由于对由具有 Serde Deserialize 的枚举组成的结构的要求相互冲突，因此无法为生命周期参数“de”推断合适的生命周期

Question

Why can I auto derive serde::Deserialize for my WidgetValue enum, but not for a struct made up entirely of WidgetValue fields?

This seems counterintuitive to me.

EDIT: For various reasons, I'm using the WidgetValue enum because I'd like to send different values through a function with the same type signature. See Vector store mixed types of data in Rust , How do I create a heterogeneous collection of objects? , etc.

serde = { version = "1.0.126", features = ["derive"] }

//works fine
#[derive(Clone, Copy, Serialize, Deserialize, Debug)]
pub enum WidgetValue{
    Integer32(i32),
    Unsized32(u32),
    CString(&'static str),
}

//lifetime error
#[derive(Clone, Copy, Serialize, Deserialize, Debug)]
pub struct DraggableInfo{
    parent: WidgetValue,
    index: WidgetValue,
    draggable_id: WidgetValue,
}

Error:

cannot infer an appropriate lifetime for lifetime parameter `'de` due to conflicting requirements

Answer 1

You can fix the lifetime error, by manually providing a bound on the 'de lifetime during deserialization. Due to the &'static str the enum WidgetValue only implements Deserialize<'static> since no other lifetime works to borrow a &'static str . This leads to an error for DraggableInfo since by default the 'de lifetime is not constrained. With this manual bound you declare 'de to outlive 'static , making them equal.

#[derive(Clone, Copy, Serialize, Deserialize, Debug)]
#[serde(bound(deserialize = "'de: 'static"))]
pub struct DraggableInfo{
    parent: WidgetValue,
    index: WidgetValue,
    draggable_id: WidgetValue,
}

#[derive(Clone, Copy, Serialize, Deserialize, Debug)]
pub enum WidgetValue{
    Integer32(i32),
    Unsized32(u32),
    CString(&'static str),
}

You likely do not want to use it like this though, since you can only derive from 'static data. You can redefine CString like this CString(Cow<'static, str>) to allow both static string literals and deserializing from non-static data.