add method to update env variables from custom source

[robertkiel]

See #4607

tldr

Adds a method that allows parsing environment variables from a custom source.

Detailed description

When building Rust application that need to run without full access to std::env, e.g. when building an application that is supposed to run as a WebAssembly module as well as a standalone application, it becomes necessary to parse arguments from a custom source, such as cli_args iterator.

Currently, it is possible to pass in cli-args from a custom source but not environment variables, which means the WebAssembly module requires additional logic to parse arguments from environment variables. By having an update_env_from function, the standalone client and the WebAssembly module can share the same CLI code.

Example:

use std::env;
use std::collections::HashMap;
use clap::{Command, Arg};

let mut cmd = Command::new("myprog")
    .arg(Arg::new("data_dir")
        .long("data_dir")
        .env("MYPROG_DATA_DIR"));

env::set_var("MYPROG_DATA_DIR", "/home/user/Work/clap");

let env_vars = HashMap::from_iter(env::vars_os());
cmd.get_env_from(env_vars);

let m = cmd.get_matches_from(vec!["foo"]);
assert_eq!("/home/user/Work/clap", m.get_one::<String>("data_dir").unwrap());

[epage]

Could you open an issue for us to discuss the problem and possible solutions, per CONTRIBUTING.md?

[robertkiel]

Could you open an issue for us to discuss the problem and possible solutions, per CONTRIBUTING.md?

Thanks for you reply, created issue #4607

[epage]

Cl;osing as this doesn't align with our plans in #4793