Static UNEVALUATED_DEFAULT_FIELD_VALUE 

pub static UNEVALUATED_DEFAULT_FIELD_VALUE: &Lint

The unevaluated_default_field_value lint detects default field values that don’t get evaluated at the definition site because the corresponding struct or enum has type or const parameters, meaning they only get evaluated when the struct or enum gets constructed using that default (post-monormophization).

Example

#![feature(default_field_values)]

struct Struct<const T: u8> {
    field1: u8 = 100 + T, // none of these values will be checked until `Struct` is
    field2: u8 = 100,     // constructed, even if they don't refer to the cont param
}

{{produces}}

Explanation

Const evaluation requires all values to be known. In the case of default field values, they will get evaluated eagerly, but if there are any type or const parameters, the evaluation of the default will only occur when materializing a value of its owning struct and leaving the field with a default value unspecified. For the example above, if you wrote let _: Struct<180> = Struct { .. };, you would get a const evaluation compile error.

This inconsistency can be confusing, and surprising, because the fields are sometimes evaluated eagerly.