build: fix compilation without default features (#218)

* build: fix compilation with optional features
* Remove unused config from Cargo.toml
* Fix cache keys
Co-authored-by: Luis Moreno <morenol@users.noreply.github.com>

src/svm/mod.rs

@@ -52,6 +52,7 @@ impl<'a> Debug for dyn Kernel<'_> + 'a {
     }
 }
 
+#[cfg(feature = "serde")]
 impl<'a> Serialize for dyn Kernel<'_> + 'a {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
@@ -64,7 +65,8 @@ impl<'a> Serialize for dyn Kernel<'_> + 'a {
 }
 
 /// Pre-defined kernel functions
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Debug, Clone)]
 pub struct Kernels {}
 
 impl<'a> Kernels {

src/svm/svc.rs

@@ -133,7 +133,7 @@ pub struct SVCParameters<
 pub struct SVC<'a, TX: Number + RealNumber, TY: Number + Ord, X: Array2<TX>, Y: Array1<TY>> {
     classes: Option<Vec<TY>>,
     instances: Option<Vec<Vec<TX>>>,
-    #[serde(skip)]
+    #[cfg_attr(feature = "serde", serde(skip))]
     parameters: Option<&'a SVCParameters<'a, TX, TY, X, Y>>,
     w: Option<Vec<TX>>,
     b: Option<TX>,

src/svm/svr.rs

@@ -92,7 +92,7 @@ pub struct SVRParameters<'a, T: Number + RealNumber> {
     pub c: T,
     /// Tolerance for stopping criterion.
     pub tol: T,
-    #[serde(skip_deserializing)]
+    #[cfg_attr(feature = "serde", serde(skip_deserializing))]
     /// The kernel function.
     pub kernel: Option<&'a dyn Kernel<'a>>,
 }