You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: images.rs
    path of file: ./repos/cross/src/bin/commands
    
    the code of the file until where you have to start completion: use std::collections::{BTreeMap, BTreeSet};

use clap::builder::PossibleValue;
use clap::{Args, Subcommand};
use cross::docker::{self, CROSS_CUSTOM_DOCKERFILE_IMAGE_PREFIX};
use cross::shell::MessageInfo;
use cross::{CommandExt, TargetList};

// known image prefixes, with their registry
// the docker.io
    
fn rustembedded_target(tag: &str) -> String {
    let is_target_char = |c: char| c == '_' || c.is_ascii_alphanumeric();
    let mut components = vec![];
    for (index, component) in tag.split('-').enumerate() {
        if index <= 2 || (!component.is_empty() && component.chars().all(is_target_char)) {
            components.push(component)
        } else {
            break;
        }
    }

    components.join("-")
}