Issue #15538 has been reported by kke (Kimmo Lehto).

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Feature #15538: Erb indenting / unindenting trim mode
https://bugs.ruby-lang.org/issues/15538

* Author: kke (Kimmo Lehto)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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In `Erb`, would it be possible to add a new trim mode that would indent the following content to match the depth of the tag? The tag could for example be `<%|` and it would be enabled using `Erb.new(template, trim_mode: '|')`

## Reason

Something like this looks nice:

``` ruby
- horse
<% ['cat', 'dog', 'goldfish'].each do |pet| %>
  <% unless pet == 'goldfish' %>
    - <%= pet %>
  <% end %>
<% end %>
```

But unfortunately it will render with "extra" indentation:

```
- horse
   - cat
   - dog
```

With the `|`  trim mode it could be written as:

```ruby
- horse
<%| ['cat', 'dog', 'goldfish'].each do |pet| %>
....
<% end %>
```

And it would output as desired without the "extra" indentation. Currently, to produce such output, you have write your erb template without indentation or using a jumpy mixed indentation (code and content have different indentation levels but are interleaved) and it's not a very user/editor friendly way to work.

## Using with `=`

It would also be handy if the `|` could be used with `<%=`, perhaps `<%|=`, this would be excellent for example when templating YAML's:

```yaml
<% bar_content = "- something\n- something else" %>
foo:
  bar: 
    <%|= bar_content %>
```

Which would produce something like:

```yaml
foo:
  bar:
    - something
    - something else
```

Instead of the invalid:

```yaml
foo:
  bar:
    - something
- something else
```

Currently there's no way to accomplish this without manually counting the leading spaces, as it's not possible to detect the column where `<%=` appeared.



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