Rust: What is the difference between String and &str?

String — owned, heap-allocated, growable string type. &str — borrowed, immutable string slice.

String:

• Owns its data — stored on the heap.
• Mutable — can modify, append, and insert.
• Growable — dynamic sizing with push, push_str, insert.
• Created via String::from(), .to_string(), or format!().
• Must be freed when the owner goes out of scope.

1let mut s = String::new();
2s.push_str("hello");
3s.push('!');
4s.insert_str(5, " beautiful");
5println!("{}", s);  // "hello beautiful!"

&str:

• Borrowed reference (slice) — no ownership.
• Immutable — cannot modify contents.
• Can point to string literals, heap data, or static data.
• Zero-cost — just a pointer and length.
• String literals ("hello") have static lifetime.

1let literal: &str = "hello";         // Static
2let owned = String::from("hello");
3let slice: &str = &owned;            // Borrowed
4let sub: &str = &owned[0..3];        // Partial: "hel"

When to use:

• Use String for owned, mutable string data.
• Use &str for read-only views and function parameters.
• Prefer &str in API design for maximum flexibility.

Common mistakes:

• Trying to index a string: s[0] — not allowed (UTF-8).
• Forgetting &String auto-derefs to &str.
• Cloning unnecessarily when borrowing would work.