linux_raw: inline __NR_getrandom into ARM assembly (#688)

`asm_const` was stabilized only in Rust 1.82, so inlining the constant
relaxes MSRV for the `linux_raw` backend on ARM targets.

Author: newpavlov

CHANGELOG.md

@@ -6,10 +6,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [0.3.4] - UNRELEASED
 
+### Changed
+- Relax MSRV for the `linux_raw` opt-in backend on ARM targets [#688]
+
 ### Added
 - `unsupported` opt-in backend [#667]
 
 [#667]: https://github.com/rust-random/getrandom/pull/667
+[#688]: https://github.com/rust-random/getrandom/pull/688
 
 ## [0.3.3] - 2025-05-09
 

src/backends/linux_raw.rs

@@ -13,7 +13,6 @@ unsafe fn getrandom_syscall(buf: *mut u8, buflen: usize, flags: u32) -> isize {
     // Based on `rustix` and `linux-raw-sys` code.
     cfg_if! {
         if #[cfg(target_arch = "arm")] {
-            const __NR_getrandom: u32 = 384;
             // In thumb-mode, r7 is the frame pointer and is not permitted to be used in
             // an inline asm operand, so we have to use a different register and copy it
             // into r7 inside the inline asm.
@@ -22,10 +21,10 @@ unsafe fn getrandom_syscall(buf: *mut u8, buflen: usize, flags: u32) -> isize {
             // bother with it.
             core::arch::asm!(
                 "mov {tmp}, r7",
-                "mov r7, {nr}",
+                // TODO(MSRV-1.82): replace with `nr = const __NR_getrandom,`
+                "mov r7, #384",
                 "svc 0",
                 "mov r7, {tmp}",
-                nr = const __NR_getrandom,
                 tmp = out(reg) _,
                 inlateout("r0") buf => r0,
                 in("r1") buflen,