Fill in match arms helper damages the structure when nested matches are present

before adding missing arms

        match &msg {
            Event::OmsMsg(OmsMsg { severity, message }) => match severity {
                Severity::Info => info!(%message),
                Severity::Warning => warn!(%message),
                Severity::Error => error!(%message),
                Severity::Success | Severity::Silent => {}
            },
            Event::OmsOrderMsg(OmsOrderMsg {
                severity,
                message,
                order_id: oid,
                request_id: _,
            }) => match severity {
                Severity::Info => info!(?oid, %message),
                Severity::Warning => warn!(?oid, %message),
                Severity::Error => error!(?oid, %message),
                Severity::Success | Severity::Silent => {}
            },
            Event::ExchangeAck(ack) => trace!(?ack),
            Event::Exec(exec) => trace!(?exec),
            Event::TimerExpired(tid) => {
                trace!(tid = tid.get(), "Timer expired")
            }
        }

after adding missing arms

        match &msg {Event::OmsMsg(OmsMsg{severity,message})=>matchseverity{Severity::Info=>info!(%message),Severity::Warning=>warn!(%message),Severity::Error=>error!(%message),Severity::Success|Severity::Silent=>{}},Event::OmsOrderMsg(OmsOrderMsg{severity,message,order_id:oid,request_id:_,})=>matchseverity{Severity::Info=>info!(?oid, %message),Severity::Warning=>warn!(?oid, %message),Severity::Error=>error!(?oid, %message),Severity::Success|Severity::Silent=>{}},Event::ExchangeAck(ack)=>trace!(?ack),Event::Exec(exec)=>trace!(?exec),Event::TimerExpired(tid)=>{trace!(tid=tid.get(),"Timer expired")}

note matchseverity in two places. With spaces added everything works as expected.

I wasn't able to construct a simple standalone test case but saw this problem multiple times in different places.

The issue names the fill-in-match-arms helper but no file or test; start by reproducing the nested-match example and tracing how the resulting structure is assembled. Done means nested matches retain their spacing and structure, without tokens such as matchseverity, with a regression test covering the case.