Introduce truncate function

src/backends/rdrand.rs

@@ -124,7 +124,7 @@ unsafe fn rdrand_exact(dest: &mut [MaybeUninit<u8>]) -> Option<()> {
 #[cfg(target_arch = "x86_64")]
 #[target_feature(enable = "rdrand")]
 unsafe fn rdrand_u32() -> Option<u32> {
-    rdrand().map(|val| u32::try_from(val & u64::from(u32::MAX)).unwrap())
+    rdrand().map(crate::utils::truncate)
 }
 
 #[cfg(target_arch = "x86_64")]

src/backends/rndr.rs

@@ -2,7 +2,10 @@
 //!
 //! Arm Architecture Reference Manual for A-profile architecture:
 //! ARM DDI 0487K.a, ID032224, D23.2.147 RNDR, Random Number
-use crate::{util::slice_as_uninit, Error};
+use crate::{
+    util::{slice_as_uninit, truncate},
+    Error,
+};
 use core::arch::asm;
 use core::mem::{size_of, MaybeUninit};
 
@@ -105,8 +108,7 @@ pub fn inner_u32() -> Result<u32, Error> {
     if is_rndr_available() {
         // SAFETY: after this point, we know the `rand` target feature is enabled
         let res = unsafe { rndr() };
-        res.map(|val| u32::try_from(val & u64::from(u32::MAX)).unwrap())
-            .ok_or(Error::RNDR_FAILURE)
+        res.map(truncate).ok_or(Error::RNDR_FAILURE)
     } else {
         Err(Error::RNDR_NOT_AVAILABLE)
     }

src/backends/wasi_p2.rs

@@ -4,7 +4,8 @@ use core::mem::MaybeUninit;
 use wasi::random::random::get_random_u64;
 
 pub fn inner_u32() -> Result<u32, Error> {
-    Ok(get_random_u64() as u64)
+    let val = get_random_u64();
+    Ok(crate::utils::truncate(val))
 }
 
 pub fn inner_u64() -> Result<u32, Error> {

src/util.rs

@@ -75,3 +75,8 @@ pub fn inner_u64() -> Result<u64, Error> {
     // since it returned `Ok`.
     Ok(unsafe { res.assume_init() })
 }
+
+/// Truncates `u64` and returns the lower 32 bits as `u32`
+pub(crate) fn truncate(val: u64) -> u32 {
+    u32::try_from(val & u64::from(u32::MAX)).expect("The higher 32 bits are masked")
+}