[ruby-core:91492] [Ruby trunk Feature#15538] Erb indenting / unindenting trim mode

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

Description updated

Yes, it appears I made an indentation error in my foo-bar examples. The indentation of `bar` was supposed to be as deep as on `foo`.

I added a tag summary.


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Feature #15538: Erb indenting / unindenting trim mode
https://bugs.ruby-lang.org/issues/15538#change-76753

* Author: kke (Kimmo Lehto)
* Status: Feedback
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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 would be easy to follow:

``` ruby
1
<%- [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
    <%= num %>
  <%- end -%>
<%- end -%>
5
```

But unfortunately it will render with "extra" indentation:

```
1
    2
    4
5
```

Currently, to avoid this, you have to write your template using either no indentation:

```
1
<%- [2, 3, 4].each do |num| -%>
<%- unless num == 3 -%>
<%= num %>
<%- end -%>
<%- end -%>
5
```

Or a weird jumpy indentation:

```
1
<%- [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
<%= num %>
  <%- end -%>
<%- end -%>
5
```

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

```ruby
1
<%|- [2, 3, 4].each do |num| -%>
  <%- unless num == 3 -%>
    <%= num %>
  <%- end -%>
<%|- end -%>
5
```

And it would output as desired without the "extra" indentation:

```
1
2
4
5
```

## 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 = "- abc\n- " -def" -%>
foo:
  bar: 
    <%|= bar_content |%>
```

Which would produce something like:

```yaml
foo:
  bar:
    - abc
    - def
```

This would require these new tags:

1. `<%|` begin a capturing a block which will be reindented to the column of the `<` character
2. `|%>` end the capturing block
3. `<%|=` like regular `<%=` but multiline strings will be reindented to the column of the `<` character

Optionally versions of the block tags with whitespace trim control:

4. `<%-|` like `<%|` but if block outputs nothing, remove leading whitespace
5. `|-%>`like `|%>` but remove trailing whitespace and linefeed from the line this tag appears on




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