Linter Rule: Disallow `local_assigns` reads that the strict locals declaration already answers

Rule: actionview-no-redundant-local-assigns

Description

Detects local_assigns lookups in a partial that already has a <%# locals: (...) %> declaration, where the declaration makes the lookup either redundant or dead.

Rationale

local_assigns is the documented way to read locals in a partial that has no strict locals declaration, and it stays useful in a partial that has one. Once a declaration exists, though, some of those lookups are answered by the declaration itself and only obscure what the template does.

A required local is already a local variable, so reading it back out of the hash is a longer way to write the name. A required local is also always present, so asking local_assigns.key? about it is a condition that can only take one branch. And a name the declaration does not mention can never arrive, because Rails raises ActionView::StrictLocalsError for callers that pass an undeclared local, so a lookup for it is dead code.

Optional locals are left alone. local_assigns.key?(:size) is the only way to tell "not passed" apart from "passed as nil", which a default value cannot express, so that check is a legitimate pattern rather than an offense. Partials with a ** keyword rest in the declaration are skipped as well, since undeclared locals can legitimately arrive there.

Partials without a strict locals declaration are not checked at all.

Examples

✅ Good

erb

<%# locals: (user:) %>

<%= user.name %>

erb

<%# locals: (user:, size: nil) %>

<%= user.name %>
<% if local_assigns.key?(:size) %>
  <span><%= size %></span>
<% end %>

erb

<%# locals: (user:, **) %>

<%= render "row", **local_assigns %>

🚫 Bad

erb

<%# locals: (user:) %>

<%= local_assigns[:user].name %>Strict local `user` is already a local variable in this partial, so `local_assigns[:user]` reads back a value that is already in scope. Use `user` instead. (actionview-no-redundant-local-assigns)

erb

<%# locals: (user:) %>

<% if local_assigns.key?(:user) %>Strict local `user` is required, so `local_assigns.key?(:user)` is always `true`. Remove the condition, or give `user` a default value to make it optional. (actionview-no-redundant-local-assigns)
  <%= user.name %>
<% end %>

erb

<%# locals: (user:) %>

<%= user.name %>
<%= local_assigns.fetch(:size, "large") %>`size` is not declared in the `locals:` declaration, so Rails raises if a caller passes it and `local_assigns.fetch(:size, ...)` can never find it. Declare `size:` in the declaration, or remove the lookup. (actionview-no-redundant-local-assigns)

Linter Rule: Disallow local_assigns reads that the strict locals declaration already answers ​

Description ​

Rationale ​

Examples ​

✅ Good ​

🚫 Bad ​

References ​