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

[dtolnay]

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:

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

After:

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

FYI @Hoverbear

[rust-highfive]

r? @lcnr

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[Hoverbear]

This is a nice improvment @dtolnay !

[lcnr]

lgtm 👍

@bors r+ rollup

[bors]

📌 Commit b005950 has been approved by lcnr

[bors]

🌲 The tree is currently closed for pull requests below priority 1000, this pull request will be tested once the tree is reopened