Fix root crate compilation dependencies to force sha1*

crates/sha1w/Cargo.toml

@@ -16,5 +16,6 @@ sha1-crypto-hash = ["crypto-hash"]
 sha1-ring = ["ring"]
 
 [dependencies]
+assert_cfg = "0.1.0"
 crypto-hash = { version = "0.3", optional = true }
 ring = { version = "0.17", optional = true }

crates/sha1w/src/lib.rs

@@ -11,6 +11,11 @@ pub trait ISha1 {
     fn finish(self) -> [u8; 20];
 }
 
+assert_cfg::exactly_one! {
+    feature = "sha1-crypto-hash",
+    feature = "sha1-ring",
+}
+
 #[cfg(feature = "sha1-crypto-hash")]
 mod crypto_hash_impl {
     use super::ISha1;