serde_keyvalue: add a few tests

Add tests for a unit type and an untagged enum.

TEST=cargo test -p serde_keyvalue

Change-Id: Ifb840e3270624ef0e54d69b7eba2e631ffcac4e5
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3906752
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>

serde_keyvalue/src/key_values.rs

@@ -922,6 +922,16 @@ mod tests {
         assert_eq!(res.m, r#"Escaped slash\\"#.to_string());
     }
 
+    #[test]
+    fn deserialize_unit() {
+        from_key_values::<SingleStruct<()>>("m").unwrap();
+        from_key_values::<SingleStruct<()>>("m=").unwrap();
+
+        from_key_values::<SingleStruct<()>>("").unwrap_err();
+        from_key_values::<SingleStruct<()>>("p").unwrap_err();
+        from_key_values::<SingleStruct<()>>("m=10").unwrap_err();
+    }
+
     #[test]
     fn deserialize_bool() {
         let res = from_key_values::<SingleStruct<bool>>("m=true").unwrap();
@@ -1108,6 +1118,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn deserialize_untagged_enum() {
+        #[derive(Deserialize, PartialEq, Debug)]
+        #[serde(untagged)]
+        enum TestEnum {
+            FirstVariant { first: u32 },
+            SecondVariant { second: bool },
+        }
+
+        #[derive(Deserialize, PartialEq, Debug)]
+        struct TestStruct {
+            #[serde(flatten)]
+            variant: TestEnum,
+        }
+
+        let res: TestStruct = from_key_values("first=10").unwrap();
+        assert_eq!(res.variant, TestEnum::FirstVariant { first: 10 });
+
+        let res: TestStruct = from_key_values("second=false").unwrap();
+        assert_eq!(res.variant, TestEnum::SecondVariant { second: false },);
+
+        let res: TestStruct = from_key_values("second").unwrap();
+        assert_eq!(res.variant, TestEnum::SecondVariant { second: true },);
+
+        from_key_values::<TestStruct>("third=10").unwrap_err();
+        from_key_values::<TestStruct>("first=some_string").unwrap_err();
+        from_key_values::<TestStruct>("second=10").unwrap_err();
+    }
+
     #[test]
     fn deserialize_first_arg_string() {
         #[derive(Deserialize, PartialEq, Debug)]