assumption. Done means [u8] receives the same guaranteed-true optimization as [u16], with coverage for both cases.