gflags-derive

Derive command line arguments from struct fields using gflags.

This is an alternative to the "Defining flags" section of the gflags manual.

Defining flags

Create a struct to contain the configuration data for your library or binary.

For example, this hypothetical logging library that defines two configuration options.

struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    dir: String,
}

Flags are added to the registry by deriving gflags_derive::Gflags on the struct.

use gflags_derive::GFlags;

#[derive(GFlags)]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    dir: String,
}

You now have two new flags, as if you had written:

gflags::define! {
    /// True if log messages should also be sent to STDERR
    --to_stderr: bool
}

gflags::define! {
    /// The directory to write log files to
    --dir: &str
}

Note that:

• The comment on each struct field is also the documentation comment for the flag, which becomes its help text.
• The type for the --dir flag has been converted from String to &str.

Defining a flag prefix

You might want all the flag names to have the same prefix, without needing to use that prefix on the field names. For example, a logging module might want all the flags to start log- or log_.

To support this, use the #[gflags(prefix = "...")] attribute on the struct.

use gflags_derive::GFlags;

#[derive(GFlags)]
#[gflags(prefix = "log_")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    dir: String,
}

The flag definitions now include the prefix, as if you had written:

gflags::define! {
    /// True if log messages should also be sent to STDERR
    --log_to_stderr: bool
}

gflags::define! {
    /// The directory to write log files to
    --log_dir: &str
}

If the flag prefix ends with - then the macro converts the flag names to kebab-case instead of snake_case. So writing:

use gflags_derive::GFlags;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    dir: String,
}

generates the following flags:

gflags::define! {
    /// True if log messages should also be sent to STDERR
    --log-to-stderr: bool
}

gflags::define! {
    /// The directory to write log files to
    --log-dir: &str
}

Handling Option<T>

Your configuration struct may have fields that have Option<T> types. For these fields gflags_derive creates a flag of the inner type T.

Customising the default value

To specify a default value for the flag add a #[gflags(default = ...)] attribute to the field.

The value for the attribute is the literal value, not a quoted value. Only quote the value if the type of the field is a string or can be created from a string.

For example, to set the default value of the --log-to-stderr flag to true:

use gflags_derive::GFlags;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    #[gflags(default = true)]
    to_stderr: bool,

    /// The directory to write log files to
    dir: String,
}

Specifying this with quotes, #[gflags(default = "true")] will give a compile time error:

expected `bool`, found `&str`

Important: This does not change the default value when an instance of the Config struct is created. It only changes the default value of the LOG_TO_STDERR.flag variable.

Customising the type

To use a different type for the field and the command line flag add a #[gflags(type = "...")] attribute to the field. For example, to store the log directory as a PathBuf but accept a string on the command line:

use gflags_derive::GFlags;
use std::path::PathBuf;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    #[gflags(type = "&str")]
    dir: PathBuf,
}

Customising the visibility

To use a different visibility for the flags add a #[gflags(visibility = "...")] attribute to the field and give a Rust visibility specifier.

In this example the LOG_DIR flag variable will be visible in the parent module.

use gflags_derive::GFlags;
use std::path::PathBuf;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    #[gflags(visibility = "pub(super)")]
    #[gflags(type = "&str")]
    dir: PathBuf,
}

Specifying a placeholder

To give a placeholder that will appear in the flag's help output add a #[gflags(placeholder = "...")] attribute to the field. This will be wrapped in <...> for display.

use gflags_derive::GFlags;
use std::path::PathBuf;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    #[gflags(placeholder = "DIR")]
    #[gflags(type = "&str")]
    dir: PathBuf,
}

In the help output the --log-dir flag will appear as:

--log-dir <DIR>
        The directory to write log files to

Skipping flags

To skip flag generation for a field add a #[gflags(skip)] attribute to the field.

use gflags_derive::GFlags;
use std::path::PathBuf;

#[derive(GFlags)]
#[gflags(prefix = "log-")]
struct Config {
    /// True if log messages should also be sent to STDERR
    to_stderr: bool,

    /// The directory to write log files to
    #[gflags(skip)]
    dir: PathBuf,
}

No --log-dir flag will be generated.

Providing multiple attributes

If you want to provide multiple attributes on a field then you can mix and match specifing multiple options in a single #[gflags(...)] attribute and specifying multiple #[gflags(...)] attributes. The following examples are identical.

...
    /// The directory to write log files to
    #[gflags(type = "&str", visibility = "pub(super)")]
    dir: PathBuf,
...

...
    /// The directory to write log files to
    #[gflags(type = "&str")]
    #[gflags(visibility = "pub(super)")]
    dir: PathBuf,
...

Deserializing and merging flags

This supports a powerful pattern for configuring an application that is composed of multiple crates, where each crate exports a configuration and supports multiple flags, and the application crate defines a configuration that imports the configuration structs from the component crates.

This master configuration can be deserialized from e.g. a JSON file, and then each component crate can have the opportunity to override the loaded configuration with information from the command line flags that are specific to that crate.

See the examples/json directory for a complete application that does this.

Use with prost

This macro can be used to derive flags for structs generated from Protobuffer schemas using prost and prost-build.

Given this .proto file

syntax = "proto3"

package log.config.v1;

message Config {
    // True if log messages should also be sent to STDERR
    bool to_stderr = 1;

    // The directory to write log files to
    string dir = 2;
}

This build.rs file will add the relevant attributes to add the log- prefix and skip the dir field.

fn main() {
    let mut config = prost_build::Config::new();

    config.type_attribute(".log.config.v1.Config", "#[derive(gflags_derive::GFlags)]");
    config.type_attribute(".log.config.v1.Config", "#[gflags(prefix=\"log-\")]");

    config.field_attribute(".log.config.v1.Config.dir", "#[gflags(skip)]");

    config
        .compile_protos(&["proto/log/config/v1/config.proto"], &["proto"])
        .unwrap();
}

See the examples/protobuf directory for a complete application that does this.

License: MIT OR Apache-2.0