research build wiring extconf rb

extconf.rb Pattern

Current state

ext/duckling/extconf.rb exists in the gem but is empty (0 bytes). The gemspec already declares spec.extensions = ["ext/duckling/extconf.rb"], so RubyGems and rake-compiler know to invoke it.

Verified reference: rust_blank example

Source: examples/rust_blank/ext/rust_blank/extconf.rb

Exact content:

require "mkmf"
require "rb_sys/mkmf"

create_rust_makefile("rust_blank/rust_blank")

Three lines. This is the complete file.

What create_rust_makefile does

create_rust_makefile is provided by the rb_sys gem (version 0.9.128 in Gemfile.lock). It:

1. Locates the Cargo workspace/crate in the same directory as extconf.rb
2. Generates a Makefile that delegates to cargo build
3. Sets CARGO_BUILD_TARGET and Ruby-specific environment variables so the native extension links against the currently running Ruby

The string argument "rust_blank/rust_blank" is the output path for the compiled shared library relative to lib/. So "duckling/duckling" would produce lib/duckling/duckling.bundle (macOS) or lib/duckling/duckling.so (Linux).

This file is generated during rake compile and also during gem install when the gem is installed from source. RubyGems invokes extconf.rb via ruby extconf.rb, which writes the Makefile, and then make is run in the extension directory.

Content needed for ext/duckling/extconf.rb

require "mkmf"
require "rb_sys/mkmf"

create_rust_makefile("duckling/duckling")

The argument "duckling/duckling" matches:

• The lib_dir = "lib/duckling" setting in Rake::ExtensionTask (see rakefile-setup.md)
• The require "duckling/duckling" call expected inside lib/duckling.rb

Relationship to gemspec

duckling.gemspec declares:

spec.extensions = ["ext/duckling/extconf.rb"]
spec.add_dependency "rb_sys", "~> 0.9.39"

The rb_sys runtime dependency is what provides rb_sys/mkmf to end-users who install the gem from source. Gemfile.lock pins rb_sys to 0.9.128.

Resolved (were open questions; the gem has since shipped as v0.2.0)

1. Does create_rust_makefile need Cargo in PATH at gem install time? Yes — the Makefile it generates invokes cargo build. End-users installing from source must have Rust/Cargo installed. The gem shipped as a source-only gem (see CI Configuration), so this requirement is real for every installer, not just a hypothetical.

2. Does rb_sys/mkmf need to be required before mkmf? Yes, confirmed — ext/duckling/extconf.rb on main requires mkmf first, then rb_sys/mkmf, exactly matching the "Content needed" prediction above and the rust_blank reference ordering.