Update CH-14, Memory Management to add LazyCell and LazyLock

[JayanAXHF]

fixes #744

Things to check before submitting a PR

• [] the tests are passing locally with cargo xtask test all (I'm pretty sure the link test fails for an unrelated reason)
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
  • if issue does not exist consider creating it or remove the clause
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

Things to do after submitting PR

• check if CI is happy with your PR

Thank you for reading, you may now delete this text! Thank you! 😄

[JayanAXHF]

Output for link test:

❯❯ rust-cookbook git:(update_ch-14) 00:58 cargo xtask test link
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
     Running `target/debug/xtask test link`
lychee 0.22.0
   [ERROR] Error while loading config: Cannot load configuration file `./ci/lychee.toml`: Failed to parse configuration file

Caused by:
    TOML parse error at line 26, column 1
       |
    26 | headers = []
       | ^^^^^^^
    unknown field `headers`, expected one of `files_from`, `verbose`, `no_progress`, `extensions`, `default_extension`, `cache`, `max_cache_age`, `cache_exclude_status`, `dump`, `dump_inputs`, `archive`, `suggest`, `max_redirects`, `max_retries`, `min_tls`, `max_concurrency`, `threads`, `user_agent`, `insecure`, `scheme`, `offline`, `include`, `exclude`, `exclude_file`, `exclude_path`, `exclude_all_private`, `exclude_private`, `exclude_link_local`, `exclude_loopback`, `include_mail`, `remap`, `fallback_extensions`, `index_files`, `header`, `accept`, `include_fragments`, `timeout`, `retry_wait_time`, `method`, `base_url`, `root_dir`, `basic_auth`, `github_token`, `skip_missing`, `no_ignore`, `hidden`, `include_verbatim`, `glob_ignore_case`, `output`, `mode`, `format`, `generate`, `require_https`, `cookie_jar`, `include_wikilinks`, `preprocess`

See: https://github.com/lycheeverse/lychee/blob/lychee-v0.22.0/lychee.example.toml
Failed to run link checker!

[AndyGauge]

OK, so ideally we would have an example that demonstrates using LazyLock to solve a real problem, not to print. Here's an example I'd rather showcase

Deferring Expensive Computations in a Struct
This is the most common use case. Imagine a User struct where calculating "Permissions" requires a heavy database query or complex logic. You don't want to run that logic every time you load a user—only if you actually check their permissions.

use std::cell::LazyCell;

struct User {
    id: u32,
    username: String,
    // We defer the expensive permission calculation
    permissions: LazyCell<Vec<String>>,
}

impl User {
    fn new(id: u32, username: String) -> Self {
        Self {
            id,
            username,
            permissions: LazyCell::new(|| {
                println!("--- Fetching permissions from database for ID {} ---", id);
                // Simulate a heavy operation
                vec!["read".to_string(), "write".to_string()]
            }),
        }
    }
}

fn main() {
    let user = User::new(1, "ferris_rustacean".into());

    println!("User {} loaded.", user.username);

    // If we never access user.permissions, the closure is never run.
    
    if true { // Imagine a conditional check here
        println!("Permissions: {:?}", *user.permissions);
    }
}

[AndyGauge]

Oh I see you already upated, I realized I made a mistake and provided 2 LazyLock examples

[JayanAXHF]

I updated both the examples

cc: @AndyGauge

[AndyGauge]

Ideally the descriptions are bit more explanative, but the comments do the job.

[AndyGauge]

Thanks for the report on the links, I'll try to update them and the tests to get a deployment done by the end of the month. Thank you for your contribution!