Add alloc feature

Author: samuelburnham

Cargo.toml

@@ -17,6 +17,7 @@ resolver = "2"
 [features]
 default = ["std", "derive", "multihash-impl", "secure-hashes"]
 std = ["unsigned-varint/std", "multihash-derive/std"]
+alloc = []
 multihash-impl = ["derive"]
 derive = ["multihash-derive"]
 arb = ["quickcheck", "rand"]

src/lib.rs

@@ -52,7 +52,7 @@
 #![deny(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
-#[cfg(not(feature = "std"))]
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
 #[macro_use]
 extern crate alloc;
 

src/multihash.rs

@@ -1,6 +1,6 @@
 use crate::hasher::{Digest, Size};
 use crate::Error;
-#[cfg(feature = "alloc")]
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 
@@ -160,7 +160,6 @@ impl<S: Size> Multihash<S> {
         write_multihash(w, self.code(), self.size(), self.digest())
     }
 
-  
   #[cfg(any(feature = "std", feature = "alloc"))]
     /// Returns the bytes of a multihash.
     pub fn to_bytes(&self) -> Vec<u8> {