Fix displaying empty arrays

Closes #145

src/value.rs

@@ -366,21 +366,19 @@ impl ser::Serialize for Value {
                 // Be sure to visit non-tables first (and also non
                 // array-of-tables) as all keys must be emitted first.
                 for (k, v) in t {
-                    if !v.is_array() && !v.is_table() {
+                    if !v.is_table() && !v.is_array() ||
-                        map.serialize_key(k)?;
+                       (v.as_array().map(|a| a.len() == 0).unwrap_or(false)) {
-                        map.serialize_value(v)?;
+                        map.serialize_entry(k, v)?;
                     }
                 }
                 for (k, v) in t {
-                    if v.is_array() {
+                    if v.as_array().map(|a| a.len() > 0).unwrap_or(false) {
-                        map.serialize_key(k)?;
+                        map.serialize_entry(k, v)?;
-                        map.serialize_value(v)?;
                     }
                 }
                 for (k, v) in t {
                     if v.is_table() {
-                        map.serialize_key(k)?;
+                        map.serialize_entry(k, v)?;
-                        map.serialize_value(v)?;
                     }
                 }
                 map.end()

tests/display-tricky.rs

@@ -0,0 +1,49 @@
+extern crate toml;
+#[macro_use] extern crate serde_derive;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Recipe {
+    pub name: String,
+    pub description: Option<String>,
+    #[serde(default)]
+    pub modules: Vec<Modules>,
+    #[serde(default)]
+    pub packages: Vec<Packages>
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Modules {
+    pub name: String,
+    pub version: Option<String>
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Packages {
+    pub name: String,
+    pub version: Option<String>
+}
+
+#[test]
+fn both_ends() {
+    let recipe_works = toml::from_str::<Recipe>(r#"
+        name = "testing"
+        description = "example"
+        modules = []
+
+        [[packages]]
+        name = "base"
+    "#).unwrap();
+    toml::to_string(&recipe_works).unwrap();
+
+    let recipe_fails = toml::from_str::<Recipe>(r#"
+        name = "testing"
+        description = "example"
+        packages = []
+
+        [[modules]]
+        name = "base"
+    "#).unwrap();
+
+    let recipe_toml = toml::Value::try_from(recipe_fails).unwrap();
+    recipe_toml.to_string();
+}