add support for changing enum variant from the last source

examples/hierarchical-env/config/default.toml

@@ -1,3 +1,5 @@
+cert.content = "/tmp/cert.pem"
+
 [database]
 url = "postgres://postgres@localhost"
 

examples/hierarchical-env/settings.rs

@@ -3,6 +3,14 @@ use std::env;
 use config::{Config, ConfigError, Environment, File};
 use serde_derive::Deserialize;
 
+#[derive(Debug, Deserialize)]
+#[allow(unused)]
+#[serde(rename_all = "lowercase")]
+enum Cert {
+    Path(String),
+    Value(String),
+}
+
 #[derive(Debug, Deserialize)]
 #[allow(unused)]
 struct Database {
@@ -42,6 +50,7 @@ pub(crate) struct Settings {
     sparkpost: Sparkpost,
     twitter: Twitter,
     braintree: Braintree,
+    cert: Cert,
 }
 
 impl Settings {

src/de.rs

@@ -281,7 +281,18 @@ impl EnumAccess {
         if table.len() == 1 {
             self.variant_deserializer(table.iter().next().unwrap().0)
         } else {
-            Err(self.structural_error())
+            if table.is_empty() {
+                return Err(self.structural_error());
+            }
+            #[cfg(feature = "preserve_order")]
+            {
+                self.variant_deserializer(table.last().unwrap().0)
+            }
+
+            #[cfg(not(feature = "preserve_order"))]
+            {
+                Err(self.structural_error())
+            }
         }
     }