ci: install qemu on Ubuntu 26.04 runners; fix the rustdoc link error

.github/workflows/ci.yaml

@@ -187,7 +187,7 @@ jobs:
       # pinned toolchain too (dtolnay installs for `stable`, which may differ).
       - run: rustup target add aarch64-unknown-linux-gnu
       - name: install aarch64 cross toolchain + qemu-user
-        run: sudo apt-get update && sudo apt-get install -y gcc-aarch64-linux-gnu qemu-user-static
+        run: scripts/install-qemu-user.sh gcc-aarch64-linux-gnu
       - name: NEON SIMD parity (cross-build + qemu run)
         run: ./scripts/neon-parity.sh
 

.github/workflows/simd-matrix.yaml

@@ -236,7 +236,7 @@ jobs:
         with:
           key: cpu-guard-library
       - name: install qemu-user
-        run: sudo apt-get update && sudo apt-get install -y qemu-user-static
+        run: scripts/install-qemu-user.sh
       - name: library warns, executable exits 132 (qemu -cpu Haswell)
         run: bash scripts/cpu-guard-library-probe.sh
 
@@ -250,7 +250,12 @@ jobs:
     # AVX2 for this target, so any `vp*` on a ymm register other than the
     # AVX1 `vptest` came from an intrinsic that escaped its gate), and the
     # SIMD unit tests under emulation.
-    runs-on: ubuntu-latest
+    # Pinned to 24.04: the ubuntu-latest image (Ubuntu 26.04, amd64v3) ships a
+    # glibc built for x86-64-v3, so no host-linked binary starts on an emulated
+    # pre-AVX2 CPU ("Fatal glibc error: CPU does not support x86-64-v3"). This
+    # arm's whole point is running on such a CPU. Move it off 24.04 only with a
+    # static (musl) build of both binaries it runs.
+    runs-on: ubuntu-24.04
     name: realization/avx × x86_64
     steps:
       - uses: actions/checkout@v4
@@ -261,7 +266,7 @@ jobs:
         with:
           key: avx
       - name: install qemu-user
-        run: sudo apt-get update && sudo apt-get install -y qemu-user-static
+        run: scripts/install-qemu-user.sh
       - name: masking parity (sandybridge build, qemu -cpu SandyBridge)
         run: bash scripts/masking-parity.sh avx-qemu
       - name: no AVX2 instruction in the parity binary
@@ -295,7 +300,12 @@ jobs:
     # Nehalem`, so any AVX instruction on the executed path faults. Three
     # rungs, as for `avx`: parity, an assembly witness that the parity binary
     # has no AVX encoding at all, and the SIMD unit tests under emulation.
-    runs-on: ubuntu-latest
+    # Pinned to 24.04: the ubuntu-latest image (Ubuntu 26.04, amd64v3) ships a
+    # glibc built for x86-64-v3, so no host-linked binary starts on an emulated
+    # pre-AVX2 CPU ("Fatal glibc error: CPU does not support x86-64-v3"). This
+    # arm's whole point is running on such a CPU. Move it off 24.04 only with a
+    # static (musl) build of both binaries it runs.
+    runs-on: ubuntu-24.04
     name: realization/v2 × x86_64
     steps:
       - uses: actions/checkout@v4
@@ -306,7 +316,7 @@ jobs:
         with:
           key: v2
       - name: install qemu-user
-        run: sudo apt-get update && sudo apt-get install -y qemu-user-static
+        run: scripts/install-qemu-user.sh
       - name: masking parity (x86-64-v2 build, qemu -cpu Nehalem)
         run: bash scripts/masking-parity.sh v2-qemu
       - name: no AVX instruction in the parity binary
@@ -349,7 +359,7 @@ jobs:
         with:
           key: aarch64
       - name: install aarch64 cross toolchain + qemu-user
-        run: sudo apt-get update && sudo apt-get install -y gcc-aarch64-linux-gnu qemu-user-static
+        run: scripts/install-qemu-user.sh gcc-aarch64-linux-gnu
       - name: masking parity (neon, qemu)
         run: bash scripts/masking-parity.sh neon-qemu
       - name: codegen witness (neon)

CLAUDE.md

@@ -273,14 +273,23 @@ available here" and is in fact a **missing linker**, then a missing
 interpreter, in two separate steps:
 
 ```sh
-sudo apt-get update
-sudo apt-get install -y gcc-aarch64-linux-gnu qemu-user-static
+scripts/install-qemu-user.sh gcc-aarch64-linux-gnu
 ```
 
-`qemu-user` alone is NOT enough: the script invokes `qemu-aarch64-static`,
-and the dynamic `qemu-aarch64` from `qemu-user` leaves a second, differently
-worded failure (`command not found`) that looks like a fresh problem rather
-than the same one. Install `qemu-user-static`.
+The scripts invoke `qemu-aarch64-static` / `qemu-x86_64-static`, so plain
+`qemu-user` alone leaves a second, differently worded failure
+(`command not found`) that looks like a fresh problem rather than the same one.
+
+> ⊘ **CORRECTED 2026-11-05.** This read *"Install `qemu-user-static`"* with
+> `sudo apt-get install -y gcc-aarch64-linux-gnu qemu-user-static`. Ubuntu 26.04
+> (the GitHub `ubuntu-latest` image since early November) dropped that package:
+> it is now virtual with no installation candidate and `apt-get install` exits
+> 100, which turned `neon-simd/parity-qemu` red on PR #408 before anything was
+> built. `scripts/install-qemu-user.sh` installs `qemu-user-static` where it
+> still exists, else `qemu-user` plus links for the `-static` names; every CI
+> qemu step calls it. Both branches were run on Ubuntu 24.04
+> (`QEMU_INSTALL_FORCE_PLAIN=1` takes the 26.04 branch), and
+> `scripts/neon-parity.sh` passed through the linked plain `qemu-aarch64`.
 
 **And `native` is the HOST arm — it names no tier at all.**
 

scripts/install-qemu-user.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Install the qemu user-mode emulators so `qemu-aarch64-static` and
+# `qemu-x86_64-static` resolve on every Ubuntu runner image.
+#
+# Ubuntu 26.04 dropped the `qemu-user-static` package (it is now a virtual
+# package with no installation candidate, and `apt-get install` exits 100).
+# There, `qemu-user` carries the emulators under their plain names, so this
+# script links the `-static` names the parity scripts call. On images that
+# still ship `qemu-user-static` it installs that, as before.
+#
+# Usage: scripts/install-qemu-user.sh [extra apt packages...]
+# Set QEMU_INSTALL_FORCE_PLAIN=1 to take the `qemu-user` path on any image.
+set -euo pipefail
+
+sudo apt-get update
+if [ "${QEMU_INSTALL_FORCE_PLAIN:-0}" != 1 ] \
+   && apt-cache show qemu-user-static 2>/dev/null | grep -q '^Filename:'; then
+  sudo apt-get install -y "$@" qemu-user-static
+else
+  sudo apt-get install -y "$@" qemu-user
+fi
+
+for arch in aarch64 x86_64; do
+  if [ "${QEMU_INSTALL_FORCE_PLAIN:-0}" = 1 ] || ! command -v "qemu-$arch-static" >/dev/null; then
+    real=$(command -v "qemu-$arch") || { echo "qemu-$arch is not installed" >&2; exit 1; }
+    sudo ln -sf "$real" "/usr/local/bin/qemu-$arch-static"
+  fi
+  echo "qemu-$arch-static -> $(readlink -f "$(command -v "qemu-$arch-static")")"
+  "qemu-$arch-static" --version | head -1
+done

src/simd_stride.rs

@@ -70,7 +70,7 @@ pub fn lut_u8(values: &[u8], table: &[u8; 256], out: &mut [u8]) {
 /// authorization or alpha-claim from these bits.
 ///
 /// # Panics
-/// If table[0] != 0 or input/output slices are too short.
+/// If `table[0] != 0` or input/output slices are too short.
 ///
 /// # Examples
 /// ```