Fix undefined behaviour.

Cargo.toml

@@ -1,10 +1,13 @@
 [package]
 name = "field-offset"
-version = "0.1.1"
+version = "0.2.0"
 authors = ["Diggory Blake <[email protected]>"]
 description = "Safe pointer-to-member implementation"
 repository = "https://github.com/Diggsey/rust-field-offset"
 readme = "README.md"
 license = "MIT OR Apache-2.0"
 
 [dependencies]
+
+[build-dependencies]
+rustc_version = "0.2.3"

build.rs

@@ -0,0 +1,12 @@
+extern crate rustc_version;
+use rustc_version::{version, Version};
+
+fn main() {
+    // Assert we haven't travelled back in time
+    assert!(version().unwrap().major >= 1);
+
+    // Check for a minimum version
+    if version().unwrap() >= Version::parse("1.36.0").unwrap() {
+        println!("cargo:rustc-cfg=fieldoffset_maybe_uninit");
+    }
+}

src/lib.rs

@@ -5,94 +5,109 @@ use std::ops::Add;
 use std::fmt;
 
 /// Represents a pointer to a field of type `U` within the type `T`
+#[repr(transparent)]
 pub struct FieldOffset<T, U>(
     /// Offset in bytes of the field within the struct
     usize,
     /// A pointer-to-member can be thought of as a function from
     /// `&T` to `&U` with matching lifetimes
-    PhantomData<for<'a> Fn(&'a T) -> &'a U>
+    PhantomData<dyn for<'a> Fn(&'a T) -> &'a U>
 );
 
 impl<T, U> FieldOffset<T, U> {
+    // Use MaybeUninit to get a fake T
+    #[cfg(fieldoffset_maybe_uninit)]
+    #[inline]
+    fn with_uninit_ptr<R, F: FnOnce(*const T) -> R>(f: F) -> R {
+        let uninit = mem::MaybeUninit::<T>::uninit();
+        f(uninit.as_ptr())
+    }
+
+    // Use a dangling pointer to get a fake T
+    #[cfg(not(fieldoffset_maybe_uninit))]
+    #[inline]
+    fn with_uninit_ptr<R, F: FnOnce(*const T) -> R>(f: F) -> R {
+        f(mem::align_of::<T>() as *const T)
+    }
+
     /// Construct a field offset via a lambda which returns a reference
     /// to the field in question.
     ///
     /// The lambda *must not* access the value passed in.
-    pub unsafe fn new<F: for<'a> FnOnce(&'a T) -> &'a U>(f: F) -> Self {
-        // Construct a "fake" T. It's not valid, but the lambda shouldn't
-        // actually access it (which is why this is unsafe)
-        let x = mem::zeroed();
-        let offset = {
-            let x = &x;
-            // Pass a reference to the zeroed T to the lambda
-            // The lambda gives us back a reference to (what we hope is)
-            // a field of T, of type U
-            let y = f(x);
-            // Compute the offset of the field via the difference between the
-            // references `x` and `y`. Overflow is an error: in debug builds it
-            // will be caught here, in release it will wrap around and be caught
-            // on the next line.
-            (y as *const U as usize) - (x as *const T as usize)
-        };
-        // Don't run destructor on "fake" T
-        mem::forget(x);
+    pub unsafe fn new<F: for<'a> FnOnce(*const T) -> *const U>(f: F) -> Self {
+        let offset = Self::with_uninit_ptr(|base_ptr| {
+            let field_ptr = f(base_ptr);
+            (field_ptr as usize).wrapping_sub(base_ptr as usize)
+        });
+
+        // Construct an instance using the offset
+        Self::new_from_offset(offset)
+    }
+    /// Construct a field offset directly from a byte offset.
+    #[inline]
+    pub unsafe fn new_from_offset(offset: usize) -> Self {
         // Sanity check: ensure that the field offset plus the field size
         // is no greater than the size of the containing struct. This is
         // not sufficient to make the function *safe*, but it does catch
         // obvious errors like returning a reference to a boxed value,
         // which is owned by `T` and so has the correct lifetime, but is not
         // actually a field.
         assert!(offset + mem::size_of::<U>() <= mem::size_of::<T>());
-        // Construct an instance using the offset
-        Self::new_from_offset(offset)
-    }
-    /// Construct a field offset directly from a byte offset.
-    pub unsafe fn new_from_offset(offset: usize) -> Self {
+
         FieldOffset(offset, PhantomData)
     }
     // Methods for applying the pointer to member
     /// Apply the field offset to a native pointer.
+    #[inline]
     pub fn apply_ptr<'a>(&self, x: *const T) -> *const U {
         ((x as usize) + self.0) as *const U
     }
     /// Apply the field offset to a native mutable pointer.
+    #[inline]
     pub fn apply_ptr_mut<'a>(&self, x: *mut T) -> *mut U {
         ((x as usize) + self.0) as *mut U
     }
     /// Apply the field offset to a reference.
+    #[inline]
     pub fn apply<'a>(&self, x: &'a T) -> &'a U {
         unsafe { &*self.apply_ptr(x) }
     }
     /// Apply the field offset to a mutable reference.
+    #[inline]
     pub fn apply_mut<'a>(&self, x: &'a mut T) -> &'a mut U {
         unsafe { &mut *self.apply_ptr_mut(x) }
     }
     /// Get the raw byte offset for this field offset.
+    #[inline]
     pub fn get_byte_offset(&self) -> usize {
         self.0
     }
     // Methods for unapplying the pointer to member
     /// Unapply the field offset to a native pointer.
     ///
     /// *Warning: very unsafe!*
+    #[inline]
     pub unsafe fn unapply_ptr<'a>(&self, x: *const U) -> *const T {
         ((x as usize) - self.0) as *const T
     }
     /// Unapply the field offset to a native mutable pointer.
     ///
     /// *Warning: very unsafe!*
+    #[inline]
     pub unsafe fn unapply_ptr_mut<'a>(&self, x: *mut U) -> *mut T {
         ((x as usize) - self.0) as *mut T
     }
     /// Unapply the field offset to a reference.
     ///
     /// *Warning: very unsafe!*
+    #[inline]
     pub unsafe fn unapply<'a>(&self, x: &'a U) -> &'a T {
         &*self.unapply_ptr(x)
     }
     /// Unapply the field offset to a mutable reference.
     ///
     /// *Warning: very unsafe!*
+    #[inline]
     pub unsafe fn unapply_mut<'a>(&self, x: &'a mut U) -> &'a mut T {
         &mut *self.unapply_ptr_mut(x)
     }
@@ -107,6 +122,7 @@ impl<T, U> FieldOffset<T, U> {
 impl<T, U, V> Add<FieldOffset<U, V>> for FieldOffset<T, U> {
     type Output = FieldOffset<T, V>;
 
+    #[inline]
     fn add(self, other: FieldOffset<U, V>) -> FieldOffset<T, V> {
         FieldOffset(self.0 + other.0, PhantomData)
     }
@@ -140,12 +156,20 @@ impl<T, U> Clone for FieldOffset<T, U> {
 /// `offset_of!(Foo => bar: Bar => x)`
 #[macro_export]
 macro_rules! offset_of {
-    ($t: path => $f: ident) => {
-        unsafe { $crate::FieldOffset::<$t, _>::new(|x| {
-            let $t { ref $f, .. } = *x;
-            $f
-        }) }
-    };
+    ($t: tt => $f: tt) => {{
+        // Make sure the field exists, and is not being accessed via Deref.
+        let $t { $f: _, .. };
+
+        // Construct the offset
+        #[allow(unused_unsafe)]
+        unsafe {
+            $crate::FieldOffset::<$t, _>::new(|x| {
+                // This is UB unless/until the compiler special-cases it to
+                // not enforce the validity constraint on `x`.
+                &(*x).$f as *const _
+            })
+        }
+    }};
     ($t: path => $f: ident: $($rest: tt)*) => {
         offset_of!($t => $f) + offset_of!($($rest)*)
     };
@@ -167,6 +191,9 @@ mod tests {
         y: Foo,
     }
 
+    #[derive(Debug)]
+    struct Tuple(i32, f64);
+
     #[test]
     fn test_simple() {
         // Get a pointer to `b` within `Foo`
@@ -193,6 +220,28 @@ mod tests {
         assert!(x.b == 42.0);
     }
 
+    #[test]
+    fn test_tuple() {
+        // Get a pointer to `b` within `Foo`
+        let tuple_1 = offset_of!(Tuple => 1);
+
+        // Construct an example `Foo`
+        let mut x = Tuple(1, 42.0);
+
+        // Apply the pointer to get at `b` and read it
+        {
+            let y = tuple_1.apply(&x);
+            assert!(*y == 42.0);
+        }
+
+        // Apply the pointer to get at `b` and mutate it
+        {
+            let y = tuple_1.apply_mut(&mut x);
+            *y = 5.0;
+        }
+        assert!(x.1 == 5.0);
+    }
+
     #[test]
     fn test_nested() {
         // Construct an example `Foo`