Lesson 10 of 20

6 mins read

Slices & String Types (&str vs String)

In Plain English

A slice is a view into a contiguous sequence of elements without copying. Rust distinguishes between owned, growable heap strings (String) and borrowed read-only string slices (&str).

The Mental Model

"A String is a full book you own on your bookshelf. A string slice (&str) is an index bookmark pointing to chapters 2 through 4 of that book—it doesn’t copy the pages, just references them."

Deep Dive: How It Works

String vs &str: String is a heap-allocated, growable buffer. &str is a pointer and length pointing to UTF-8 bytes (in the binary or on the heap).

Range Syntax: &s[0..5] borrows the first 5 bytes of string s.

Array Slices: &[i32] borrows a slice of an array or vector without allocation.

Syntax Blueprint

let s = String::from("hello world"); let hello: &str = &s[0..5];

Slices store a starting memory pointer and a length, granting zero-copy sub-sequence views.

Core Rules to Remember

&str: Zero-cost borrowed view into UTF-8 text.

String: Owned, dynamically growable heap-allocated string buffer.

Slice Ranges: &arr[1..4] views indices 1, 2, and 3 without copying.

Common Beginner Traps & How to Fix Them

Mistake:Slicing across multi-byte UTF-8 character boundaries.

Why it happens: String slicing indexes raw bytes, not characters. Slicing in the middle of a multi-byte emoji causes a runtime panic.

How to fix: Ensure slice indices align with valid UTF-8 boundaries, or use .chars().

Pro Tip: In function arguments, always prefer &str over &String because &String coerces into &str automatically (deref coercion).

Live Interactive Example

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String and Array Slicing

Rust 1.78 / rustc

Output Console

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How it works: Demonstrates zero-copy string and array slicing.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Extract slice from a string

Given let greeting = String::from("Hello Rustaceans");
Extract the slice let rust = &greeting[6..16];
Print "Target: Rustaceans".

Sandbox Output

Click "Run & Check" to test your solution.

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