Detect arm64-ilp32 just enough to say that it's not supported.

arm64-ilp32 is a 32-bit ABI on a 64-bit platform, just like x32 is a
32-bit abi on x86-64.

build.rs

@@ -26,7 +26,9 @@ fn main() {
     let arch = var("CARGO_CFG_TARGET_ARCH").unwrap();
     let asm_name = format!("{}/{}.s", OUTLINE_PATH, arch);
     let os_name = var("CARGO_CFG_TARGET_OS").unwrap();
-    let is_x32 = arch == "x86_64" && var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap() == "32";
+    let pointer_width = var("CARGO_CFG_TARGET_POINTER_WIDTH").unwrap();
+    let is_x32 = arch == "x86_64" && pointer_width == "32";
+    let is_arm64_ilp32 = arch == "aarch64" && pointer_width == "32";
     println!("cargo:rerun-if-env-changed=CARGO_CFG_TARGET_ARCH");
 
     // If rustix_use_libc is set, or if we're on an architecture/OS that doesn't
@@ -35,6 +37,7 @@ fn main() {
         || os_name != "linux"
         || std::fs::metadata(&asm_name).is_err()
         || is_x32
+        || is_arm64_ilp32
     {
         use_feature("libc");
     } else {

src/imp/linux_raw/arch/inline/aarch64.rs

@@ -3,6 +3,9 @@
 use crate::imp::reg::{ArgReg, FromAsm, RetReg, SyscallNumber, ToAsm, A0, A1, A2, A3, A4, A5, R0};
 use core::arch::asm;
 
+#[cfg(target_pointer_width = "32")]
+compile_error!("arm64-ilp32 is not supported yet");
+
 #[inline]
 #[must_use]
 pub(in crate::imp) unsafe fn syscall0_readonly(nr: SyscallNumber<'_>) -> RetReg<R0> {

src/imp/linux_raw/arch/outline/aarch64.s

@@ -6,6 +6,8 @@
 //
 // outline.rs takes care of reordering the nr argument to the end for us,
 // so we only need to move nr into w8.
+//
+// arm64-ilp32 is not yet supported.
 
     .file       "aarch64.s"
     .arch       armv8-a