`erb-no-unused-expressions`: false positive on ViewComponent slot setters called on a nested slot's yielded builder

[andrewmcodes]

Summary

erb-no-unused-expressions already knows that ViewComponent slot setters are intentional side-effecting calls: when the receiver is the object yielded by a render(...) do |component| block, a block-less <% component.with_foo(...) %> is correctly not flagged.

The exemption is too narrow. It's scoped to the receiver yielded by the render block, so it doesn't apply to slot setters called on a builder object yielded by a nested slot. ViewComponent slots can themselves yield a builder (via renders_one/renders_many with a block), and you populate that builder's slots the same way — <% item.with_icon(...) %>. Those nested setters get flagged as unused expressions even though they're identical, valid usage.

Rule: erb-no-unused-expressions

Minimal reproduction

Components (sketch):

class ListComponent < ViewComponent::Base
  renders_many :items, ItemComponent   # `with_item` yields an ItemComponent
end

class ItemComponent < ViewComponent::Base
  renders_one :icon, IconComponent     # `with_icon` sets a slot, no block needed
end

Template:

<%= render ListComponent.new do |list| %>
  <% list.with_item do |item| %>
    <% item.with_icon("home") %>   <%# ← flagged, but this is correct usage %>
  <% end %>
<% end %>

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

Note that list.with_item (the outer slot setter) is not flagged — only the nested item.with_icon.

Isolating the trigger

The exemption keys purely on the receiver being a render block's yielded parameter — not on the with_* name, not on block presence, and not on trailing text. The following cases make the boundary explicit:

<% bare_call("z") %>                       <%# flagged — outside any render block %>

<%= render Outer.new do |outer| %>
  <% outer.slot_setter("a") %>             <%# NOT flagged — receiver is render block param %>
  <% other_receiver.slot_setter("a2") %>   <%# flagged — receiver is NOT the render block param %>

  <%= render Inner.new do |inner| %>
    <% inner.slot_setter("b") %>           <%# NOT flagged — receiver is (nested) render block param %>
  <% end %>

  <% outer.with_item do |item| %>
    <% item.slot_setter("c") %>            <%# flagged — receiver is a slot-block builder, not a render param %>
  <% end %>
<% end %>

<% plain_method do |x| %>
  <% x.slot_setter("d") %>                 <%# flagged — receiver is an arbitrary block param %>
<% end %>

Receiver origin	Flagged?
render(...) do |x| block param (outer, inner)	No
Any other receiver inside a render block	Yes
Builder yielded by a nested slot block (item from with_item do |item|)	Yes (false positive)
Block param of a non-render method block	Yes
No enclosing block	Yes

The same slot_setter method is exempt on outer.slot_setter and flagged on item.slot_setter — confirming the check is structural/receiver-based, and that the nested-slot builder simply isn't recognized.

Expected behavior

Slot setters invoked on a builder object yielded by a nested slot block should be treated the same as slot setters on the top-level render block param: not flagged. <% item.with_icon("home") %> is the correct, documented way to populate a slot — the return value is meant to be discarded, which is exactly why it belongs in a silent <% %> tag. The rule's suggested fix (switch to <%= %>) would be wrong here: it would output the setter's return value and produce duplicated/malformed markup.

Suggested fix

Extend the existing render-block exemption so it also recognizes the builder objects yielded by slot blocks. Concretely, when a <% receiver.method(...) %> call's receiver is the block parameter of an enclosing block that is itself a slot setter (with_*) — at any nesting depth — apply the same exemption already used for render(...) do |x| params.

Environment

• herb gem v0.10.1, libprism v1.9.0, libherb v0.10.1
• @herb-tools/linter v0.10.1
• Node.js v24.13.0
• Reproduced via herb lint