fix UB =)

src/lib.rs

@@ -25,8 +25,16 @@ pub struct Dyn<'a, Trait: ?Sized + 'a> {
 impl<'a, Trait: ?Sized + 'a> Dyn<'a, Trait> {
 	pub type Target = Trait;
 
-	fn as_ptr(&self) -> *const Self::Target {
-		ptr::from_raw_parts::<Trait>(&self.data, self.meta)
+	/// SAFETY:
+	/// The `meta` field in `this` must be valid for reading.
+	///
+	/// The returned pointer inherits validity from the input pointer.
+	unsafe fn ptr(this: *mut Self) -> *mut Self::Target {
+		let pmeta = &raw const (*this).meta;
+		// SAFETY: `meta` is valid for reading, per the function safety requirement.
+		let meta = unsafe { *pmeta };
+		let pdata = &raw mut (*this).data;
+		ptr::from_raw_parts_mut::<Trait>(pdata, meta)
 	}
 }
 
@@ -34,13 +42,15 @@ impl<'a, Trait: ?Sized + 'a> ops::Deref for Dyn<'a, Trait> {
 	type Target = Trait;
 
 	fn deref(&self) -> &Self::Target {
-		unsafe { self.as_ptr().as_ref_unchecked() }
+		// SAFETY: `self` is valid for reading.
+		unsafe { Self::ptr(self as *const Self as *mut Self).as_ref_unchecked() }
 	}
 }
 
 impl<'a, Trait: ?Sized + 'a> ops::DerefMut for Dyn<'a, Trait> {
 	fn deref_mut(&mut self) -> &mut Self::Target {
-		unsafe { self.as_ptr().cast_mut().as_mut_unchecked() }
+		// SAFETY: `self` is valid for writing.
+		unsafe { Self::ptr(self as *mut Self).as_mut_unchecked() }
 	}
 }