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u64 is a primitive unsigned integer type in Rust that occupies exactly 64 bits (8 bytes) of memory. It represents non-negative whole numbers strictly within the range of 0 to 18,446,744,073,709,551,615 (264−12^{64} - 1). Because it is unsigned, it cannot represent negative values and does not allocate a sign bit.

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Constants and Bounds

The standard library provides associated constants to access the boundaries of the type: 
let min_val: u64 = u64::MIN; // 0
let max_val: u64 = u64::MAX; // 18446744073709551615

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Syntax and Instantiation

Rust allows u64 initialization through explicit type annotation, literal suffixes, and various radix prefixes. Underscores can be inserted anywhere within numeric literals for visual grouping. 
// Explicit type annotation
let decimal: u64 = 1_000_000;

// Literal suffix (type inference)
let suffixed = 42u64;

// Radix prefixes
let hex: u64 = 0xDEAD_BEEF_0123_4567; // Base 16
let octal: u64 = 0o777_644;           // Base 8
let binary: u64 = 0b1010_1100_0011;   // Base 2

// Byte literal cast
let from_byte = b'A' as u64;          // 65

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Type Casting

Casting to and from u64 is performed using the as keyword.
  • Casting from a smaller unsigned integer (u8, u16, u32) results in zero-extension.
  • Casting from a smaller signed integer (i8, i16, i32) results in sign-extension before interpreting the bits as unsigned.
  • Casting to a smaller integer type results in truncation of the most significant bits.
let small: u32 = 42;
let widened: u64 = small as u64; // Zero-extended to 64 bits

let large: u64 = 0x1122_3344_5566_7788;
let truncated: u32 = large as u32; // 0x5566_7788 (Higher 32 bits discarded)

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Integer Overflow Behavior

Rust handles u64 arithmetic overflows differently depending on the compilation profile:
  • Debug mode (dev): Integer operations that overflow will trigger a panic.
  • Release mode (release): Integer operations silently perform two’s complement wrapping.
To enforce specific overflow behaviors regardless of the compilation profile, u64 provides explicit arithmetic methods: 
let val = u64::MAX;

// Checked: Returns Option::None on overflow
let checked: Option<u64> = val.checked_add(1); 

// Wrapping: Wraps around using two's complement (returns 0)
let wrapped: u64 = val.wrapping_add(1); 

// Saturating: Clamps to the numeric bound (returns u64::MAX)
let saturated: u64 = val.saturating_add(1); 

// Overflowing: Returns a tuple (result, boolean_flag)
let (result, did_overflow) = val.overflowing_add(1); // (0, true)

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Memory Layout and Endianness

u64 is stored in memory as a contiguous 8-byte sequence. The standard library provides methods to manipulate and convert the byte order (endianness) of the integer, which is critical for network serialization or FFI. 
let num: u64 = 0x1234_5678_90AB_CDEF;

// Convert to a byte array in Big-Endian (network byte order)
let be_bytes: [u8; 8] = num.to_be_bytes(); 
// [0x12, 0x34, 0x56, 0x78, 0x90, 0xAB, 0xCD, 0xEF]

// Convert to a byte array in Little-Endian
let le_bytes: [u8; 8] = num.to_le_bytes(); 
// [0xEF, 0xCD, 0xAB, 0x90, 0x78, 0x56, 0x34, 0x12]

// Reconstruct from bytes
let reconstructed = u64::from_be_bytes(be_bytes);

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Trait Implementations

As a primitive, u64 implements several core traits by default:
  • Copy and Clone: Passed by value rather than by reference.
  • Eq, PartialEq, Ord, PartialOrd: Fully comparable and sortable.
  • Hash: Can be used as keys in HashMap or HashSet.
  • Default: The default value is 0.
  • Bitwise traits (BitAnd, BitOr, BitXor, Shl, Shr): Supports standard bitwise operations.
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