Function lower_body_to_mir 

pub fn lower_body_to_mir<'db>(
    db: &'db (dyn HirDatabase + 'static),
    owner: InferBodyId<'db>,
    store: &ExpressionStore,
    infer: &InferenceResult<'db>,
    root_expr: Idx<Expr>,
    self_param: Option<Idx<Binding>>,
    params: &[Param<Idx<Pat>>],
) -> Result<MirBody<'db>, MirLowerError<'db>>

Extracts params from body.params/body.self_param and the callable signature, then delegates to lower_to_mir_with_store.