Function lower_to_mir 

pub fn lower_to_mir<'db>(
    db: &'db (dyn HirDatabase + 'static),
    owner: DefWithBodyId,
    body: &Body,
    infer: &InferenceResult<'db>,
    root_expr: Idx<Expr>,
) -> Result<MirBody<'db>, MirLowerError<'db>>