Reinstantiate no_std cargo feature flag.

The feature was removed in d4c189a due to heavy refactoring of libcore
before rust 1.0.

.travis.yml

@@ -10,3 +10,5 @@ script:
 - (test $TRAVIS_RUST_VERSION != "nightly" || (cd phf_macros && cargo bench))
 - (cd phf_codegen && cargo test)
 - (cd phf_codegen/test && cargo test)
+- (cd phf_shared && cargo build --features core)
+- (cd phf && cargo build --features core)

README.md

@@ -24,6 +24,8 @@ PHF data structures can be constucted via either the compiler plugins in the
 Compiler plugins are not a stable part of Rust at the moment, so `phf_macros`
 can only be used with nightlies.
 
+The `phf/core` feature will compile the `phf` crate with a dependency on libcore instead of libstd, enabling use in environments where libstd will not work.
+
 phf_macros
 ===========
 

phf/Cargo.toml

@@ -12,6 +12,9 @@ name = "phf"
 path = "src/lib.rs"
 test = false
 
+[features]
+core = ["phf_shared/core"]
+
 [dependencies]
 debug-builders = "0.1"
 phf_shared = { version = "=0.7.5", path = "../phf_shared" }

phf/src/lib.rs

@@ -5,10 +5,20 @@
 //! the documentation of those crates for more details.
 #![doc(html_root_url="https://sfackler.github.io/rust-phf/doc/v0.7.5")]
 #![warn(missing_docs)]
+#![cfg_attr(feature = "core", feature(no_std))]
+#![cfg_attr(feature = "core", no_std)]
 
 extern crate debug_builders;
 extern crate phf_shared;
 
+#[cfg(feature = "core")]
+mod std {
+    pub use core::{borrow, fmt, iter, option, ops, slice};
+    pub mod prelude {
+        pub use core::prelude as v1;
+    }
+}
+
 pub use phf_shared::PhfHash;
 #[doc(inline)]
 pub use map::Map;

phf_shared/Cargo.toml

@@ -11,3 +11,6 @@ documentation = "https://sfackler.github.io/rust-phf/doc/v0.7.5/phf_shared"
 name = "phf_shared"
 path = "src/lib.rs"
 test = false
+
+[features]
+core = []

phf_shared/src/lib.rs

@@ -1,4 +1,11 @@
 #![doc(html_root_url="http://sfackler.github.io/rust-phf/doc/v0.7.5")]
+#![cfg_attr(feature = "core", feature(no_std, core_slice_ext, core_str_ext))]
+#![cfg_attr(feature = "core", no_std)]
+
+#[cfg(feature = "core")]
+mod std {
+    pub use core::{slice, str, hash};
+}
 
 use std::hash::{Hasher, Hash, SipHasher};