build: gate regex API on 'std' feature

This commit adds a new 'std' feature and enables it by default. This
permits us to one day add support for building regex without '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 'std'
feature.

Closes #457

Author: BurntSushi

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 = ["std"]
+# The '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 'std'
+# feature will prevent regex from compiling.
+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.

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

src/lib.rs

@@ -530,11 +530,17 @@ extern crate quickcheck;
 extern crate regex_syntax as syntax;
 extern crate utf8_ranges;
 
+#[cfg(feature = "std")]
 pub use error::Error;
+#[cfg(feature = "std")]
 pub use re_builder::unicode::*;
+#[cfg(feature = "std")]
 pub use re_builder::set_unicode::*;
+#[cfg(feature = "std")]
 pub use re_set::unicode::*;
+#[cfg(feature = "std")]
 pub use re_trait::Locations;
+#[cfg(feature = "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 = "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 = "std")]
 pub mod internal {
     pub use compile::Compiler;
     pub use exec::{Exec, ExecBuilder};