Cleanly error out when building without the alloc feature

.travis.yml

@@ -11,7 +11,7 @@ script:
   - if [[ "$TRAVIS_NIGHTLY" == "true" ]]; then cargo test --verbose --all-features; fi
   - if [[ "$TRAVIS_NIGHTLY" == "true" ]]; then cargo bench --verbose --all-features --no-run; fi
   - if [[ "$TRAVIS_NIGHTLY" == "true" ]]; then rustup target add thumbv7m-none-eabi; fi
-  - if [[ "$TRAVIS_NIGHTLY" == "true" ]]; then cargo build --verbose --no-default-features --target thumbv7m-none-eabi; fi
+  - if [[ "$TRAVIS_NIGHTLY" == "true" ]]; then cargo build --verbose --no-default-features --features=alloc --target thumbv7m-none-eabi; fi
   - if [[ "$TRAVIS_NIGHTLY" != "true" ]]; then cargo test --verbose; fi
 
 cache: cargo

src/lib.rs

@@ -49,6 +49,9 @@
 //! ```
 #![cfg_attr(not(test), no_std)]
 
+#[cfg(not(feature = "alloc"))]
+compile_error!("This crate does not yet support environments without liballoc. See https://github.com/RustCrypto/RSA/issues/51.");
+
 #[cfg(feature = "alloc")]
 extern crate alloc;
 #[cfg(feature = "std")]
@@ -67,40 +70,55 @@ extern crate hex;
 #[cfg(all(test, feature = "serde"))]
 extern crate serde_test;
 
+#[cfg(feature = "alloc")]
 pub use num_bigint::BigUint;
 
 /// Useful algorithms.
+#[cfg(feature = "alloc")]
 pub mod algorithms;
 
 /// Error types.
+#[cfg(feature = "alloc")]
 pub mod errors;
 
 /// Supported hash functions.
+#[cfg(feature = "alloc")]
 pub mod hash;
 
 /// Supported padding schemes.
+#[cfg(feature = "alloc")]
 pub mod padding;
 
 #[cfg(feature = "pem")]
 pub use pem;
 
+#[cfg(feature = "alloc")]
 mod key;
+#[cfg(feature = "alloc")]
 mod oaep;
 #[cfg(feature = "std")]
 mod parse;
+#[cfg(feature = "alloc")]
 mod pkcs1v15;
+#[cfg(feature = "alloc")]
 mod pss;
+#[cfg(feature = "alloc")]
 mod raw;
 
+#[cfg(feature = "alloc")]
 pub use self::hash::Hash;
+#[cfg(feature = "alloc")]
 pub use self::key::{PublicKey, PublicKeyParts, RSAPrivateKey, RSAPublicKey};
+#[cfg(feature = "alloc")]
 pub use self::padding::PaddingScheme;
 
 // Optionally expose internals if requested via feature-flag.
 
 #[cfg(not(feature = "expose-internals"))]
+#[cfg(feature = "alloc")]
 mod internals;
 
 /// Internal raw RSA functions.
 #[cfg(feature = "expose-internals")]
+#[cfg(feature = "alloc")]
 pub mod internals;