set_verbose does not interact with help text

[chrysn]

The new set_default mechanism (and the combination with --quiet) allows setting the default level to warn -- something I highly appreciate.

The help text of --verbose, however, is built earlier than the set_default is evaluated (much earlier, in fact -- at build time). This means that programs that use set_default with anything but the built-in default Error get inconsistent help messages, ie. always

By default, it'll only report errors. Passing -v one time also prints
warnings, -vv enables info logging, -vvv debug, and -vvvv trace.

and not, say

By default, it'll only report errors and warnings. Passing -v one time
enables info logging, -vv debug, and -vvv trace.

I'm not well enough versed in structopt to propose any thought-through way toward resolution. One possibility would to add a generic argument to Verbosity that'd indicate the default and serve as help text; could look like

#[derive(StructOpt, Debug)]
struct CliArgsWithVerbosity {
    ...,
    #[structopt(flatten)]
    verbose: clap_verbosity_flag::Verbosity<DefaultVerbosity=clap_verbosity_flag::DefaultWarning>,
}

and where the DefaultWarning struct provides something like

impl VerbosityDefault for DefaultWarning {
    const fn level() -> Option<Level> {
        Some(log::Level::Warn)
    }
    const fn text() -> &'static str {
        "By default, it'll only report errors and warnings [...]"
    }
}

but as said, I don't know whether using that text would be doable with structopt.

[epage]

Once we bump our MSRV to 1.51, we could use const-generics but there isn't a way to customize the help string based on that.

At best, we can make the help string more generic.

[chrysn]

The current min-const-generics would require that there's a numeric level (and not an enum log::Level let alone Optionlog::Level) for some time -- and if it needs to go though a conversion (Verbosyty<DefaultVerbosity=default_level_from(Some(log::level::Warning))>), a "type-level enum" aka implementation-of-a-custom-trait might be more ergonomic still.