Day 24: Attributes

Attributes are metadata applied to modules, crates, items, and expressions.

Attribute Syntax

#[attribute]           // Outer attribute (applies to next item)
#![attribute]          // Inner attribute (applies to enclosing item)

Common Attributes

Conditional Compilation

#[cfg(target_os = "linux")]
fn linux_only() { }

#[cfg(test)]
mod tests { }

#[cfg(feature = "advanced")]
fn advanced_feature() { }

Linting

#[allow(dead_code)]
fn unused_function() { }

#[warn(unused_variables)]
fn strict_function() { }

Memory Layout: #[repr]

Critical for Solana! Account data must have predictable memory layouts for serialization.

#[repr(C)]  // C-compatible layout
struct AccountData {
    is_initialized: bool,  // 1 byte
    bump: u8,              // 1 byte
    authority: [u8; 32],   // 32 bytes
    balance: u64,          // 8 bytes
}

Repr	Meaning
`#[repr(C)]`	C-compatible layout (predictable)
`#[repr(packed)]`	No padding between fields
`#[repr(transparent)]`	Same layout as single field
`#[repr(u8)]`	Enum discriminant is u8

Why #[repr(C)] Matters

Without #[repr(C)], Rust may reorder fields or add padding:

// Without repr(C) - Rust may optimize layout
struct Bad {
    a: bool,    // Could be anywhere
    b: u64,     // Could be anywhere
}

// With repr(C) - Predictable order
#[repr(C)]
struct Good {
    a: bool,    // Offset 0
    b: u64,     // Offset 8 (after padding)
}

The Task

Add #[repr(C)] to make AccountHeader have exactly 34 bytes:

is_initialized: bool (1 byte)
account_type: u8 (1 byte)
authority: [u8; 32] (32 bytes)

Hints

#[repr(C)]
struct AccountHeader {
    is_initialized: bool,
    account_type: u8,
    authority: [u8; 32],
}