Make VTab / VTabCursor unsafe trait as implmenting them on the wrong type is unsound

src/vtab/array.rs

@@ -71,7 +71,7 @@ struct ArrayTab {
     base: ffi::sqlite3_vtab,
 }
 
-impl VTab for ArrayTab {
+unsafe impl VTab for ArrayTab {
     type Aux = ();
     type Cursor = ArrayTabCursor;
 
@@ -149,7 +149,7 @@ impl ArrayTabCursor {
         }
     }
 }
-impl VTabCursor for ArrayTabCursor {
+unsafe impl VTabCursor for ArrayTabCursor {
     fn filter(&mut self, idx_num: c_int, _idx_str: Option<&str>, args: &Values<'_>) -> Result<()> {
         if idx_num > 0 {
             self.ptr = args.get_array(0)?;

src/vtab/csvtab.rs

@@ -95,7 +95,7 @@ impl CSVTab {
     }
 }
 
-impl VTab for CSVTab {
+unsafe impl VTab for CSVTab {
     type Aux = ();
     type Cursor = CSVTabCursor;
 
@@ -296,7 +296,7 @@ impl CSVTabCursor {
     }
 }
 
-impl VTabCursor for CSVTabCursor {
+unsafe impl VTabCursor for CSVTabCursor {
     // Only a full table scan is supported.  So `filter` simply rewinds to
     // the beginning.
     fn filter(

src/vtab/mod.rs

@@ -189,7 +189,11 @@ impl VTabConnection {
 
 /// `feature = "vtab"` Virtual table instance trait.
 ///
-/// Implementations must be like:
+/// # Safety
+///
+/// The first item in a struct implementing VTab must be
+/// `rusqlite::sqlite3_vtab`, and the struct must be `#[repr(C)]`.
+///
 /// ```rust,ignore
 /// #[repr(C)]
 /// struct MyTab {
@@ -200,7 +204,7 @@ impl VTabConnection {
 /// ```
 ///
 /// (See [SQLite doc](https://sqlite.org/c3ref/vtab.html))
-pub trait VTab: Sized {
+pub unsafe trait VTab: Sized {
     type Aux;
     type Cursor: VTabCursor;
 
@@ -465,7 +469,7 @@ impl OrderBy<'_> {
 /// ```
 ///
 /// (See [SQLite doc](https://sqlite.org/c3ref/vtab_cursor.html))
-pub trait VTabCursor: Sized {
+pub unsafe trait VTabCursor: Sized {
     /// Begin a search of a virtual table.
     /// (See [SQLite doc](https://sqlite.org/vtab.html#the_xfilter_method))
     fn filter(&mut self, idx_num: c_int, idx_str: Option<&str>, args: &Values<'_>) -> Result<()>;

src/vtab/series.rs

@@ -49,7 +49,7 @@ struct SeriesTab {
     base: ffi::sqlite3_vtab,
 }
 
-impl VTab for SeriesTab {
+unsafe impl VTab for SeriesTab {
     type Aux = ();
     type Cursor = SeriesTabCursor;
 
@@ -181,7 +181,7 @@ impl SeriesTabCursor {
         SeriesTabCursor::default()
     }
 }
-impl VTabCursor for SeriesTabCursor {
+unsafe impl VTabCursor for SeriesTabCursor {
     fn filter(&mut self, idx_num: c_int, _idx_str: Option<&str>, args: &Values<'_>) -> Result<()> {
         let idx_num = QueryPlanFlags::from_bits_truncate(idx_num);
         let mut i = 0;