Linter Rule: Disallow inline case conditions

Rule: erb-no-inline-case-conditions

Description

Disallow placing case and its first when/in condition in the same ERB tag. When a case statement and its condition appear in a single ERB tag (e.g., <% case x when y %>), the parser cannot reliably process, compile, or format the template. This rule flags such patterns and guides users toward separate ERB tags.

Rationale

ERB templates that combine case with a when or in condition in a single tag create parsing ambiguity. The parser creates synthetic condition nodes to handle this pattern in non-strict mode, but the resulting AST cannot be reliably formatted or compiled.

Using separate ERB tags for case and its conditions:

Makes the template structure unambiguous for the parser
Enables proper formatting and compilation
Improves readability by clearly separating the case expression from its branches
Follows the conventional ERB style used across the Ruby on Rails ecosystem

Examples

✅ Good

case/when in separate ERB tags:

erb

<% case variable %>
<% when "a" %>
  A
<% when "b" %>
  B
<% else %>
  Other
<% end %>

case/in (pattern matching) in separate ERB tags:

erb

<% case value %>
<% in 1 %>
  One
<% in 2 %>
  Two
<% else %>
  Other
<% end %>

🚫 Bad

Inline case/when in a single ERB tag:

erb

<% case variable when "a" %>A `case` statement with `when` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% when y %>`). (erb-no-inline-case-conditions)
A `case` statement with `when`/`in` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% when y %>`). (`ERB_CASE_WITH_CONDITIONS_ERROR`) (parser-no-errors)
  A
<% when "b" %>
  B
<% end %>

Inline case/in in a single ERB tag:

erb

<% case value in 1 %>A `case` statement with `in` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% in y %>`). (erb-no-inline-case-conditions)
A `case` statement with `when`/`in` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% when y %>`). (`ERB_CASE_WITH_CONDITIONS_ERROR`) (parser-no-errors)
  One
<% in 2 %>
  Two
<% end %>

case/when on separate lines but still in the same ERB tag:

erb

<% case variableA `case` statement with `when` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% when y %>`). (erb-no-inline-case-conditions)
A `case` statement with `when`/`in` conditions in a single ERB tag cannot be reliably parsed, compiled, and formatted. Use separate ERB tags for `case` and its conditions (e.g., `<% case x %>` followed by `<% when y %>`). (`ERB_CASE_WITH_CONDITIONS_ERROR`) (parser-no-errors)
   when "a" %>
  A
<% when "b" %>
  B
<% end %>

Linter Rule: Disallow inline case conditions ​

Description ​

Rationale ​

Examples ​

✅ Good ​

🚫 Bad ​

References ​