memoffset/src/offset_of.rs

// Copyright (c) 2017 Gilad Naaman
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

/// Macro to create a local `base_ptr` raw pointer of the given type, avoiding UB as
/// much as is possible currently.
#[cfg(maybe_uninit)]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset__let_base_ptr {
    ($name:ident, $type:ty) => {
        // No UB here, and the pointer does not dangle, either.
        // But we have to make sure that `uninit` lives long enough,
        // so it has to be in the same scope as `$name`. That's why
        // `let_base_ptr` declares a variable (several, actually)
        // instead of returning one.
        let uninit = $crate::__priv::mem::MaybeUninit::<$type>::uninit();
        let $name: *const $type = uninit.as_ptr();
    };
}
#[cfg(not(maybe_uninit))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset__let_base_ptr {
    ($name:ident, $type:ty) => {
        // No UB right here, but we will later dereference this pointer to
        // offset into a field, and that is UB because the pointer is dangling.
        let $name = $crate::__priv::mem::align_of::<$type>() as *const $type;
    };
}

/// Macro to compute the distance between two pointers.
#[cfg(any(feature = "unstable_const", stable_const))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset_offset_from_unsafe {
    ($field:expr, $base:expr) => {{
        let field = $field; // evaluate $field outside the `unsafe` block
        let base = $base; // evaluate $base outside the `unsafe` block
        // Compute offset, with unstable `offset_from` for const-compatibility.
        // (Requires the pointers to not dangle, but we already need that for `raw_field!` anyway.)
        unsafe { (field as *const u8).offset_from(base as *const u8) as usize }
    }};
}
#[cfg(not(any(feature = "unstable_const", stable_const)))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset_offset_from_unsafe {
    ($field:expr, $base:expr) => {
        // Compute offset.
        ($field as usize) - ($base as usize)
    };
}

/// Calculates the offset of the specified field from the start of the named struct.
///
/// ## Examples
/// ```
/// use memoffset::offset_of;
///
/// #[repr(C, packed)]
/// struct Foo {
///     a: u32,
///     b: u64,
///     c: [u8; 5]
/// }
///
/// fn main() {
///     assert_eq!(offset_of!(Foo, a), 0);
///     assert_eq!(offset_of!(Foo, b), 4);
/// }
/// ```
///
/// ## Notes
/// Rust's ABI is unstable, and [type layout can be changed with each
/// compilation](https://doc.rust-lang.org/reference/type-layout.html).
///
/// Using `offset_of!` with a `repr(Rust)` struct will return the correct offset of the
/// specified `field` for a particular compilation, but the exact value may change
/// based on the compiler version, concrete struct type, time of day, or rustc's mood.
///
/// As a result, the value should not be retained and used between different compilations.
#[macro_export(local_inner_macros)]
macro_rules! offset_of {
    ($parent:path, $field:tt) => {{
        // Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
        _memoffset__let_base_ptr!(base_ptr, $parent);
        // Get field pointer.
        let field_ptr = raw_field!(base_ptr, $parent, $field);
        // Compute offset.
        _memoffset_offset_from_unsafe!(field_ptr, base_ptr)
    }};
}

/// Calculates the offset of the specified field from the start of the tuple.
///
/// ## Examples
/// ```
/// use memoffset::offset_of_tuple;
///
/// fn main() {
///     assert!(offset_of_tuple!((u8, u32), 1) >= 0, "Tuples do not have a defined layout");
/// }
/// ```
#[cfg(tuple_ty)]
#[macro_export(local_inner_macros)]
macro_rules! offset_of_tuple {
    ($parent:ty, $field:tt) => {{
        // Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
        _memoffset__let_base_ptr!(base_ptr, $parent);
        // Get field pointer.
        let field_ptr = raw_field_tuple!(base_ptr, $parent, $field);
        // Compute offset.
        _memoffset_offset_from_unsafe!(field_ptr, base_ptr)
    }};
}

/// Calculates the offset of the specified union member from the start of the union.
///
/// ## Examples
/// ```
/// use memoffset::offset_of_union;
///
/// #[repr(C, packed)]
/// union Foo {
///     foo32: i32,
///     foo64: i64,
/// }
///
/// fn main() {
///     assert!(offset_of_union!(Foo, foo64) == 0);
/// }
/// ```
///
/// ## Note
/// Due to macro_rules limitations, this macro will accept structs with a single field as well as unions.
/// This is not a stable guarantee, and future versions of this crate might fail
/// on any use of this macro with a struct, without a semver bump.
#[macro_export(local_inner_macros)]
macro_rules! offset_of_union {
    ($parent:path, $field:tt) => {{
        // Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
        _memoffset__let_base_ptr!(base_ptr, $parent);
        // Get field pointer.
        let field_ptr = raw_field_union!(base_ptr, $parent, $field);
        // Compute offset.
        _memoffset_offset_from_unsafe!(field_ptr, base_ptr)
    }};
}

#[cfg(test)]
mod tests {
    #[test]
    fn offset_simple() {
        #[repr(C)]
        struct Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        assert_eq!(offset_of!(Foo, a), 0);
        assert_eq!(offset_of!(Foo, b), 4);
        assert_eq!(offset_of!(Foo, c), 8);
    }

    #[test]
    #[cfg_attr(miri, ignore)] // this creates unaligned references
    fn offset_simple_packed() {
        #[repr(C, packed)]
        struct Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        assert_eq!(offset_of!(Foo, a), 0);
        assert_eq!(offset_of!(Foo, b), 4);
        assert_eq!(offset_of!(Foo, c), 6);
    }

    #[test]
    fn tuple_struct() {
        #[repr(C)]
        struct Tup(i32, i32);

        assert_eq!(offset_of!(Tup, 0), 0);
        assert_eq!(offset_of!(Tup, 1), 4);
    }

    #[test]
    fn offset_union() {
        // Since we're specifying repr(C), all fields are supposed to be at offset 0
        #[repr(C)]
        union Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        assert_eq!(offset_of_union!(Foo, a), 0);
        assert_eq!(offset_of_union!(Foo, b), 0);
        assert_eq!(offset_of_union!(Foo, c), 0);
    }

    #[test]
    fn path() {
        mod sub {
            #[repr(C)]
            pub struct Foo {
                pub x: u32,
            }
        }

        assert_eq!(offset_of!(sub::Foo, x), 0);
    }

    #[test]
    fn inside_generic_method() {
        struct Pair<T, U>(T, U);

        fn foo<T, U>(_: Pair<T, U>) -> usize {
            offset_of!(Pair<T, U>, 1)
        }

        assert_eq!(foo(Pair(0, 0)), 4);
    }

    #[cfg(tuple_ty)]
    #[test]
    fn test_tuple_offset() {
        let f = (0i32, 0.0f32, 0u8);
        let f_ptr = &f as *const _;
        let f1_ptr = &f.1 as *const _;

        assert_eq!(
            f1_ptr as usize - f_ptr as usize,
            offset_of_tuple!((i32, f32, u8), 1)
        );
    }

    #[test]
    fn test_raw_field() {
        #[repr(C)]
        struct Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        let f: Foo = Foo {
            a: 0,
            b: [0, 0],
            c: 0,
        };
        let f_ptr = &f as *const _;
        assert_eq!(f_ptr as usize + 0, raw_field!(f_ptr, Foo, a) as usize);
        assert_eq!(f_ptr as usize + 4, raw_field!(f_ptr, Foo, b) as usize);
        assert_eq!(f_ptr as usize + 8, raw_field!(f_ptr, Foo, c) as usize);
    }

    #[cfg(tuple_ty)]
    #[test]
    fn test_raw_field_tuple() {
        let t = (0u32, 0u8, false);
        let t_ptr = &t as *const _;
        let t_addr = t_ptr as usize;

        assert_eq!(
            &t.0 as *const _ as usize - t_addr,
            raw_field_tuple!(t_ptr, (u32, u8, bool), 0) as usize - t_addr
        );
        assert_eq!(
            &t.1 as *const _ as usize - t_addr,
            raw_field_tuple!(t_ptr, (u32, u8, bool), 1) as usize - t_addr
        );
        assert_eq!(
            &t.2 as *const _ as usize - t_addr,
            raw_field_tuple!(t_ptr, (u32, u8, bool), 2) as usize - t_addr
        );
    }

    #[test]
    fn test_raw_field_union() {
        #[repr(C)]
        union Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        let f = Foo { a: 0 };
        let f_ptr = &f as *const _;
        assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, a) as usize);
        assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, b) as usize);
        assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, c) as usize);
    }

    #[cfg(any(feature = "unstable_const", stable_const))]
    #[test]
    fn const_offset() {
        #[repr(C)]
        struct Foo {
            a: u32,
            b: [u8; 2],
            c: i64,
        }

        assert_eq!([0; offset_of!(Foo, b)].len(), 4);
    }

    #[cfg(feature = "unstable_const")]
    #[test]
    fn const_offset_interior_mutable() {
        #[repr(C)]
        struct Foo {
            a: u32,
            b: core::cell::Cell<u32>,
        }

        assert_eq!([0; offset_of!(Foo, b)].len(), 4);
    }

    #[cfg(any(feature = "unstable_const", stable_const))]
    #[test]
    fn const_fn_offset() {
        const fn test_fn() -> usize {
            #[repr(C)]
            struct Foo {
                a: u32,
                b: [u8; 2],
                c: i64,
            }

            offset_of!(Foo, b)
        }

        assert_eq!([0; test_fn()].len(), 4);
    }
}