Fix provenance UB and alignment UB

A &Header cannot be used to get a useful pointer to data beyond it,
because the provenance of the pointer from the as-cast of the &Header
only has provenance over the Header.

After a set_len call that shrinks the slice, it is invalid to create a
slice then try to get_unchecked into the region between the old and new
length, because the reference in the slice that the ThinVec now Derefs
to does not have provenance over that region.

Also misaligned data pointers were produce when the gecko-ffi feature
was enabled and T has an alignment greater than 4.

And the use of align_offset in tests is subtly wrong, align_offset is
for optimizations only. The docs say that a valid implementation
may always return usize::MAX.

src/lib.rs

@@ -264,23 +264,6 @@ impl Header {
     fn set_len(&mut self, len: usize) {
         self._len = assert_size(len);
     }
-
-    fn data<T>(&self) -> *mut T {
-        let header_size = mem::size_of::<Header>();
-        let padding = padding::<T>();
-
-        let ptr = self as *const Header as *mut Header as *mut u8;
-
-        unsafe {
-            if padding > 0 && self.cap() == 0 {
-                // The empty header isn't well-aligned, just make an aligned one up
-                NonNull::dangling().as_ptr()
-            } else {
-                // This could technically result in overflow, but padding would have to be absurdly large for this to occur.
-                ptr.add(header_size + padding) as *mut T
-            }
-        }
-    }
 }
 
 #[cfg(feature = "gecko-ffi")]
@@ -321,10 +304,10 @@ impl Header {
 /// optimize everything to not do that (basically, make ptr == len and branch
 /// on size == 0 in every method), but it's a bunch of work for something that
 /// doesn't matter much.
-#[cfg(any(not(feature = "gecko-ffi"), test))]
+#[cfg(any(not(feature = "gecko-ffi"), test, miri))]
 static EMPTY_HEADER: Header = Header { _len: 0, _cap: 0 };
 
-#[cfg(all(feature = "gecko-ffi", not(test)))]
+#[cfg(all(feature = "gecko-ffi", not(test), not(miri)))]
 extern "C" {
     #[link_name = "sEmptyTArrayHeader"]
     static EMPTY_HEADER: Header;
@@ -470,7 +453,18 @@ impl<T> ThinVec<T> {
         unsafe { self.ptr.as_ref() }
     }
     fn data_raw(&self) -> *mut T {
-        self.header().data()
+        unsafe {
+            if self.header().cap() == 0 {
+                // The empty header isn't well-aligned, just make an aligned one up
+                NonNull::dangling().as_ptr()
+            } else {
+                // This could technically result in overflow, but padding would have to be absurdly large for this to occur.
+                let padding = padding::<T>();
+                let header_size = mem::size_of::<Header>();
+                let ptr = self.ptr.as_ptr() as *mut u8;
+                ptr.add(header_size + padding) as *mut T
+            }
+        }
     }
 
     // This is unsafe when the header is EMPTY_HEADER.
@@ -565,7 +559,7 @@ impl<T> ThinVec<T> {
                 // doesn't re-drop the just-failed value.
                 let new_len = self.len() - 1;
                 self.set_len(new_len);
-                ptr::drop_in_place(self.get_unchecked_mut(new_len));
+                ptr::drop_in_place(self.data_raw().add(new_len));
             }
         }
     }
@@ -915,7 +909,7 @@ impl<T> ThinVec<T> {
             (*ptr).set_cap(new_cap);
             self.ptr = NonNull::new_unchecked(ptr);
         } else {
-            let mut new_header = header_with_capacity::<T>(new_cap);
+            let new_header = header_with_capacity::<T>(new_cap);
 
             // If we get here and have a non-zero len, then we must be handling
             // a gecko auto array, and we have items in a stack buffer. We shouldn't
@@ -931,8 +925,9 @@ impl<T> ThinVec<T> {
             let len = self.len();
             if cfg!(feature = "gecko-ffi") && len > 0 {
                 new_header
-                    .as_mut()
-                    .data::<T>()
+                    .as_ptr()
+                    .add(1)
+                    .cast::<T>()
                     .copy_from_nonoverlapping(self.data_raw(), len);
                 self.set_len(0);
             }
@@ -2654,9 +2649,9 @@ mod std_tests {
         macro_rules! assert_aligned_head_ptr {
             ($typename:ty) => {{
                 let v: ThinVec<$typename> = ThinVec::with_capacity(1 /* ensure allocation */);
-                let head_ptr: *mut $typename = v.header().data::<$typename>();
+                let head_ptr: *mut $typename = v.data_raw();
                 assert_eq!(
-                    head_ptr.align_offset(std::mem::align_of::<$typename>()),
+                    head_ptr as usize % std::mem::align_of::<$typename>(),
                     0,
                     "expected Header::data<{}> to be aligned",
                     stringify!($typename)