An instance variable read inside a jbuilder partial is rewritten to a bare local that nothing defines #540

Probed at main 23b21b7, Linux x86-64, gcc 16.2.1.

Fixture = fixtures/tiny-api from roundhouse's own repo, with gadgets_controller.rb added as
app/controllers/gadgets_controller.rb, show.json.jbuilder and _gadget.json.jbuilder added
under app/views/gadgets/, and routes.rb replacing config/routes.rb (adds resources :gadgets, only: %i[show], otherwise unchanged). The locals key here deliberately matches the partial's own basename (gadget),
so the parameter-naming issue from #539 does not also fire — this fixture isolates the ivar read:

# app/controllers/gadgets_controller.rb
def show
  @widget = Widget.find_by(id: params[:id])
  @note = "hello"
end

# app/views/gadgets/show.json.jbuilder
json.partial!("gadgets/gadget", gadget: @widget)

# app/views/gadgets/_gadget.json.jbuilder
json.name(gadget.name)
json.note(@note) if @note

Real Rails renders this as {"name":"Sprocket","note":"hello"} — Rails partials see the controller's instance
variables (@note reaches _gadget.json.jbuilder the same way it reaches any other view/partial in the same
request), with no need to pass it as a local.

roundhouse --target spinel (and --target ruby) refuse to emit at all, with no --allow-unsupported:

app/views/gadgets/_gadget.json.jbuilder:2:21: error[ivar_unresolved]: @note has no known type
app/views/gadgets/_gadget.json.jbuilder:2:11: error[ivar_unresolved]: @note has no known type

With --allow-unsupported both targets emit (app/views/gadgets/_gadget_json.rb):

module Views
  module Gadgets
    def self.gadget_json(gadget)
      io = String.new
      io << "{"
      io << "\"name\":"
      io << JsonBuilder.encode_value(gadget.name)
      if note
        io << ","
        io << "\"note\":"
        io << JsonBuilder.encode_value(note)
      end
      io << "}"
      io
    end
  end
end

@note became a bare note that the method never binds — not a parameter, not a local assignment anywhere in
the body. Running the --target ruby tree against a seeded widget row (REQUEST_METHOD=GET PATH_INFO=/gadgets/<id> ruby main.rb) raises on the first request that reaches the if note line:

app/views/gadgets/_gadget_json.rb:8:in 'Views::Gadgets.gadget_json': undefined local variable or method 'note' for module Views::Gadgets (NameError)
      if note
         ^^^^
Did you mean?  not
	from app/views/gadgets/show_json.rb:7:in 'Views::Gadgets.show_json'
	from app/controllers/gadgets_controller.rb:21:in 'GadgetsController#show'

Expected: @note resolves inside the partial the same way it would in Rails — either genuinely plumbed through
(the controller's ivar reaching the partial method, e.g. as an extra parameter the same way index/show templates
already receive their ivar-derived args) or, short of that, a survey/unsupported diagnostic at transpile
time instead of a confident wrong emit that only fails at request time.

Cause: src/lower/jbuilder_to_library/mod.rs, jbuilder_method_parts (around line 315):

let ivar_params: Vec<Symbol> = if is_partial {
    Vec::new()
} else {
    crate::lower::view_to_library::view_read_ivars(&view.body)
};

The surrounding comment states the assumption driving this: "A PARTIAL keeps the convention: its record arrives
positionally from the parent's collection, and it reads no ivar at all." For a non-partial template,
view_read_ivars walks the body and turns every @ivar read into a parameter; for a partial this is
unconditionally skipped, so any @ivar a partial body does read is still rewritten to a bare local by
rewrite_ivars_to_locals (the same ivar→local pass runs regardless of is_partial) but is never added as a
parameter anywhere — the two passes disagree about whether partials can read ivars at all. The strict-mode
error[ivar_unresolved] is the type checker catching the output of that disagreement, not a separate bug.

Duplicate search: gh search issues -R rubys/roundhouse for "jbuilder partial instance variable", "jbuilder ivar undefined", "partial @note", "jbuilder partial ivar scope", "json.partial! @" — no match other than
the unrelated json.partial! @record positional-record case (#322, closed, fixed by merged PR #356) and #360 /
#323 (open, different jbuilder shapes: a runtime-built partial path, and a collection+block/array!/
helper_method cluster). None covers an ivar read inside a partial's own body. PRs touching
jbuilder_to_library (#355, #359, #361, #367, #356 — all merged) address collection blocks, template locals,
conditionals, rescue blocks and the @record positional case; none touches ivar visibility inside partials.
Not a duplicate.

json.name(gadget.name)
json.note(@note) if @note

class GadgetsController < ApplicationController
  def show
    @widget = Widget.find_by(id: params[:id])
    @note = "hello"
  end
end

Rails.application.routes.draw do
  resources :widgets, only: %i[index show create update]
  resources :gadgets, only: %i[show]
  get "/feed.json", to: "formats#literal"
  get "/formats", to: "formats#index"
  get "/formats/:id", to: "formats#show"
end

json.partial!("gadgets/gadget", gadget: @widget)

Found while compiling a Rails API app with --target spinel. Happy to send a PR for this if the direction above sounds right.