Add test for overwriting key from env

tests/overwrite_key.rs

@@ -0,0 +1,47 @@
+use config::{Config, File, Environment};
+use serde::{Serialize,  Deserialize};
+
+const CONFIG: &str = r#"
+name = "foo"
+[v4]
+ca_path = "ca"
+cert_path = "bar"
+key_path = "baz"
+"#;
+
+#[derive(Debug, Deserialize, Clone)]
+pub struct Settings {
+    pub name: String,
+    pub v4: TlsConfig,
+}
+
+#[derive(Debug, Serialize, Deserialize, Clone)]
+pub struct TlsConfig {
+    pub ca_path: String,
+    #[serde(alias = "certpath")]
+    pub cert_path: String,
+    // #[serde(alias = "keypath")]
+    pub key_path: String,
+}
+
+#[test]
+fn overwrite_key() {
+    std::env::set_var("V4_CERTPATH", "Hello World");
+
+    let s = Config::builder()
+        .add_source(File::from_str(CONFIG, config::FileFormat::Toml))
+        .add_source(Environment::default().separator("_"))
+        .build();
+
+    assert!(s.is_ok(), "build failed: {:?}", s);
+    let s = s.unwrap();
+
+    let v: Result<Settings, _> = s.try_deserialize();
+
+    // This is expected to error because the key `certpath` is specified by ENV and `cert_path`
+    // from the TOML.
+    // This should work, but does not because of the way this crate deserializes into T.
+    //
+    // The fix is to name the "TlsConfig::cert_path" field "TlsConfig::certpath".
+    assert!(v.is_err(), "accidentially ok: {:?}", v);
+}