Add a feature for linking to proc_macro.

This feature is enabled by default for backwards compatibility, but
it can be disabled in order to break the runtime dependency on
the dynamic library libproc_macro in the rustc toolchain.

.travis.yml

@@ -10,3 +10,4 @@ rust:
 
 script:
   - cargo test
+  - cargo test --no-default-fatures

Cargo.toml

@@ -10,4 +10,11 @@ keywords = ["syn"]
 include = ["Cargo.toml", "src/**/*.rs", "tests/**/*.rs", "README.md", "LICENSE-APACHE", "LICENSE-MIT"]
 
 [dependencies]
-proc-macro2 = "0.2"
+proc-macro2 = { version = "0.2.3", default-features = false }
+
+[features]
+# Disabling the proc-macro feature removes the dynamic library dependency on
+# libproc_macro in the rustc compiler.
+proc-macro = []
+
+default = ["proc-macro", "proc-macro2/proc-macro"]

src/lib.rs

@@ -94,6 +94,7 @@
 #![doc(html_root_url = "https://docs.rs/quote/0.4.2")]
 
 extern crate proc_macro2;
+#[cfg(feature = "proc-macro")]
 extern crate proc_macro;
 
 mod tokens;
@@ -169,16 +170,19 @@ pub mod __rt {
 /// # Example
 ///
 /// ```
+/// # #[cfg(feature = "proc-macro")]
 /// extern crate proc_macro;
 ///
 /// #[macro_use]
 /// extern crate quote;
 ///
+/// # #[cfg(feature = "proc-macro")]
 /// use proc_macro::TokenStream;
 ///
 /// # const IGNORE_TOKENS: &'static str = stringify! {
 /// #[proc_macro_derive(HeapSize)]
 /// # };
+/// # #[cfg(feature = "proc-macro")]
 /// pub fn derive_heap_size(input: TokenStream) -> TokenStream {
 ///     // Parse the input and figure out what implementation to generate...
 ///     # const IGNORE_TOKENS: &'static str = stringify! {

src/tokens.rs

@@ -2,6 +2,7 @@ use super::ToTokens;
 use std::fmt::{self, Debug, Display};
 use std::hash::{Hash, Hasher};
 
+#[cfg(feature = "proc-macro")]
 use proc_macro;
 use proc_macro2::{TokenStream, TokenTree};
 
@@ -108,6 +109,7 @@ impl From<Tokens> for TokenStream {
     }
 }
 
+#[cfg(feature = "proc-macro")]
 impl From<Tokens> for proc_macro::TokenStream {
     fn from(tokens: Tokens) -> proc_macro::TokenStream {
         TokenStream::from(tokens).into()