Linter Rule: Disallow unused expressions in silent ERB tags

Rule: erb-no-unused-expressions

Description

Disallow expressions inside silent ERB tags (<% %>) whose return values are discarded. Writing expressions like <% @user.name %> or <% helper_method(arg) %> evaluates the expression but discards the result, making the line functionally useless.

This rule detects and warns about silent ERB tags containing:

• Method calls without blocks
• Instance variable reads (@user)
• Class variable reads (@@count)
• Global variable reads ($global)
• Constant reads (CONSTANT, Foo::Bar)

Rationale

These expressions are evaluated but unused, they don't produce output (not <%= ... %>), and they don't perform meaningful side effects (no assignments, no blocks, no control flow). They're likely a mistake where the developer meant to use an output tag (<%= ... %>) instead, or they're leftover from refactoring.

Examples

✅ Good

<%= @user.name %>

<%= helper_method(arg) %>

<% @name = @user.name %>

<% if logged_in? %>
  <p>Welcome!</p>
<% end %>

<% @users.each do |user| %>
  <%= user.name %>
<% end %>

ViewComponent slot setters are intentional side effects and are not flagged, including on builder objects yielded by nested slots:

<%= render ListComponent.new do |list| %>
  <% list.with_item do |item| %>
    <% item.with_icon("home") %>
  <% end %>
<% end %>

🚫 Bad

<% @user.name %>
Avoid unused expressions in silent ERB tags. `<% @user.name %>` is evaluated but its return value is discarded. Use `<%= @user.name %>` to output the value or remove the expression. (erb-no-unused-expressions)

<% helper_method(arg) %>
Avoid unused expressions in silent ERB tags. `<% helper_method(arg) %>` is evaluated but its return value is discarded. Use `<%= helper_method(arg) %>` to output the value or remove the expression. (erb-no-unused-expressions)

<% foo.bar.baz %>
Avoid unused expressions in silent ERB tags. `<% foo.bar.baz %>` is evaluated but its return value is discarded. Use `<%= foo.bar.baz %>` to output the value or remove the expression. (erb-no-unused-expressions)

<% @user %>
Avoid unused expressions in silent ERB tags. `<% @user %>` is evaluated but its return value is discarded. Use `<%= @user %>` to output the value or remove the expression. (erb-no-unused-expressions)

<% CONSTANT %>
Avoid unused expressions in silent ERB tags. `<% CONSTANT %>` is evaluated but its return value is discarded. Use `<%= CONSTANT %>` to output the value or remove the expression. (erb-no-unused-expressions)

<% User.count %>
Avoid unused expressions in silent ERB tags. `<% User.count %>` is evaluated but its return value is discarded. Use `<%= User.count %>` to output the value or remove the expression. (erb-no-unused-expressions)