Pass the {curve, x}25519-dalek features through the build

Cargo.toml

@@ -10,13 +10,15 @@ edition = "2018"
 readme = "README.md"
 
 [features]
-default = []
+default = ["u64_backend"]
 slow-hash = ["scrypt"]
 bench = []
+u64_backend = ["curve25519-dalek/u64_backend", "x25519-dalek/u64_backend"]
+u32_backend = ["curve25519-dalek/u32_backend", "x25519-dalek/u32_backend"]
 
 [dependencies]
 aead = "0.3.1"
-curve25519-dalek = "2.1.0"
+curve25519-dalek = { version = "2.1.0", default-features = false, features = ["std"]}
 displaydoc = "0.1"
 generic-array = "0.14.2"
 hkdf = "0.9.0-alpha.0"
@@ -25,7 +27,7 @@ rand_core = "0.5.1"
 scrypt = { version = "0.3.0", optional = true }
 sha2 = "0.9.0"
 thiserror = "1.0.19"
-x25519-dalek = "0.6.0"
+x25519-dalek = { version = "0.6.0", default-features = false, features = ["std"]}
 zeroize = "1.1"
 
 [dev-dependencies]

src/lib.rs

@@ -386,6 +386,12 @@
 #![cfg_attr(not(feature = "bench"), deny(missing_docs))]
 #![deny(unsafe_code)]
 
+#[cfg(not(any(feature = "u64_backend", feature = "u32_backend",)))]
+compile_error!(
+    "no dalek arithmetic backend cargo feature enabled! \
+     please enable one of: u64_backend, u32_backend"
+);
+
 // Error types
 pub mod errors;