#struct #layout #explicit

macro struct_layout

Customize your struct layout with this one weird trick

1 unstable release

0.1.0 Apr 20, 2019

#607 in Procedural macros

MIT license

668 lines

Struct layout

MIT License crates.io docs.rs

Customize your struct layout with explicit control over the fields.


This proc-macro is available on crates.io.

Documentation can be found on docs.rs.

In your Cargo.toml, put

struct_layout = "0.1"


Apply the #[struct_layout::explicit] attribute to a struct definition and put #[field] attributes on every field declaration.

The syntax takes inspiration from C# [StructLayout] attribute.

/// Doc comments are allowed.
#[struct_layout::explicit(size = 32, align = 4)]
#[derive(Copy, Clone, Debug, Default)]
pub struct Foo {
	#[field(offset = 21, get, set)]
	pub unaligned: f32,

	/// Documenting the fields works too.
	#[field(offset = 4)]
	pub int: i32,

There's a lot to unpack here, let's go through it step by step.

The struct_layout::explicit attribute

This attribute must be applied to a struct definition, the attribute arguments order is fixed:

The size and alignment of the structure are required and follow the format size = <usize> and align = <usize>.

Following is an optional check(..) argument which specifies a trait bound which all field members must implement. This allows a custom trait to guarantee that all field types are safe to be used. If absent all fields are required to implement Copy.

Additional attributes

Because of the invasive nature of the transformation, only a small set of whitelisted attributes are supported:

  • doc comments are allowed and applied to the final generated structure.
  • derive with a set of whitelisted identifiers.

The generated structure

The proc-macro generates a newtype tuple structure wrapping a byte array with length specified by the size argument. The structure is adorned by the C and align representation with the alignment specified by the align argument. The visibility specified is applied to the generated structure. Any doc comments are also added.

Basically, this:

/// Doc comments are allowed.
#[repr(C, align(4))]
pub struct Foo([u8; 32]);

Supported auto derived traits

The only supported traits to be auto derived are Copy, Clone, Debug and Default. Future extensions may allow more traits to be supported.

Don't forget you can implement additional methods and traits on the generated type!

Structure field syntax

Every field in the structure must be accompanied by a single #[field(..)] attribute. No other attributes except doc comments are allowed.

The field attribute must start with specifying the offset of the field using offset = <usize>. Followed by a list of methods for how to implement access to the field.

Supported methods are get, set, ref or mut. If no methods are specified, they will all be implemented for this field. The accessor methods have where clause requiring the field type to implement the trait specified by the check argument of the struct_layout::explicit attribute.

The accessors are implemented via methods with the following signatures:

  • get: fn field(&self) -> T
  • set: fn set_field(&mut self, value: T) -> &mut Self
  • ref: fn field_ref(&self) -> &T
  • mut: fn field_mut(&mut self) -> &mut T

Get and set allow unaligned offsets. If any of the fields have ref or mut accessors then the field offset and the structure specified alignment must be a multiple of the field type's alignment.

If the field offset is out of bounds or unaligned (where required) an incomprehensible error is generated complaining about const evaluation failing. This is unfortunate due to how these constraints are statically asserted (using the array size trick).


All fields are required to implement Copy or the trait bound specified by the check argument. This restriction makes things safer, but by no means perfect. By using this library you commit to not doing stupid things and perhaps drop an issue in the bugtracker where safety can be improved.

Under no circumstance should a field be allowed to implement Drop or be a reference type. It is not supported.

How to construct an instance

If requested the Default trait may be auto derived filling in the fields with their type's default value. You may add additional associated methods to the generated structure.

It is also possible to use the unsafe std::mem::zeroed to create a zero initialized instance if this makes sense.

Compatibility with no_std

The generated code is compatible with no_std!


Licensed under MIT License, see license.txt.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, shall be licensed as above, without any additional terms or conditions.

No runtime deps