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

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

String:

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

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

&str:

• Borrowed reference (slice) to string data.
• Immutable — cannot modify contents.
• Can point to any string data (heap, stack, or static).
• String literals ("hello") are &str with static lifetime.
• Zero-cost abstraction — just a pointer + length.

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

When to use:

• Use String when you need ownership and mutation.
• Use &str for read-only access (function params, comparisons).
• Prefer &str in API design for flexibility.

Common mistakes:

• Not understanding UTF-8: s[0] doesn't work — use .chars().
• Forgetting that &String auto-derefs to &str.
• Unnecessary cloning when borrowing would suffice.