Function lower_body 

pub(super) fn lower_body(
    db: &dyn SourceDatabase,
    owner: DefWithBodyId,
    syntax_node: SyntaxNodePtr,
    current_file_id: HirFileId,
    module: ModuleId,
    parameters: Option<ParamList>,
    body: Option<Expr>,
    is_async_fn: bool,
    is_gen_fn: bool,
) -> (Body, BodySourceMap)