Linter Rule: Enforce consistent right-trimming syntax

Rule: erb-right-trim

Description

This rule enforces the use of -%> for right-trimming ERB output tags (like <%= %>) instead of =%>. It also warns when right-trimming syntax (-%> or =%>) is used on non-output ERB tags (like <% %>, <% if %>, etc.) where it has no effect.

Rationale

While =%> can be used for right-trimming whitespace in some ERB engines (like Erubi), it is an obscure and not well-defined syntax that lacks consistent support across most ERB implementations. The -%> syntax is the standard, well-documented approach for right-trimming that is universally supported and consistent with left-trimming syntax (<%-).

Additionally, right-trimming syntax only has an effect on ERB output tags (<%= and <%==). Using -%> or =%> on non-output ERB tags (control flow like <% if %>, <% each %>, etc.) has no effect and is misleading.

Using -%> for output tags ensures compatibility across different ERB engines, improves code clarity, and aligns with established Rails and ERB conventions.

Examples

✅ Good

<%= title -%>

<% if true %>
  <h1>Content</h1>
<% end %>

<% items.each do |item| %>
  <li><%= item -%></li>
<% end %>

🚫 Bad

<%= title =%>
Use `-%>` instead of `=%>` for right-trimming. The `=%>` syntax is obscure and not well-supported in most ERB engines (erb-right-trim)


<% title =%>
Right-trimming with `=%>` has no effect on non-output ERB tags. Use `%>` instead (erb-right-trim)


<% title -%>
Right-trimming with `-%>` has no effect on non-output ERB tags. Use `%>` instead (erb-right-trim)


<% if true -%>
Right-trimming with `-%>` has no effect on non-output ERB tags. Use `%>` instead (erb-right-trim)
  <h1>Content</h1>
<% end %>


<% items.each do |item| =%>
Right-trimming with `=%>` has no effect on non-output ERB tags. Use `%>` instead (erb-right-trim)
  <li><%= item %></li>
<% end %>

References

• Inspiration: ERB Lint RightTrim rule