Use MaybeUninit registers

.github/workflows/ci.yml

@@ -111,6 +111,8 @@ jobs:
             target: i686-unknown-linux-gnu
           - rust: nightly
             target: hexagon-unknown-linux-musl
+          - rust: nightly-2023-08-23 # The last nightly version that doesn't support MaybeUninit registers.
+            target: hexagon-unknown-linux-musl
           - rust: '1.72' # inline asm for loongarch has been stabilized in Rust 1.72
             target: loongarch64-unknown-linux-gnu
           - rust: stable
@@ -163,6 +165,8 @@ jobs:
             target: powerpc64le-unknown-linux-gnu
           - rust: nightly
             target: riscv32gc-unknown-linux-gnu
+          - rust: nightly-2023-08-23 # The last nightly version that doesn't support MaybeUninit registers.
+            target: riscv32gc-unknown-linux-gnu
           - rust: '1.59'
             target: riscv64gc-unknown-linux-gnu
           - rust: stable
@@ -246,6 +250,12 @@ jobs:
           RUSTFLAGS: ${{ env.RUSTFLAGS }} -C target-feature=+v8
           RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} -C target-feature=+v8
         if: matrix.target == 'armv7-unknown-linux-gnueabi'
+      # arm v6 with legacy cp15_barrier
+      - run: tools/test.sh -vv --tests $TARGET $BUILD_STD $RELEASE
+        env:
+          RUSTFLAGS: ${{ env.RUSTFLAGS }} --cfg atomic_maybe_uninit_use_cp15_barrier
+          RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} --cfg atomic_maybe_uninit_use_cp15_barrier
+        if: matrix.target == 'arm-unknown-linux-gnueabi'
       # arm v7 big endian
       # armeb-unknown-linux-gnueabi is v8 by default, use custom target instead
       - run: tools/test.sh -vv --tests --target armebv7-unknown-linux-gnueabi -Z build-std $RELEASE
@@ -292,6 +302,8 @@ jobs:
           - '1.59'
           - stable
           - beta
+          - nightly-2023-08-23 # The last nightly version that doesn't support MaybeUninit registers.
+          - nightly-2023-08-24 # The oldest nightly version that supports MaybeUninit registers: https://github.com/rust-lang/rust/pull/114790
           - nightly
     runs-on: ubuntu-latest
     timeout-minutes: 60

build.rs

@@ -45,6 +45,10 @@ fn main() {
     if !version.probe(61, 2022, 3, 7) {
         println!("cargo:rustc-cfg=atomic_maybe_uninit_no_const_fn_trait_bound");
     }
+    // https://github.com/rust-lang/rust/pull/114790 merged in nightly-2023-08-24
+    if !version.probe(74, 2023, 8, 23) {
+        println!("cargo:rustc-cfg=atomic_maybe_uninit_no_asm_maybe_uninit");
+    }
 
     match target_arch {
         "loongarch64" => {
@@ -70,8 +74,8 @@ fn main() {
         _ => {}
     }
 
-    let is_apple =
-        target_os == "macos" || target_os == "ios" || target_os == "tvos" || target_os == "watchos";
+    let is_macos = target_os == "macos";
+    let is_apple = is_macos || target_os == "ios" || target_os == "tvos" || target_os == "watchos";
     match target_arch {
         "x86_64" => {
             // x86_64 Apple targets always support CMPXCHG16B:
@@ -126,7 +130,6 @@ fn main() {
         "aarch64" => {
             // aarch64 macOS always supports FEAT_LSE/FEAT_LSE2/FEAT_LRCPC because it is armv8.5-a:
             // https://github.com/llvm/llvm-project/blob/llvmorg-17.0.0-rc2/llvm/include/llvm/TargetParser/AArch64TargetParser.h#L494
-            let is_macos = target_os == "macos";
             let mut has_lse = is_macos;
             let mut has_rcpc = is_macos;
             // FEAT_LSE2 doesn't imply FEAT_LSE. FEAT_LSE128 implies FEAT_LSE but not FEAT_LSE2. FEAT_LRCPC3 implies FEAT_LRCPC.