run Miri on CI

.github/workflows/main.yml

@@ -160,6 +160,21 @@ jobs:
         rm -rf /tmp/.buildx-cache
         mv /tmp/.buildx-cache-new /tmp/.buildx-cache
 
+  miri:
+    name: Miri
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: true
+    - name: Install Rust (rustup)
+      run: rustup update nightly --no-self-update && rustup default nightly
+      shell: bash
+    - run: rustup component add miri
+    - run: cargo miri setup
+    - uses: Swatinem/rust-cache@v2
+    - run: ./ci/miri.sh
+
   rustfmt:
     name: Rustfmt
     runs-on: ubuntu-latest
@@ -190,6 +205,7 @@ jobs:
       - test
       - rustfmt
       - clippy
+      - miri
     runs-on: ubuntu-latest
     # GitHub branch protection is exceedingly silly and treats "jobs skipped because a dependency
     # failed" as success. So we have to do some contortions to ensure the job fails if any of its

ci/miri.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -ex
+
+# We need Tree Borrows as some of our raw pointer patterns are not
+# compatible with Stacked Borrows.
+export MIRIFLAGS="-Zmiri-tree-borrows"
+
+# One target that sets `mem-unaligned` and one that does not,
+# and a big-endian target.
+TARGETS=(x86_64-unknown-linux-gnu
+    armv7-unknown-linux-gnueabihf
+    s390x-unknown-linux-gnu)
+for TARGET in "${TARGETS[@]}"; do
+    # Only run the `mem` tests to avoid this taking too long.
+    cargo miri test --manifest-path testcrate/Cargo.toml --features no-asm --target $TARGET -- mem
+done

testcrate/tests/mem.rs

@@ -128,11 +128,13 @@ fn memcmp_eq() {
 #[test]
 fn memcmp_ne() {
     let arr1 @ arr2 = gen_arr::<256>();
-    for i in 0..256 {
+    // Reduce iteration count in Miri as it is too slow otherwise.
+    let limit = if cfg!(miri) { 64 } else { 256 };
+    for i in 0..limit {
         let mut diff_arr = arr1;
         diff_arr.0[i] = 127;
         let expect = diff_arr.0[i].cmp(&arr2.0[i]);
-        for k in i + 1..256 {
+        for k in i + 1..limit {
             let result = unsafe { memcmp(diff_arr.0.as_ptr(), arr2.0.as_ptr(), k) };
             assert_eq!(expect, result.cmp(&0));
         }