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

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

String:

• Owns its data on the heap.
• Mutable — can modify, append, insert.
• Growable — dynamic sizing.
• Created via String::from(), .to_string(), format!().
• Freed when owner goes out of scope.

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

&str:

• Borrowed reference — no ownership.
• Immutable — cannot modify.
• Can point to any string data.
• Zero-cost — pointer + length.
• String literals have static lifetime.

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

When to use:

• Use String for owned, mutable data.
• Use &str for read-only views.
• Prefer &str in APIs.

Common mistakes:

• Indexing strings: s[0] — not allowed.
• Forgetting &String auto-derefs to &str.
• Cloning when borrowing works.