const

Applies to: structs functions methods

Marks all generated builder functions and methods as const fn. See the limitations of this feature.

Examples

Struct

use bon::Builder;

#[derive(Builder)]
#[builder(const)]
struct Example {
    x1: u32,

    // Must specify the default value explicitly because `Default::default()`
    // is not callable in `const` context
    #[builder(default = 0)]
    x2: u32
}

const {
    Example::builder()
        .x1(32)
        .build();
}

Function

use bon::builder;

#[builder(const)]
const fn example(
    x1: u32,

    // Must specify the default value explicitly because `Default::default()`
    // is not callable in `const` context
    #[builder(default = 0)]
    x2: u32
) {}

const {
    example()
        .x1(32)
        .call();
}

Method

use bon::bon;

struct Example;

#[bon]
impl Example {
    #[builder(const)]
    const fn example(
        x1: u32,

        // Must specify the default value explicitly because `Default::default()`
        // is not callable in `const` context
        #[builder(default = 0)]
        x2: u32
    ) {}
}

const {
    Example::example()
        .x1(32)
        .call();
}

Limitations

Limitation	Reason
const must be the first attribute parameter: #[builder(const, ...other params)]	syn::Meta limitation
No member types with non-trivial Drop implementations such as String, Vec, Box	Requires const_precise_live_drops
No bare default and skip	Default::default() can't be called in const context. Specify the value explicitly via default/skip = ...
Only simple expressions are allowed in default, skip, with	Full support requires const_closures. In the meantime move your expression into a separate function if bon rejects it
No attributes requiring non-const functions (into, with = <_>::from_iter, getter(clone), etc.)	Non-const functions and trait methods can't be called in const context
1.61.0 MSRV	Requires const_fn_trait_bound stabilized in 1.61.0