You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: item_tree.rs
    path of file: ./repos/rust-analyzer/crates/hir-def/src
    
    the code of the file until where you have to start completion: //! A simplified AST that only contains items.
//!
//! This is the primary IR used throughout `hir_def`. It is the input to the name resolution
//! algorithm, as well as to the queries defined in `adt.rs`, `data.rs`, and most things in
//! `attr.
    
    pub(crate) fn block_item_tree_query(db: &dyn DefDatabase, block: BlockId) -> Arc<ItemTree> {
        let loc = block.lookup(db);
        let block = loc.ast_id.to_node(db.upcast());

        let ctx = lower::Ctx::new(db, loc.ast_id.file_id);
        let mut item_tree = ctx.lower_block(&block);
        item_tree.shrink_to_fit();
        Arc::new(item_tree)
    }