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//! Functions for calculating field offsets. use crate::utils::{self, Mem}; /// Calculates the offset of a field in bytes,given the previous field. /// /// # Parameters /// /// `Struct` is the struct that contains the field that this calculates the offset for. /// /// `Prev` is the type of the previous field. /// /// `Next` is the type of the field that this calculates the offset for. /// /// `previous_offset` is the offset in bytes of the previous field,of `Prev` type. /// /// # Example /// /// ``` /// use repr_offset::offset_calc::next_field_offset; /// /// #[repr(C, packed)] /// struct Foo(u8, u16, u32, u64); /// /// assert_eq!( OFFSET_1, 1 ); /// assert_eq!( OFFSET_2, 3 ); /// assert_eq!( OFFSET_3, 7 ); /// /// const OFFSET_0: usize = 0; /// const OFFSET_1: usize = next_field_offset::<Foo, u8, u16>(OFFSET_0); /// const OFFSET_2: usize = next_field_offset::<Foo, u16, u32>(OFFSET_1); /// const OFFSET_3: usize = next_field_offset::<Foo, u32, u64>(OFFSET_2); /// /// ``` #[inline(always)] pub const fn next_field_offset<Struct, Prev, Next>(previous_offset: usize) -> usize { GetNextFieldOffset { previous_offset, previous_size: Mem::<Prev>::SIZE, container_alignment: Mem::<Struct>::ALIGN, next_alignment: Mem::<Next>::ALIGN, } .call() } /// Calculates the offset (in bytes) of a field, with the `call` method. /// /// # Example /// /// ``` /// use repr_offset::offset_calc::GetNextFieldOffset; /// /// use std::mem; /// /// #[repr(C, packed)] /// struct Foo(u8, u16, u32, u64); /// /// assert_eq!( OFFSET_1, 1 ); /// assert_eq!( OFFSET_2, 3 ); /// assert_eq!( OFFSET_3, 7 ); /// /// const OFFSET_0: usize = 0; /// /// const OFFSET_1: usize = GetNextFieldOffset{ /// previous_offset: OFFSET_0, /// previous_size: mem::size_of::<u8>(), /// container_alignment: mem::align_of::<Foo>(), /// next_alignment: mem::align_of::<u16>(), /// }.call(); /// /// const OFFSET_2: usize = GetNextFieldOffset{ /// previous_offset: OFFSET_1, /// previous_size: mem::size_of::<u16>(), /// container_alignment: mem::align_of::<Foo>(), /// next_alignment: mem::align_of::<u32>(), /// }.call(); /// /// const OFFSET_3: usize = GetNextFieldOffset{ /// previous_offset: OFFSET_2, /// previous_size: mem::size_of::<u32>(), /// container_alignment: mem::align_of::<Foo>(), /// next_alignment: mem::align_of::<u64>(), /// }.call(); /// /// ``` pub struct GetNextFieldOffset { /// The offset in bytes of the previous field. pub previous_offset: usize, /// The size of the previous field. pub previous_size: usize, /// The alignment of the type that contains the field. pub container_alignment: usize, /// The alignment of the field that this calculates the offset for. pub next_alignment: usize, } impl GetNextFieldOffset { /// Calculates the offset (in bytes) of a field. pub const fn call(self) -> usize { let middle_offset = self.previous_offset + self.previous_size; let padding = { let alignment = utils::min_usize(self.next_alignment, self.container_alignment); let misalignment = middle_offset % alignment; // Workaround for `if` in const contexts not being stable on Rust 1.34 let mask = ((misalignment == 0) as usize).wrapping_sub(1); (alignment - misalignment) & mask }; middle_offset + padding } }