Assertion failed when using QueryCursor::matches

[serbanrobu]

Assertion failed when using QueryCursor::matches

[WillLillis]

This is really interesting! I took a look at this with a debugger and I think I found the problem. Namely, this line:

let mut query_matches = QueryCursor::new().matches(&query, tree.root_node(), text.as_bytes());

With the code written this way, drop() is called for the QueryCursor object. The drop implementation calls ts_query_cursor_delete() via ffi, which clears the contents of the array. Then, when you call query_matches.next();, the underlying cursor object has been deallocated, leading to the assertion triggering due to an array with zero length:

/// Get a pointer to the element at a given `index` in the array.
#define array_get(self, _index) \
  (assert((uint32_t)(_index) < (self)->size), &(self)->contents[_index])

Splitting the cursor's creation and the .matches() call into separate lines, or just referencing the cursor later on to keep the object in scope for the matches() call can work around the problem. For a more permanent fix, adding a lifetime annotation for the cursor in matches()'s definition appears to fix things:

#[doc(alias = "ts_query_cursor_exec")]
pub fn matches<'query, 'cursor: 'query, 'tree, T: TextProvider<I>, I: AsRef<[u8]>>(
    &'cursor mut self,
    query: &'query Query,
    node: Node<'tree>,
    text_provider: T,
) -> QueryMatches<'query, 'tree, T, I> {
    let ptr = self.ptr.as_ptr();
    unsafe { ffi::ts_query_cursor_exec(ptr, query.ptr.as_ptr(), node.0) };
    QueryMatches {
        ptr,
        query,
        text_provider,
        buffer1: Vec::default(),
        buffer2: Vec::default(),
        _phantom: PhantomData,
    }
}

Attempting to build your file with the above changes to tree-sitter yields the following error, which seems to correctly describe the problem:

error[E0716]: temporary value dropped while borrowed
  --> src/main.rs:10:29
   |
10 |     let mut query_matches = QueryCursor::new().matches(&query, tree.root_node(), text.as_bytes());
   |                             ^^^^^^^^^^^^^^^^^^                                                   - temporary value is freed at the end of this statement
   |                             |
   |                             creates a temporary value which is freed while still in use
...
13 |     query_matches.next();
   |     ------------- borrow later used here
   |
help: consider using a `let` binding to create a longer lived value
   |
10 ~     let binding = QueryCursor::new();
11 ~     let mut query_matches = binding.matches(&query, tree.root_node(), text.as_bytes());
   |

For more information about this error, try `rustc --explain E0716`.
error: could not compile `ts_test` (bin "ts_test") due to 1 previous error

I'll open up a PR with the fix :)

[serbanrobu]

Wow! I didn't think of that and this isn't the first time I've used Tree-sitter, but probably the first time I've put QueryCursor::new().matches on the same line.

Thank you!