E0490: Field ptr requires field

Explanation

@field_ptr asks the compiler for the address of a field under its chosen struct layout, so its operand must syntactically be a field-access place such as value.field. A whole variable, array element, or computed value does not identify a struct field for this layout-mediated operation.

Likely cause

Code passed a local or another non-field expression to @field_ptr. Select the intended struct field, or use @raw or @raw_mut when the goal is to address a whole place. Raw-pointer intrinsics must appear in a checked block.

Examples

Request a field pointer for a whole value

struct Pair { value: i32 }
fn main() -> i32 {
    let pair = Pair { value: 42 };
    let pointer = checked { @field_ptr(pair) };
    checked { @ptr_read(pointer) }
}

Request a pointer to a named field

struct Pair { value: i32 }
fn main() -> i32 {
    let pair = Pair { value: 42 };
    let pointer: ptr mut i32 = checked { @field_ptr(pair.value) };
    checked { @ptr_read(pointer) }
}

References

• Field pointer operands