Rollup merge of #78255 - dtolnay:match, r=lcnr

Rollup merge of #78255 - dtolnay:match, r=lcnr

Reduce diagram mess in 'match arms have incompatible types' error

I noticed this wild diagram in https://twitter.com/a_hoverbear/status/1318960787105353728 which I think does not benefit from the big outer vertical span.

This PR shrinks the outer span to cover just the `match` keyword and scrutinee expression *if* at least one of the highlighted match arms involved in the error is multiline.

**Before:**

<pre>
<b>error[E0308]: `match` arms have incompatible types</b>
   <b>--&gt;</b> src/topology/builder.rs:141:35
    <b>|</b>
<b>120 |</b>             let transform = match transform {
    <b>|    _________________________-</b>
<b>121 |   |</b>             Transform::Function(t) =&gt; {
    <b>|  _|_______________________________________-</b>
<b>122 | | |</b>                 filter_event_type(input_rx, input_type).compat().flat_map(|v| {
<b>123 | | |</b>                     futures::stream::iter(match v {
<b>124 | | |</b>                         Err(e) =&gt; {
<b>...   | |</b>
<b>139 | | |</b>                 .compat();
<b>140 | | |</b>             }
    <b>| |_|_____________- this is found to be of type `()`</b>
<b>141 |   |</b>             Transform::Task(t) =&gt; t
    <b>|  _|___________________________________^</b>
<b>142 | | |</b>                 .transform(filter_event_type(input_rx, input_type))
<b>143 | | |</b>                 .forward(output)
<b>144 | | |</b>                 .map(|_| debug!("Finished"))
<b>145 | | |</b>                 .compat(),
    <b>| |_|_________________________^ expected `()`, found struct `futures::compat::Compat01As03`</b>
<b>146 |   |</b>         };
    <b>|   |_________- `match` arms have incompatible types</b>
    <b>|</b>
    <b>= note:</b> expected type `<b>()</b>`
             found struct `<b>futures::compat::Compat01As03&lt;futures::Map&lt;futures::stream::Forward&lt;std::boxed::Box&lt;dyn futures::Stream&lt;Error = (), Item = event::Event&gt; + std::marker::Send&gt;, topology::fanout::Fanout&gt;, [closure@src/topology/builder.rs:144:22: 144:44]&gt;&gt;</b>`
</pre>

**After:**

<pre>
<b>error[E0308]: `match` arms have incompatible types</b>
   <b>--&gt;</b> src/topology/builder.rs:141:35
    <b>|</b>
<b>120 |</b>             let transform = match transform {
    <b>|                             --------------- `match` arms have incompatible types</b>
<b>121 |</b>                 Transform::Function(t) =&gt; {
    <b>|  _________________________________________-</b>
<b>122 | |</b>                   filter_event_type(input_rx, input_type).compat().flat_map(|v| {
<b>123 | |</b>                       futures::stream::iter(match v {
<b>124 | |</b>                           Err(e) =&gt; {
<b>...   |</b>
<b>139 | |</b>                   .compat();
<b>140 | |</b>               }
    <b>| |_______________- this is found to be of type `()`</b>
<b>141 |</b>                 Transform::Task(t) =&gt; t
    <b>|  _____________________________________^</b>
<b>142 | |</b>                   .transform(filter_event_type(input_rx, input_type))
<b>143 | |</b>                   .forward(output)
<b>144 | |</b>                   .map(|_| debug!("Finished"))
<b>145 | |</b>                   .compat(),
    <b>| |___________________________^ expected `()`, found struct `futures::compat::Compat01As03`</b>
    <b>|</b>
    <b>= note:</b> expected type `<b>()</b>`
             found struct `<b>futures::compat::Compat01As03&lt;futures::Map&lt;futures::stream::Forward&lt;std::boxed::Box&lt;dyn futures::Stream&lt;Error = (), Item = event::Event&gt; + std::marker::Send&gt;, topology::fanout::Fanout&gt;, [closure@src/topology/builder.rs:144:22: 144:44]&gt;&gt;</b>`
</pre>

FYI @Hoverbear