Bug: f64 within flattened HashMap throws error on deserialization

[smessmer]

serde = { version = "^1.0.204", features = ["derive"] }
serde_json = { version = "^1.0.120", features = ["arbitrary_precision"] }

Code:

#[derive(Serialize, Deserialize)]
struct MainStruct {
    #[serde(flatten)]
    key_value_pairs: HashMap<String, f64>,
}

let obj = MainStruct {
    key_value_pairs: [("key".to_string(), 1.0f64)].into_iter().collect(),
};

let serialized = serde_json::to_string(&obj).unwrap();
println!("{:?}", serialized);
let _parsed: MainStruct = serde_json::from_str(&serialized).unwrap();

This prints the correctly serialized string

{"key":1.0}

But when it tries to deserialize it again, it panics with:

called `Result::unwrap()` on an `Err` value: Error("invalid type: map, expected f64", line: 1, column: 11)

[purplesyringa]

This seems to be a variation of #959.