Não é possível derivar `Default`: campo sem padrão

Why this fires¶

A struct decorated with @derive(Default) has a field whose type has no zero value and no explicit default — @derive(Default) cannot synthesize a Default impl for it.

struct Point { x: int, y: int }

@derive(Default)
struct Shape {
    label: str,
    anchor: Point,
    //      ^^^^^ error[TE108]: cannot derive Default for 'Shape': field 'anchor'
    //             has no Default (give it an explicit default or implement
    //             Default for its type)
}

Point has no zero value of its own (it isn't a primitive and has no @derive(Default)/impl Default of its own), so Shape can't derive one either.

Fix it¶

1. Give the field an explicit default¶

@derive(Default)
struct Shape {
    label: str,
    anchor: Point = Point { x: 0, y: 0 },   // ok
}

2. Derive (or implement) Default for the field's type too¶

@derive(Default)
struct Point { x: int, y: int }

@derive(Default)
struct Shape {
    label: str,
    anchor: Point,                          // ok — Point now has a Default
}

See also¶

• TE109 — a field's type can't be inferred from its default value.
• TE107 — a struct-literal call site omits a required field.
• /docs/data-structures#structs — field defaults and @derive(Default).