do detection only once

sha2/Cargo.toml

@@ -16,6 +16,13 @@ fake-simd = "0.1"
 opaque-debug = "0.2"
 sha2-asm = { version="0.5", optional=true }
 
+[dependencies.lazy_static]
+version = "1.4.0"
+default-features = false
+# no_std feature is an anti-pattern. Why, lazy_static, why?
+# See https://github.com/rust-lang-nursery/lazy-static.rs/issues/150
+features = ["spin_no_std"]
+
 [dev-dependencies]
 digest = { version = "0.8", features = ["dev"] }
 hex-literal = "0.1"

sha2/src/sha256.rs

@@ -10,25 +10,25 @@ use platform::Implementation;
 type BlockSize = U64;
 type Block = GenericArray<u8, BlockSize>;
 
+lazy_static::lazy_static! {
+    static ref IMPL: Implementation = Implementation::detect();
+}
+
 /// A structure that represents that state of a digest computation for the
 /// SHA-2 512 family of digest functions
 #[derive(Clone)]
 struct Engine256State {
     h: [u32; 8],
-    implementation: Implementation,
 }
 
 impl Engine256State {
     fn new(h: &[u32; STATE_LEN]) -> Engine256State {
-        Engine256State {
-            h: *h,
-            implementation: Implementation::detect(),
-        }
+        Engine256State { h: *h }
     }
 
     pub fn process_block(&mut self, block: &Block) {
         let block = unsafe { &*(block.as_ptr() as *const [u8; 64]) };
-        self.implementation.compress256(&mut self.h, block);
+        IMPL.compress256(&mut self.h, block);
     }
 }