feat(serde): Make serde optional

Fixes #113

Cargo.toml

@@ -18,12 +18,16 @@ appveyor  = { repository = "cobalt-org/liquid-rust" }
 
 [[bin]]
 name = "liquid-dbg"
-required-features = ["cli"]
+required-features = ["cli", "serde"]
 test = false
 doctest = false
 bench = false
 doc = false
 
+[[test]]
+name = "liquid"
+required-features = ["serde"]
+
 [features]
 default = ["extra-filters"]
 cli = ["clap", "error-chain", "serde_yaml"]
@@ -37,9 +41,8 @@ chrono = "0.3"
 unicode-segmentation = "1.1"
 itertools = "0.6.0"
 url = "1.4"
-serde = "1.0"
-serde_derive = "1.0"
 
+serde = { version = "1.0", optional = true, features = ["derive"] }
 clap = { version = "2.22.0", optional = true }
 error-chain = { version = "0.10.0", optional = true }
 serde_yaml = { version = "0.7", optional = true }

src/lib.rs

@@ -39,10 +39,9 @@ extern crate url;
 
 #[macro_use]
 extern crate lazy_static;
-
-extern crate serde;
+#[cfg(feature = "serde")]
 #[macro_use]
-extern crate serde_derive;
+extern crate serde;
 
 use std::collections::HashMap;
 use lexer::Element;

src/value.rs

@@ -8,8 +8,8 @@ use token::Token::*;
 
 /// An enum to represent different value types
 #[derive(Clone, Debug)]
-#[derive(Serialize, Deserialize)]
-#[serde(untagged)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", serde(untagged))]
 pub enum Value {
     Num(f32),
     Bool(bool),

tests/liquid.rs

@@ -0,0 +1 @@
+