The layout of a type is its size, alignment, and the relative offsets of its fields. For enums, how the discriminant is laid out and interpreted is also part of type layout.

Type layout can be changed with each compilation. Instead of trying to document exactly what is done, we only document what is guaranteed today.

All values have an alignment and size.

The alignment of a value specifies what addresses are valid to store the value at. A value of alignment n must only be stored at an address that is a multiple of n. For example, a value with an alignment of 2 must be stored at an even address, while a value with an alignment of 1 can be stored at any address. Alignment is measured in bytes, and must be at least 1, and always a power of 2. The alignment of a value can be checked with the align_of_val function.

The size of a value is the offset in bytes between successive elements in an array with that item type including alignment padding. The size of a value is always a multiple of its alignment. The size of a value can be checked with the size_of_val function.

Types where all values have the same size and alignment known at compile time implement the Sized trait and can be checked with the size_of and align_of functions. Types that are not Sized are known as dynamically sized types. Since all values of a Sized type share the same size and alignment, we refer to those shared values as the size of the type and the alignment of the type respectively.

The size of most primitives is given in this table.

usize and isize have a size big enough to contain every address on the target platform. For example, on a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.

Most primitives are generally aligned to their size, although this is platform-specific behavior. In particular, on x86 u64 and f64 are only aligned to 32 bits.

Pointers and references have the same layout. Mutability of the pointer or reference does not change the layout.

Pointers to sized types have the same size and alignment as usize.

Pointers to unsized types are sized. The size and alignemnt is guaranteed to be at least equal to the size and alignment of a pointer.

Note: Though you should not rely on this, all pointers to DSTs are currently twice the size of the size of usize and have the same alignment.

Arrays are laid out so that the nth element of the array is offset from the start of the array by n * the size of the type bytes. An array of [T; n] has a size of size_of::<T>() * n and the same alignment of T.

Slices have the same layout as the section of the array they slice.

Note: This is about the raw [T] type, not pointers (&[T], Box<[T]>, etc.) to slices.

Tuples do not have any guarantes about their layout.

The exception to this is the unit tuple (()) which is guaranteed as a zero-sized type to have a size of 0 and an alignment of 1.

Trait objects have the same layout as the value the trait object is of.

Note: This is about the raw trait object types, not pointers (&Trait, Box<Trait>, etc.) to trait objects.

Closures have no layout guarantees.

All user-defined composite types (structs, enum, and unions) have a representation that specifies what the layout is for the type.

The possible representations for a type are the default representation, C, the primitive representations, and packed. Multiple representations can be applied to a single type.

The representation of a type can be changed by applying the [repr attribute] to it. The following example shows a struct with a C representation.

#[repr(C)]
struct ThreeInts {
    first: i16,
    second: i8,
    third: i32
}

Note: As a consequence of the representation being an attribute on the item, the representation does not depend on generic parameters. Any two types with the same name have the same representation. For example, Foo<Bar> and Foo<Baz> both have the same representation.

The representation of a type does not change the layout of its fields. For example, a struct with a C representation that contains a struct Inner with the default representation will not change the layout of Inner.

Nominal types without a repr attribute have the default representation. Informally, this representation is also called the rust representation.

There are no guarantees of data layout made by this representation.

The C representation is designed for dual purposes. One purpose is for creating types that are interoptable with the C Language. The second purpose is to create types that you can soundly performing operations that rely on data layout such as reinterpreting values as a different type.

Because of this dual purpose, it is possible to create types that are not useful for interfacing with the C programming language.

This representation can be applied to structs, unions, and enums.

The alignment of the struct is the alignment of the most-aligned field in it.

The size and offset of fields is determined by the following algorithm.

Start with a current offset of 0 bytes.

For each field in declaration order in the struct, first determine the size and alignment of the field. If the current offset is not a multiple of the field's alignment, then add padding bytes to the current offset until it is a multiple of the field's alignment. The offset for the field is what the current offset is now. Then increase the current offset by the size of the field.

Finally, the size of the struct is the current offset rounded up to the nearest multiple of the struct's alignment.

Here is this algorithm described in psudeocode.

struct.alignment = struct.fields().map(|field| field.alignment).max();

let current_offset = 0;

for field in struct.fields_in_declaration_order() {
    // Increase the current offset so that it's a multiple of the alignment
    // of this field. For the first field, this will always be zero.
    // The skipped bytes are called padding bytes.
    current_offset += field.alignment % current_offset;

    struct[field].offset = current_offset;

    current_offset += field.size;
}

struct.size = current_offset + current_offset % struct.alignment;

Note: This algorithm can produce zero-sized structs. This differs from C where structs without data still have a size of one byte.

A union declared with #[repr(C)] will have the same size and alignment as an equivalent C union declaration in the C language for the target platform. The union will have a size of the maximum size of all of its fields rounded to its alignment, and an alignment of the maximum alignment of all of its fields. These maximums may come from different fields.

#[repr(C)]
union Union {
    f1: u16,
    f2: [u8; 4],
}

assert_eq!(std::mem::size_of::<Union>(), 4);  // From f2
assert_eq!(std::mem::align_of::<Union>(), 2); // From f1

#[repr(C)]
union SizeRoundedUp {
   a: u32,
   b: [u16; 3],
}

assert_eq!(std::mem::size_of::<SizeRoundedUp>(), 8);  // Size of 6 from b,
                                                      // rounded up to 8 from
                                                      // alignment of a.
assert_eq!(std::mem::align_of::<SizeRoundedUp>(), 4); // From a

For C-like enumerations, the C representation has the size and alignment of the default enum size and alignment for the target platform's C ABI.

Note: The enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags.

Warning: There are crucial differences between an enum in the C language and Rust's C-like enumerations with this representation. An enum in C is mostly a typedef plus some named constants; in other words, an object of an enum type can hold any integer value. For example, this is often used for bitflags in C. In contrast, Rust’s C-like enumerations can only legally hold the discrimnant values, everything else is undefined behaviour. Therefore, using a C-like enumeration in FFI to model a C enum is often wrong.

It is an error for zero-variant enumerations to have the C representation.

For all other enumerations, the layout is unspecified.

Likewise, combining the C representation with a primitive representation, the layout is unspecified.

The primitive representations are the representations with the same names as the primitive integer types. That is: u8, u16, u32, u64, usize, i8, i16, i32, i64, and isize.

Primitive representations can only be applied to enumerations.

For C-like enumerations, they set the size and alignment to be the same as the primitive type of the same name. For example, a C-like enumeration with a u8 representation can only have discriminants between 0 and 255 inclusive.

It is an error for zero-variant enumerations to have a primitive representation.

For all other enumerations, the layout is unspecified.

Likewise, combining two primitive representations together is unspecified.

The align representation can be used on structs and unions to raise the alignment of the type to a given value.

Alignment is specified as a parameter in the form of #[repr(align(x))]. The alignment value must be a power of two of type u32. The align representation can raise the alignment of a type to be greater than it's primitive alignment, it cannot lower the alignment of a type.

The align and packed representations cannot be applied on the same type and a packed type cannot transitively contain another aligned type.

The packed representation can only be used on structs and unions.

It modifies the representation (either the default or C) by removing any padding bytes and forcing the alignment of the type to 1.

The align and packed representations cannot be applied on the same type and a packed type cannot transitively contain another aligned type.

Warning: Dereferencing an unaligned pointer is [undefined behaviour] and it is possible to safely create unaligned pointers to packed fields. Like all ways to create undefined behavior in safe Rust, this is a bug.