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A slice in Rust is a dynamically-sized type (DST) representing a contiguous view into a block of memory. It allows safe, borrow-checked access to a portion of a collection—such as an array, Vec<T>, or String—without copying the underlying data. Because the slice itself ([T]) is dynamically sized, its size is not known at compile time. Consequently, slices are almost exclusively interacted with via references (&[T] for shared access or &mut [T] for mutable access). Under the hood, a slice reference is a “fat pointer” consisting of exactly two machine words:
  1. A pointer to the memory address of the first element in the slice.
  2. A usize representing the length of the slice (the number of elements it contains).

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Syntax and Instantiation

Slices are constructed using the borrowing operator (& or &mut) combined with Rust’s range syntax (start..end), where start is inclusive and end is exclusive. 
let array: [i32; 5] = [10, 20, 30, 40, 50];

// Shared slice reference (&[i32])
let slice: &[i32] = &array[1..4]; // Contains [20, 30, 40]

// Mutable slice reference (&mut [i32])
let mut mut_array: [i32; 5] = [10, 20, 30, 40, 50];
let mut_slice: &mut [i32] = &mut mut_array[1..4];
Rust provides syntactic sugar for omitting bounds when slicing from the beginning or to the end of a collection: 
let collection = [0, 1, 2, 3, 4];

let a = &collection[..];    // Full range: index 0 to len
let b = &collection[2..];   // From index 2 to len
let c = &collection[..3];   // From index 0 to 3 (exclusive)
let d = &collection[1..=3]; // Inclusive range: index 1 to 3 (inclusive)

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String Slices (&str)

A string slice (&str) is a specialized, primitive slice type. While &[T] is a slice of arbitrary elements, &str is fundamentally a slice of bytes (&[u8]) with a strict type system and standard library guarantee that the underlying byte sequence is valid UTF-8. 
let heap_string: String = String::from("Compiler");
let str_slice: &str = &heap_string[0..4]; // "Comp"

// String literals are inherently string slices pointing to static memory
let literal: &'static str = "Static data";
Note: Slicing a string must occur at valid UTF-8 character boundaries. Slicing in the middle of a multi-byte character will cause a runtime panic.

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Type System Mechanics

  • [T]: The slice type itself. It does not implement the Sized trait (making it an unsized type), meaning it cannot be stored directly on the stack or passed by value.
  • &[T]: The slice reference. It implements the Sized trait because the fat pointer has a fixed, known size at compile time (16 bytes on a 64-bit architecture).
  • Deref Coercion: Collections that manage contiguous memory implement the Deref trait targeting their respective slice types. Vec<T> implements Deref<Target = [T]>, and String implements Deref<Target = str>. This allows a &Vec<T> to implicitly coerce into a &[T] when passed to functions or methods.
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