fix: add architecture gating for inline-assembly

Signed-off-by: Brandon H. Gomes <bhgomes@pm.me>

field/src/goldilocks_field.rs

@@ -305,10 +305,9 @@ impl DivAssign for GoldilocksField {
 #[inline(always)]
 #[cfg(target_arch = "x86_64")]
 unsafe fn add_no_canonicalize_trashing_input(x: u64, y: u64) -> u64 {
-    use core::arch::asm;
     let res_wrapped: u64;
     let adjustment: u64;
-    asm!(
+    core::arch::asm!(
         "add {0}, {1}",
         // Trick. The carry flag is set iff the addition overflowed.
         // sbb x, y does x := x - y - CF. In our case, x and y are both {1:e}, so it simply does

util/Cargo.toml

@@ -3,5 +3,3 @@ name = "plonky2_util"
 description = "Utilities used by Plonky2"
 version = "0.1.0"
 edition = "2021"
-
-[dependencies]

util/src/lib.rs

@@ -9,8 +9,6 @@
 extern crate alloc;
 
 use alloc::vec::Vec;
-use core::arch::asm;
-use core::convert::Infallible;
 use core::hint::unreachable_unchecked;
 use core::mem::size_of;
 use core::ptr::{swap, swap_nonoverlapping};
@@ -19,15 +17,6 @@ use crate::transpose_util::transpose_in_place_square;
 
 mod transpose_util;
 
-/// Converts `result` into the [`Ok`] variant of [`Result`].
-#[inline]
-pub fn into_ok<T>(result: Result<T, Infallible>) -> T {
-    match result {
-        Ok(value) => value,
-        _ => unreachable!("The `Infallible` value cannot be constructed."),
-    }
-}
-
 pub fn bits_u64(n: u64) -> usize {
     (64 - n.leading_zeros()) as usize
 }
@@ -281,8 +270,19 @@ pub fn assume(p: bool) {
 /// This function has no semantics. It is a hint only.
 #[inline(always)]
 pub fn branch_hint() {
+    // NOTE: These are the currently supported assembly architectures. See the
+    // [nightly reference](https://doc.rust-lang.org/nightly/reference/inline-assembly.html) for
+    // the most up-to-date list.
+    #[cfg(any(
+        target_arch = "aarch64",
+        target_arch = "arm",
+        target_arch = "riscv32",
+        target_arch = "riscv64",
+        target_arch = "x86",
+        target_arch = "x86_64",
+    ))]
     unsafe {
-        asm!("", options(nomem, nostack, preserves_flags));
+        core::arch::asm!("", options(nomem, nostack, preserves_flags));
     }
 }