crc-fast-rust: testing the no_std fix against CI; really gating spin dep in no_std builds

Author: loadingalias

.gitignore

@@ -5,3 +5,4 @@
 .DS_Store
 .git
 .vscode
+walkthrough.md

Cargo.toml

@@ -35,11 +35,22 @@ rustversion = "1.0"
 indexmap = { version = ">=2.11.0, <2.12.0", optional = true }
 
 # no_std support
-# spin is always required for no_std builds (feature detection synchronization)
-spin = { version = "0.10.0", default-features = false, features = ["once", "rwlock", "mutex", "spin_mutex"] }
+# spin is optional - only needed on architectures with SIMD (for feature detection)
+# or when cache feature is enabled
+spin = { version = "0.10.0", default-features = false, features = ["once", "rwlock", "mutex", "spin_mutex"], optional = true }
 # hashbrown is only needed when caching is enabled in no_std
 hashbrown = { version = "0.16.0", optional = true }
 
+# Target-specific dependencies: spin is needed for feature detection on SIMD-capable architectures
+[target.'cfg(target_arch = "aarch64")'.dependencies]
+spin = { version = "0.10.0", default-features = false, features = ["once", "rwlock", "mutex", "spin_mutex"] }
+
+[target.'cfg(target_arch = "x86")'.dependencies]
+spin = { version = "0.10.0", default-features = false, features = ["once", "rwlock", "mutex", "spin_mutex"] }
+
+[target.'cfg(target_arch = "x86_64")'.dependencies]
+spin = { version = "0.10.0", default-features = false, features = ["once", "rwlock", "mutex", "spin_mutex"] }
+
 [dev-dependencies]
 criterion = "0.7"
 cbindgen = "0.29"
@@ -84,7 +95,7 @@ ffi = [] # C/C++ compatible dynamic/static library, planned to become optional i
 
 # optional features
 cli = ["std"] # command line interface binaries (checksum, arch-check, get-custom-params)
-cache = ["alloc", "hashbrown"] # no_std caching requires alloc + hashbrown HashMap
+cache = ["alloc", "hashbrown", "spin"] # no_std caching requires alloc + hashbrown HashMap + spin for synchronization
 
 # the features below are deprecated, aren't in use, and will be removed in the next MAJOR version (v2)
 vpclmulqdq = [] # deprecated, VPCLMULQDQ stabilized in Rust 1.89.0

src/arch/x86_64/avx512_vpclmulqdq.rs

@@ -66,6 +66,7 @@ impl Simd512 {
         ))
     }
 
+    #[cfg(feature = "std")]
     #[inline]
     #[target_feature(enable = "avx512f")]
     unsafe fn extract_u64s(&self) -> [u64; 8] {
@@ -93,6 +94,7 @@ impl Simd512 {
         Self(_mm512_xor_si512(self.0, other.0))
     }
 
+    #[cfg(feature = "std")]
     #[inline]
     #[target_feature(enable = "avx512f")]
     #[allow(unused)]
@@ -382,7 +384,7 @@ unsafe fn reflect_bytes512(reflector: &Reflector512, data: Simd512) -> Simd512 {
 // pre-compute the reverse indices for 512-bit shuffling
 #[rustversion::since(1.89)]
 static REVERSE_INDICES_512: __m512i =
-    unsafe { std::mem::transmute([7u64, 6u64, 5u64, 4u64, 3u64, 2u64, 1u64, 0u64]) };
+    unsafe { core::mem::transmute([7u64, 6u64, 5u64, 4u64, 3u64, 2u64, 1u64, 0u64]) };
 
 // Implement a 512-bit byte shuffle function
 #[rustversion::since(1.89)]

src/crc32/mod.rs

@@ -5,5 +5,8 @@
 pub mod algorithm;
 pub mod consts;
 
-#[cfg(any(target_arch = "x86", target_arch = "x86_64", target_arch = "aarch64"))]
+#[cfg(all(
+    feature = "std",
+    any(target_arch = "x86", target_arch = "x86_64", target_arch = "aarch64")
+))]
 pub(crate) mod fusion;