feat: add Cow

Author: katopz

src/SUMMARY.md

@@ -42,6 +42,7 @@
     - [Associated Type Bounds](rust/r3/associated-type-bounds.md)
     - [BoxStream](rust/r3/futures-boxstream.md)
     - [IndexMap](rust/r3/indexmap.md)
+    - [Cow](rust/r3/cow.md)
   - [R2 - Expert](rust/r2/mod.md)
     - [Hello Github Action](rust/r2/hello-github-action.md)
     - [Hello Actix CloudRun](rust/r2/hello-actix-cloudrun.md)

src/rust/r3/cow.md

@@ -0,0 +1,46 @@
+# `Cow` 🐄 aka `Copy on write`
+
+![](/assets/kat.png) <span class="speech-bubble"> Let's talk about `Cow` 🐄 aka `Copy on write`</span>
+
+```rust,editable
+use std::borrow::Cow;
+
+fn main() {
+    println!("🛸💨 Welcome to the Farm (and Alien Abduction) Fun Zone! 🐄\n");
+
+    // --- 🐄 Our First Cow: Just Grazing Peacefully ---
+    // This cow is just chillin' in the field, like a borrowed string.
+    let mut peaceful_cow: Cow<'static, str> = Cow::Borrowed("Mooo, just grazing... 🌿");
+    println!("1. Peaceful Cow: {}", peaceful_cow);
+    println!("   (It's borrowed! Just observing this cow. 🧐)\n");
+
+    // --- 👽 UFO Appears! Time to SUCK the Cow! ---
+    // The UFO wants to "modify" the cow (i.e., abduct it!).
+    // If it's a borrowed cow, the UFO needs to make its OWN copy to take!
+    // (This simulates `to_mut()` forcing a copy for modification)
+    println!("2. OH NO! A UFO appears! 👽");
+    peaceful_cow.to_mut().push_str(" AIEEEE! I'm being beamed up! ⬆️");
+    println!("   Cow after abduction attempt: {}", peaceful_cow);
+    println!("   (Since it was borrowed, the UFO made a *copy* to abduct. The original is safe... for now! 🚀)\n");
+
+
+    // --- 🐄 Our Second Cow: Already in Its Own Barn ---
+    // This cow is already "owned" by us, like an owned String.
+    let mut owned_cow: Cow<'_, str> = Cow::Owned(String::from("Mooo, I'm safe in MY barn! 🏡"));
+    println!("3. Owned Cow: {}", owned_cow);
+    println!("   (This cow is already owned! No initial copy needed. 😎)\n");
+
+    // --- 👽 UFO Appears AGAIN! ---
+    // The UFO wants to abduct THIS cow too!
+    // But since this cow is *already owned*, the UFO can just grab *this specific cow*.
+    // No new copy is needed because it's already "owned" for modification.
+    println!("4. Another UFO! This one's persistent! 🛸");
+    owned_cow.to_mut().push_str(" Help! They're taking MY barn too! 😵‍💫");
+    println!("   Owned Cow after abduction: {}", owned_cow);
+    println!("   (It was already owned, so the UFO just took *that* cow. Efficient abduction! 💯)\n");
+
+    println!("🛸🐄💨 CoW: Keeping your original data safe from alien abduction, unless they need to make a copy! ✨");
+}
+```
+
+> 💡 You can read more about `Cow` here 👉 [6 things you can do with the Cow 🐄 in Rust 🦀](https://dev.to/kgrech/6-things-you-can-do-with-the-cow-in-rust-4l55)

src/rust/r3/create-lib.md

@@ -79,5 +79,3 @@ fn hello(foo: &str) -> Cow<'_, str>
 // Take impl, return impl.
 fn hello(foo: impl AsRef<str>) -> impl AsRef<str>
 ```
-
-> 💡 You can read more about `Cow` here 👉 [6 things you can do with the Cow 🐄 in Rust 🦀](https://dev.to/kgrech/6-things-you-can-do-with-the-cow-in-rust-4l55)