Add support for overriding optional variables with custom null sentinel

examples/hierarchical-env/config/default.toml

@@ -10,6 +10,7 @@ version = 1
 [twitter]
 consumer_token = "twitter-dev-consumer-key"
 consumer_secret = "twitter-dev-consumer-secret"
+avatar = "avatar-default"
 
 [braintree]
 merchant_id = "braintree-merchant-id"

examples/hierarchical-env/config/production.toml

@@ -7,6 +7,7 @@ token = "sparkpost-prod-token"
 [twitter]
 consumer_token = "twitter-prod-consumer-key"
 consumer_secret = "twitter-prod-consumer-secret"
+avatar = "avatar-prod"
 
 [braintree]
 public_key = "braintree-prod-public-key"

examples/hierarchical-env/settings.rs

@@ -23,6 +23,7 @@ struct Sparkpost {
 struct Twitter {
     consumer_token: String,
     consumer_secret: String,
+    avatar: Option<String>,
 }
 
 #[derive(Debug, Deserialize)]
@@ -62,7 +63,12 @@ impl Settings {
             .add_source(File::with_name("examples/hierarchical-env/config/local").required(false))
             // Add in settings from the environment (with a prefix of APP)
             // Eg.. `APP_DEBUG=1 ./target/app` would set the `debug` key
-            .add_source(Environment::with_prefix("app"))
+            .add_source(
+                Environment::with_prefix("app")
+                    .separator("__")
+                    // APP__TWITTER__AVATAR=null will override the default value with null
+                    .with_null_as("null"),
+            )
             // You may also programmatically change settings
             .set_override("database.url", "postgres://")?
             .build()?;

src/env.rs

@@ -52,6 +52,9 @@ pub struct Environment {
     /// Ignore empty env values (treat as unset).
     ignore_empty: bool,
 
+    /// Treat the given value as null
+    null_as: Option<String>,
+
     /// Parses booleans, integers and floats if they're detected (can be safely parsed).
     try_parsing: bool,
 
@@ -168,6 +171,11 @@ impl Environment {
         self
     }
 
+    pub fn with_null_as(mut self, null_as: &str) -> Self {
+        self.null_as = Some(null_as.into());
+        self
+    }
+
     /// Note: enabling `try_parsing` can reduce performance it will try and parse
     /// each environment variable 3 times (bool, i64, f64)
     pub fn try_parsing(mut self, try_parsing: bool) -> Self {
@@ -316,11 +324,17 @@ impl Source for Environment {
                             .collect();
                         ValueKind::Array(v)
                     }
+                } else if Some(value.to_lowercase()) == self.null_as {
+                    ValueKind::Nil
                 } else {
                     ValueKind::String(value)
                 }
             } else {
-                ValueKind::String(value)
+                if Some(value.to_lowercase()) == self.null_as {
+                    ValueKind::Nil
+                } else {
+                    ValueKind::String(value)
+                }
             };
 
             m.insert(key, Value::new(Some(&uri), value));

tests/testsuite/env.rs

@@ -63,6 +63,14 @@ fn test_empty_value_is_ignored() {
     });
 }
 
+#[test]
+fn test_null_is_parsed() {
+    temp_env::with_var("C_A_B", Some("<null>"), || {
+        let environment = Environment::default().with_null_as("<null>");
+        assert!(environment.collect().unwrap().contains_key("c_a_b"));
+    });
+}
+
 #[test]
 fn test_keep_prefix() {
     temp_env::with_var("C_A_B", Some(""), || {