Rust String vs str (&str) | Ownership, Slices, and API Design

Introduction

“String or &str?” is the classic Rust beginner question. This article explains both and how to choose in APIs.

What you will learn

• Memory layout (heap vs slice)
• Ownership vs borrowing
• Conversion patterns
• Common lifetime mistakes

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Table of contents

1. Quick comparison
2. Memory layout
3. Ownership and borrowing
4. Performance
5. Conversions
6. Practical guide
7. Common mistakes
8. Closing thoughts

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1. Quick comparison

	String	&str
Kind	Owned UTF-8	Borrowed slice
Storage	Heap + stack fat pointer	Stack pointer + len (often into static or heap)
Mutability	Can mutate buffer	View-only
Typical args	Move ownership	Borrow (&str)
Typical returns	Owned data	&str tied to input lifetime

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2. Memory layout

String is a growable UTF-8 buffer on the heap. &str is a slice—pointer + length—into UTF-8 bytes elsewhere (literal, String, etc.).

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3. Ownership and borrowing

fn take_ownership(s: String) {
    println!("{}", s);
} // drop

let s = String::from("hello");
take_ownership(s);
// println!("{}", s); // error: moved

fn borrow(s: &str) {
    println!("{}", s);
}

let s = String::from("hello");
borrow(&s);
println!("{}", s); // OK

Prefer &str parameters unless you need to consume or store the string without a clear lifetime.

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4. Performance

Cloning String allocates; copying &str copies two words. Hot paths should avoid unnecessary to_string().

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5. Conversions

• String → &str: &s, s.as_str(), &s[..]
• &str → String: to_string(), String::from, to_owned()

Build String only when you must own or mutate.

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6. Practical guide

Parameters

fn print_message(msg: &str) {
    println!("{}", msg);
}

Accepts literals, String slices, etc.

Struct fields

Usually String for owned data. &str fields need lifetimes:

struct UserRef<'a> {
    name: &'a str,
}

Returns

Returning &str from local String is invalid—return String or Cow or borrow from input with correct lifetime.

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7. Common mistakes

• Taking String by value when &str suffices
• Extra to_string() in hot paths
• Returning &str to dropped data

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Closing thoughts

1. Arguments: &str by default
2. String when you need ownership
3. Read-only views: &str
4. If lifetimes get painful, prefer owning String

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FAQ

See frontmatter.

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Related posts

• Rust ownership
• Rust borrowing
• Rust lifetimes

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Keywords

Rust, String, str, slice, ownership, borrowing, lifetimes, performance, comparison