WASM Build Notes

July 19, 2026 · View on GitHub

This document records the issues encountered and workarounds applied when building Ruby as WebAssembly for Rubree.

Table of Contents

Ruby 3.4 Issues

These issues were first discovered during Ruby 3.4.8 WASM support (PR #527). Issues 1–2 also apply to Ruby 4.0.

1. Prism parser crashes in WASM (Closed)

  • Status: No longer reproducible. The original memory fault was due to a misconfiguration in wasmify-rails (source URL override and missing patches). After correcting the setup, the crash does not occur, and the issue is considered resolved upstream.
  • Upstream: ruby/prism#4065

2. RubyGems deprecate.rb block passing failure (Closed)

  • Status: No longer reproducible. The error was due to an outdated patch and/or old RubyGems version in wasmify-rails. With the latest upstream fixes and correct configuration, the problem does not occur.
  • Upstream: ruby/rubygems#9456

3. wasmify-rails source URL override broken

  • Symptom: WASM build silently uses wrong Ruby source tarball
  • Root cause: ruby_wasm 2.9.0 renamed build_source_aliases to build_config_aliases, so wasmify-rails 0.4.1's monkey-patch is silently ignored Fix: lib/tasks/wasmify_patches.rake overrides the build_config_aliases method to set correct source URLs for Ruby 4.0.6 (temporary workaround for wasmify-rails <= 0.4.1).
    • April 2026: palkan/wasmify-rails#11 has been merged to main, but the gem version is not yet updated. The patch is still required for now, and will be removed after the next gem release.

4. bigdecimal in exclude_gems causes LoadError

  • Symptom: LoadError for bigdecimal during WASM pack
  • Root cause: bigdecimal moved from ext/ to a bundled gem in Ruby 3.4
  • Fix: Removed bigdecimal from config/wasmify.yml exclude_gems
  • Upstream: palkan/wasmify-rails#7

Ruby 4.0 Additional Issues

These issues appeared when upgrading from Ruby 3.4 to 4.0 (PRs #528–#530).

5. Gem::Deprecate not loaded in WASM runtime

  • Symptom: uninitialized constant Gem::Deprecate (NameError) at boot in WASM
  • Root cause: Ruby 4.0 pre-defines the Gem module at the C level, causing require "rubygems" to be skipped (it thinks RubyGems is already loaded). But Gem::Deprecate and other classes are not actually defined Fix: Auto-patch wasmify-rails' shim.rb to add require "rubygems" before require "/bundle/setup" (idempotent, temporary workaround via lib/tasks/wasmify_patches.rake).
    • April 2026: palkan/wasmify-rails#12 has been merged to main, but the gem version is not yet updated. The patch is still required for now, and will be removed after the next gem release.

6. ruby_wasm platform gem incompatible with Ruby 4.0 (Closed)

Temporary workarounds were required due to lack of Ruby 4.0 platform gem support, but this was resolved with official support in ruby_wasm 2.9.4. (See details in ruby/ruby.wasm#636)

Patches and Workarounds Summary

FilePurposeApplies to
ruby_wasm_patches/fix-default-parser-parse-y.patchForce parse.y parser in version.cRuby 3.4, 4.0 (Closed)
ruby_wasm_patches/fix-deprecate-class-eval-block.patchConvert deprecate.rb to heredoc styleRuby 3.4, 4.0 (Closed)
lib/tasks/wasmify_patches.rakeSource URL override (build_config_aliases), parser injection, shim.rb patch (require "rubygems")Ruby 3.3, 3.4, 4.0 (PRs have been merged upstream, but the patch is still required for the current gem version; will be removed after the next release)

Upstream References

Issue / PRRepositoryStatus
#4065 Prism parser WASM crashruby/prismClosed
#9456 class_eval block passing in WASMruby/rubygemsClosed
#11 build_source_aliases renamepalkan/wasmify-railsMerged (April 2026, gem not yet updated)
#12 shim.rb missing require "rubygems" for Ruby 4.0palkan/wasmify-railsMerged (April 2026, gem not yet updated)
#7 bigdecimal compatibility with Ruby 3.4palkan/wasmify-railsOpen
#636 Platform gem Ruby 4.0 precompiled binariesruby/ruby.wasmClosed