Constant USAGE 

pub const USAGE: &str = "usage: atune-solver \
    [--sampler random|sobol|qmc|tpe|gp|cmaes|auto] [--budget <trials>] \
    [--opt-in <setting>]... [--opt-out <setting>]... [--gp-cost-budget <units>]\n\
    sampler settings (one per flag, repeatable; --opt-in of an on-by-default \
    setting is a no-op, --opt-out takes only on-by-default ones):\n  \
    tpe: on by default log-scoring, multivariate, discrete-kernels, linear-split, \
    optuna-bandwidth, sharpening\n  \
    sobol: on by default schema-order; off by default scramble, halton\n  \
    gp: on by default optuna-search; off by default ard, mixed-space\n  \
    cmaes: on by default optuna-init; off by default active, separable, margin \
    (separable excludes margin)\n  \
    auto: off by default gp-cost-budget (DEFAULT_GP_COST_BUDGET unless \
    --gp-cost-budget is given)";

The atune-solver usage line, also printed for --help.

Lists every label SamplerKind’s FromStr accepts, qmc included: a flag the parser takes but the help hides is a flag nobody can discover.