1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135

#![forbid(unsafe_code)]

/// Find the offset in bytes of the given `$field` of `$Type`. Requires an
/// already initialized `$instance` value to work with.
///
/// This is similar to the macro from [`memoffset`](https://docs.rs/memoffset),
/// however it uses no `unsafe` code.
///
/// This macro has a 3-argument and 2-argument version.
/// * In the 3-arg version you specify an instance of the type, the type itself,
///   and the field name.
/// * In the 2-arg version the macro will call the [`default`](Default::default)
///   method to make a temporary instance of the type for you.
///
/// The output of this macro is the byte offset of the field (as a `usize`). The
/// calculations of the macro are fixed across the entire program, but if the
/// type used is `repr(Rust)` then they're *not* fixed across compilations or
/// compilers.
///
/// ## Examples
///
/// ### 3-arg Usage
///
/// ```rust
/// # use bytemuck::offset_of;
/// // enums can't derive default, and for this example we don't pick one
/// enum MyExampleEnum {
///   A,
///   B,
///   C,
/// }
///
/// // so now our struct here doesn't have Default
/// #[repr(C)]
/// struct MyNotDefaultType {
///   pub counter: i32,
///   pub some_field: MyExampleEnum,
/// }
///
/// // but we provide an instance of the type and it's all good.
/// let val = MyNotDefaultType { counter: 5, some_field: MyExampleEnum::A };
/// assert_eq!(offset_of!(val, MyNotDefaultType, some_field), 4);
/// ```
///
/// ### 2-arg Usage
///
/// ```rust
/// # use bytemuck::offset_of;
/// #[derive(Default)]
/// #[repr(C)]
/// struct Vertex {
///   pub loc: [f32; 3],
///   pub color: [f32; 3],
/// }
/// // if the type impls Default the macro can make its own default instance.
/// assert_eq!(offset_of!(Vertex, loc), 0);
/// assert_eq!(offset_of!(Vertex, color), 12);
/// ```
///
/// # Usage with `#[repr(packed)]` structs
///
/// Attempting to compute the offset of a `#[repr(packed)]` struct with
/// `bytemuck::offset_of!` requires an `unsafe` block. We hope to relax this in
/// the future, but currently it is required to work around a soundness hole in
/// Rust (See [rust-lang/rust#27060]).
///
/// [rust-lang/rust#27060]: https://github.com/rust-lang/rust/issues/27060
///
/// <p style="background:rgba(255,181,77,0.16);padding:0.75em;">
/// <strong>Warning:</strong> This is only true for versions of bytemuck >
/// 1.4.0. Previous versions of
/// <code style="background:rgba(41,24,0,0.1);">bytemuck::offset_of!</code>
/// will only emit a warning when used on the field of a packed struct in safe
/// code, which can lead to unsoundness.
/// </p>
///
/// For example, the following will fail to compile:
///
/// ```compile_fail
/// #[repr(C, packed)]
/// #[derive(Default)]
/// struct Example {
///   field: u32,
/// }
/// // Doesn't compile:
/// let _offset = bytemuck::offset_of!(Example, field);
/// ```
///
/// While the error message this generates will mention the
/// `safe_packed_borrows` lint, the macro will still fail to compile even if
/// that lint is `#[allow]`ed:
///
/// ```compile_fail
/// # #[repr(C, packed)] #[derive(Default)] struct Example { field: u32 }
/// // Still doesn't compile:
/// #[allow(safe_packed_borrows)]
/// {
///   let _offset = bytemuck::offset_of!(Example, field);
/// }
/// ```
///
/// This *can* be worked around by using `unsafe`, but it is only sound to do so
/// if you can guarantee that taking a reference to the field is sound.
///
/// In practice, this means it only works for fields of align(1) types, or if
/// you know the field's offset in advance (defeating the point of `offset_of`)
/// and can prove that the struct's alignment and the field's offset are enough
/// to prove the field's alignment.
///
/// Once the `raw_ref` macros are available, a future version of this crate will
/// use them to lift the limitations of packed structs. For the duration of the
/// `1.x` version of this crate that will be behind an on-by-default cargo
/// feature (to maintain minimum rust version support).
#[macro_export]
macro_rules! offset_of {
  ($instance:expr, $Type:path, $field:tt) => {{
    #[forbid(safe_packed_borrows)]
    {
      // This helps us guard against field access going through a Deref impl.
      #[allow(clippy::unneeded_field_pattern)]
      let $Type { $field: _, .. };
      let reference: &$Type = &$instance;
      let address = reference as *const _ as usize;
      let field_pointer = &reference.$field as *const _ as usize;
      // These asserts/unwraps are compiled away at release, and defend against
      // the case where somehow a deref impl is still invoked.
      let result = field_pointer.checked_sub(address).unwrap();
      assert!(result <= $crate::__core::mem::size_of::<$Type>());
      result
    }
  }};
  ($Type:path, $field:tt) => {{
    $crate::offset_of!(<$Type as Default>::default(), $Type, $field)
  }};
}