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Diffstat (limited to 'src/offset_of.rs')
| -rw-r--r-- | src/offset_of.rs | 135 |
1 files changed, 135 insertions, 0 deletions
diff --git a/src/offset_of.rs b/src/offset_of.rs new file mode 100644 index 0000000..3de2327 --- /dev/null +++ b/src/offset_of.rs @@ -0,0 +1,135 @@ +#![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)
+ }};
+}
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