A jbuilder partial! whose path is built at run time ("widgets/#{kind}") renders {} #360

Probed with roundhouse 2026.9.18 (7fad14c0), Linux x86-64, CRuby 4.0.5, --target ruby (the spinel tree has the same view .rb).

A jbuilder partial! whose path is built when the template runs renders {}, with no diagnostic:

# app/views/widgets/_widget.json.jbuilder
json.id widget.id
json.name widget.name

# app/views/widgets/dynamic.json.jbuilder
kind = "widget"
json.partial!("widgets/#{kind}", widget: @widget)

Controller action: @widget = Widget.find(params[:id]) then render :dynamic; route get "widgets/:id/dynamic", to: "widgets#dynamic", defaults: { format: :json }; schema widgets(name string, size integer); ApplicationController < ActionController::Base.

Emitted:

def self.dynamic_json(widget)
  io = String.new
  io << "{"
  io << ""
  io << ""
  io << "}"
  io
end

The real-world form picks one of a few partials by a record attribute or a param (json.partial! "things/#{thing.kind}", thing: thing).

Where it goes

src/lower/jbuilder_to_library/mod.rs, classify: the partial! arm needs string_literal(path_arg), and an interpolated path is an ExprNode::StringInterp, so the statement is Unknown. (The kind = … assignment is a separate gap: a template local is also Unknown; #359 keeps it.)

A path can't name a method at compile time, but the candidates are known: the _*.json.jbuilder partials under the path's fixed prefix (widgets/). One way is a case over the interpolated string with one when "widgets/widget" arm per candidate calling its <name>_json, and an else that raises the ActionView::MissingTemplate Rails would raise. That's a design call (and the arms' arities must agree), so I've left it as an issue. Short of that, a diagnostic for Unknown statements would have pointed at it.

Found while compiling a Rails API app with --target spinel.