make congruence-closure (and hence impl Trait) optional

Author: nikomatsakis

Cargo.toml

@@ -7,6 +7,9 @@ repository = "https://github.com/nikomatsakis/ena"
 version = "0.5.0"
 authors = ["Niko Matsakis <[email protected]>"]
 
+[features]
+congruence-closure = [ "petgraph" ]
+
 [dependencies]
 log = "0.3"
-petgraph = "0.4.5"
+petgraph = { version = "0.4.5", optional = true }

src/cc/mod.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "congruence-closure")]
+
 //! An implementation of the Congruence Closure algorithm based on the
 //! paper "Fast Decision Procedures Based on Congruence Closure" by Nelson
 //! and Oppen, JACM 1980.

src/lib.rs

@@ -8,10 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(conservative_impl_trait)]
+// The CC code uses `impl Trait`
+#![cfg_attr(feature = "congruence-closure", feature(conservative_impl_trait))]
 
 #[macro_use]
 extern crate log;
+
+#[cfg(feature = "congruence-closure")]
 extern crate petgraph;
 
 pub mod snapshot_vec;