start with pointer

Cargo.toml

@@ -6,3 +6,4 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+sptr = "0.2.3"

src/lib.rs

@@ -1,8 +1,163 @@
-#[cfg(test)]
+mod strategies;
-mod tests {
+
-    #[test]
+use std::fmt::{Debug, Formatter};
-    fn it_works() {
+use std::marker::PhantomData;
-        let result = 2 + 2;
+use std::ops::Not;
-        assert_eq!(result, 4);
+
+use sptr::Strict;
+
+pub struct StuffedPtr<T, S>(*mut T, PhantomData<S>)
+where
+    S: StuffingStrategy;
+
+impl<T, S> StuffedPtr<T, S>
+where
+    S: StuffingStrategy,
+{
+    pub fn new_ptr(ptr: *mut T) -> Self {
+        Self(map_ptr(ptr, S::stuff_ptr), PhantomData)
+    }
+
+    pub fn new_extra(extra: S::Extra) -> Self {
+        // this doesn't have any provenance, which is ok, since it's never a pointer anyways.
+        // if the user calls `set_ptr` it will use the new provenance from that ptr
+        let ptr = std::ptr::null_mut();
+        let ptr = Strict::with_addr(ptr, S::stuff_extra(extra));
+        Self(ptr, PhantomData)
+    }
+
+    pub unsafe fn get_ptr(&self) -> Option<*mut T> {
+        self.is_extra().not().then(|| self.get_ptr_unchecked())
+    }
+
+    pub unsafe fn get_ptr_unchecked(&self) -> *mut T {
+        map_ptr(self.0, S::extract_ptr)
+    }
+
+    pub unsafe fn into_extra_unchecked(self) -> S::Extra {
+        let data = self.addr();
+        S::extract_extra(data)
+    }
+
+    pub unsafe fn get_extra_unchecked(&self) -> S::Extra {
+        let data = self.addr();
+        S::extract_extra(data)
+    }
+
+    pub unsafe fn get_extra(&self) -> Option<S::Extra> {
+        self.is_extra().then(|| self.get_extra_unchecked())
+    }
+
+    fn addr(&self) -> usize {
+        Strict::addr(self.0)
+    }
+
+    fn is_extra(&self) -> bool {
+        S::is_extra(self.addr())
+    }
+}
+
+impl<T, S> Debug for StuffedPtr<T, S>
+where
+    S: StuffingStrategy,
+    S::Extra: Debug,
+{
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        if self.is_extra() {
+            // SAFETY: We checked that self contains the extra
+            // Note: if S::Extra: !Copy, we can't just copy it out and call it a day
+            // For example, if it's a Box, not forgetting it here would lead to a double free
+            // So we just format it and forget it afterwards
+            let extra = unsafe { self.get_extra_unchecked() };
+            f.debug_struct("StuffedPtr::Extra")
+                .field("extra", &extra)
+                .finish()?;
+            std::mem::forget(extra);
+            Ok(())
+        } else {
+            let ptr = map_ptr(self.0, S::extract_ptr);
+            f.debug_struct("StuffedPtr::Ptr")
+                .field("ptr", &ptr)
+                .finish()
+        }
+    }
+}
+
+impl<T, S> Drop for StuffedPtr<T, S>
+where
+    S: StuffingStrategy,
+{
+    fn drop(&mut self) {
+        if self.is_extra() {
+            // SAFETY: We move it out here and it's never accessed again.
+            let extra = unsafe { self.get_extra_unchecked() };
+            drop(extra);
+        } else {
+            // dropping a ptr is a no-op
+        }
+    }
+}
+
+impl<T, S> From<Box<T>> for StuffedPtr<T, S>
+where
+    S: StuffingStrategy,
+{
+    fn from(boxed: Box<T>) -> Self {
+        Self::new_ptr(Box::into_raw(boxed))
+    }
+}
+
+pub unsafe trait StuffingStrategy {
+    type Extra;
+
+    fn is_extra(data: usize) -> bool;
+    fn stuff_extra(inner: Self::Extra) -> usize;
+    fn extract_extra(data: usize) -> Self::Extra;
+
+    fn stuff_ptr(inner: usize) -> usize {
+        inner
+    }
+    fn extract_ptr(inner: usize) -> usize {
+        inner
+    }
+}
+
+fn map_ptr<T>(ptr: *mut T, map: impl FnOnce(usize) -> usize) -> *mut T {
+    let int = Strict::addr(ptr);
+    let result = map(int);
+    Strict::with_addr(ptr, result)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::strategies::test_strategies::HasDebug;
+    use crate::StuffedPtr;
+
+    #[test]
+    fn set_get_ptr_no_extra() {
+        unsafe {
+            let boxed = Box::new(1);
+            let stuffed_ptr: StuffedPtr<i32, ()> = boxed.into();
+            let ptr = stuffed_ptr.get_ptr_unchecked();
+            let boxed = Box::from_raw(ptr);
+            assert_eq!(*boxed, 1);
+        }
+    }
+
+    #[test]
+    fn debug() {
+        let boxed = Box::new(1);
+        let stuffed_ptr: StuffedPtr<i32, HasDebug> = boxed.into();
+        assert!(format!("{stuffed_ptr:?}").starts_with("StuffedPtr::Ptr {"));
+
+        drop(unsafe { Box::from_raw(stuffed_ptr.get_ptr().unwrap()) });
+
+        let extra = HasDebug;
+        let stuffed_ptr: StuffedPtr<i32, HasDebug> = StuffedPtr::new_extra(extra);
+        println!("{:?} {:X}", stuffed_ptr.0, usize::MAX);
+        assert_eq!(
+            format!("{stuffed_ptr:?}"),
+            "StuffedPtr::Extra { extra: hello! }"
+        );
     }
 }

src/strategies.rs

@@ -0,0 +1,47 @@
+use crate::StuffingStrategy;
+
+unsafe impl StuffingStrategy for () {
+    type Extra = ();
+
+    fn is_extra(_data: usize) -> bool {
+        false
+    }
+
+    fn stuff_extra(_inner: Self::Extra) -> usize {
+        0
+    }
+
+    fn extract_extra(_data: usize) -> Self::Extra {
+        ()
+    }
+}
+
+#[cfg(test)]
+pub mod test_strategies {
+    use crate::StuffingStrategy;
+    use std::fmt::{Debug, Formatter};
+
+    pub struct HasDebug;
+
+    impl Debug for HasDebug {
+        fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+            f.write_str("hello!")
+        }
+    }
+
+    unsafe impl StuffingStrategy for HasDebug {
+        type Extra = Self;
+
+        fn is_extra(data: usize) -> bool {
+            data == usize::MAX
+        }
+
+        fn stuff_extra(_inner: Self::Extra) -> usize {
+            usize::MAX
+        }
+
+        fn extract_extra(_data: usize) -> Self::Extra {
+            Self
+        }
+    }
+}