test: Add 'missing field' test

Author: linyihai

tests/testsuite/config.rs

@@ -1863,3 +1863,34 @@ fn trim_paths_parsing() {
     let trim_paths: TomlTrimPaths = gctx.get("profile.dev.trim-paths").unwrap();
     assert_eq!(trim_paths, expected, "failed to parse {val}");
 }
+
+#[cargo_test]
+fn missing_fields() {
+    #[derive(Deserialize, Default, Debug)]
+    struct Foo {
+        bar: Bar,
+    }
+
+    #[derive(Deserialize, Default, Debug)]
+    struct Bar {
+        bax: bool,
+        baz: bool,
+    }
+
+    let gctx = GlobalContextBuilder::new()
+        .env("CARGO_FOO_BAR_BAZ", "true")
+        .build();
+    assert_error(gctx.get::<Foo>("foo").unwrap_err(), "missing field `bax`");
+    let gctx: GlobalContext = GlobalContextBuilder::new()
+        .env("CARGO_FOO_BAR_BAZ", "true")
+        .env("CARGO_FOO_BAR_BAX", "true")
+        .build();
+    let foo = gctx.get::<Foo>("foo").unwrap();
+    assert_eq!(foo.bar.bax, true);
+    assert_eq!(foo.bar.baz, true);
+
+    let gctx: GlobalContext = GlobalContextBuilder::new()
+        .config_arg("foo.bar.baz=true")
+        .build();
+    assert_error(gctx.get::<Foo>("foo").unwrap_err(), "missing field `bax`");
+}