Linter Rule: Disallow mistyped literals for strict locals

Rule: actionview-no-mistyped-locals

Description

Flags a literal argument in a render call whose type contradicts the strict local's declared default type in the partial, like passing "yes" where the partial declares open: false.

Rationale

A strict local's default is the one piece of type information a partial publishes, and its body is written against it. A partial comparing size == :lg never matches when the caller passed "lg", and a partial treating open as a flag renders the truthy arm for any non-empty String. Rails cannot catch this, since locals are untyped, so the mistake surfaces as wrong rendering rather than an error.

Only literals are checked. A method call or variable argument cannot be typed statically, so this is defense in depth, not a guarantee. nil is allowed against any declared type, and a local declared without a default or with a nil default accepts anything.

The check also guards state seeding. A herb:state default naming a strict local takes its type from that local's default, so a mistyped call site would seed the wrong type into client state.

Examples

Given app/views/application/_menu.html.erb declaring:

<%# locals: (open: false, size: "md") %>

<div class="menu <%= size %>">
  <% if open %>Open<% end %>
</div>

✅ Good

<%= render "application/menu", open: true, size: "lg" %>

<%= render "application/menu", open: menu_open? %>

<%= render "application/menu", size: nil %>

🚫 Bad

<%= render "application/menu", open: "yes" %>

<%= render "application/menu", size: :lg %>

References

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