build: gate regex API on 'use_std' feature

This commit adds a new 'use_std' feature and enables it by default.
This permits us to one day add support for building regex without
'use_std' (but with 'alloc', probably) by avoiding the introduction
of incompatibilities. Namely, this setup ensures that all of today's
uses of '--no-default-features' won't compile without also adding the
'use_std' feature.

Closes rust-lang#457

Author: BurntSushi

Diff for: Cargo.toml

@@ -43,10 +43,13 @@ quickcheck = { version = "0.6", default-features = false }
 rand = "0.4"
 
 [features]
-# We don't enable any features by default currently, but if the compiler
-# supports a specific type of feature, then regex's build.rs might enable
-# some default features.
-default = []
+default = ["use_std"]
+# The 'use_std' feature permits the regex crate to use the standard library.
+# This is intended to support future use cases where the regex crate may be
+# able to compile without std, and instead just rely on 'core' and 'alloc'
+# (for example). Currently, this isn't supported, and removing the 'use_std'
+# feature will prevent regex from compiling.
+use_std = []
 # A blanket feature that governs whether unstable features are enabled or not.
 # Unstable features are disabled by default, and typically rely on unstable
 # features in rustc itself.

Diff for: build.rs

@@ -12,7 +12,7 @@ fn main() {
     let version = String::from_utf8(output).unwrap();
 
     // If we're using nightly Rust, then we can enable vector optimizations.
-    // Note that these aren't actually activated unless the `nightly` feature
+    // Note that these aren't actually activated unless the `unstable` feature
     // is enabled.
     //
     // We also don't activate these if we've explicitly disabled auto

Diff for: src/lib.rs

@@ -530,11 +530,17 @@ extern crate quickcheck;
 extern crate regex_syntax as syntax;
 extern crate utf8_ranges;
 
+#[cfg(feature = "use_std")]
 pub use error::Error;
+#[cfg(feature = "use_std")]
 pub use re_builder::unicode::*;
+#[cfg(feature = "use_std")]
 pub use re_builder::set_unicode::*;
+#[cfg(feature = "use_std")]
 pub use re_set::unicode::*;
+#[cfg(feature = "use_std")]
 pub use re_trait::Locations;
+#[cfg(feature = "use_std")]
 pub use re_unicode::{
     Regex, Match, Captures,
     CaptureNames, Matches, CaptureMatches, SubCaptureMatches,
@@ -629,6 +635,7 @@ When the `s` flag is enabled, `.` matches any byte.
 In general, one should expect performance on `&[u8]` to be roughly similar to
 performance on `&str`.
 */
+#[cfg(feature = "use_std")]
 pub mod bytes {
     pub use re_builder::bytes::*;
     pub use re_builder::set_bytes::*;
@@ -664,6 +671,7 @@ mod vector;
 /// testing different matching engines and supporting the `regex-debug` CLI
 /// utility.
 #[doc(hidden)]
+#[cfg(feature = "use_std")]
 pub mod internal {
     pub use compile::Compiler;
     pub use exec::{Exec, ExecBuilder};