https://xc-h.com/2018/09/04/less-declaration-block-as-mixins-arguments/

from.less

.some-mixin (
    @titleFont: '',
    @bodyStyle: {},
) {
    & > .title {
        font: @titleFont;
    }

    & > .body {
        @bodyStyle();
    }
}

.some-component {
    .some-mixin(
        @titleFont: bold 1.5em Helvetica,
        @bodyStyle: {
            color: gray;
        },
    );
}

to.css

.some-component > .title {
    font: bold 1.5em Helvetica;
}
.some-component > .body {
    color: gray;
}