ruby-head-wasm-wasi build fails on current Ruby head because cross build runs wasm dump_ast instead of host dump_ast

[ledsun]

Summary

Building ruby-head-wasm-wasi against current Ruby head fails during .rbinc generation in the cross build.
The failure occurs because the build executes ./dump_ast, which is a wasm binary, instead of a host-built dump_ast.

Although Ruby head supports providing a host dump_ast via --with-dump-ast, the generated Makefile still uses the target-side binary when it is not specified.

Environment

The Ruby source used by the failing build was:

• ruby/ruby@251fd7a

This reproduces on a clean build with:

rm -rf build
rake npm:ruby-head-wasm-wasi

Results:

./dump_ast: 151: Syntax error: "(" unexpected
make: *** [uncommon.mk:1307: /home/ledsun/ruby.wasm/build/checkouts/head/io.rbinc] Error 1
Try running with `rake --verbose` for more complete output.
bundler: failed to load command: /home/ledsun/ruby.wasm/exe/rbwasm (/home/ledsun/ruby.wasm/exe/rbwasm)
/home/ledsun/ruby.wasm/lib/ruby_wasm/build/executor.rb:77:in 'RubyWasm::BuildExecutor#system': Command failed with status (2): 'make' '-j8' 'install' 'DESTDIR=/home/ledsun/ruby.wasm/build/wasm32-unknown-wasip1/ruby-head-wasm32-unknown-wasip1-full-61ff6b0c74de0393d023cecfb9d1d7be/install' (RuntimeError)
        from /home/ledsun/ruby.wasm/lib/ruby_wasm/build/product/crossruby.rb:238:in 'RubyWasm::CrossRubyProduct#build'

Related change

This behavior appears after:

• ruby/ruby@a70dbe1

This change introduces support for --with-dump-ast, but the fallback behavior still uses ./dump_ast even in cross builds.
The generated Makefile contains:

DUMP_AST = ./dump_ast$(EXEEXT)

As a result, .rbinc generation invokes the target-side dump_ast instead of a host executable.

Proposed fix

Pass the host-built dump_ast explicitly to configure:

--with-dump-ast=/path/to/host/baseruby-build/dump_ast

This ensures .rbinc generation uses a host executable.

ruby.wasm already passes --with-baseruby, so it can also pass --with-dump-ast.

Minimal patch:

diff --git a/lib/ruby_wasm/build/product/crossruby.rb b/lib/ruby_wasm/build/product/crossruby.rb
@@
     def baseruby_path
       File.join(@baseruby.install_dir, "bin/ruby")
     end
+
+    def dump_ast_path
+      File.join(@baseruby.product_build_dir, "dump_ast")
+    end
@@
       args << %Q(--with-zlib-dir=#{@zlib.install_root})
       args << %Q(--with-openssl-dir=#{@openssl.install_root}) if @openssl
       args << %Q(--with-baseruby=#{baseruby_path})
+      args << %Q(--with-dump-ast=#{dump_ast_path})

This resolves the issue by avoiding execution of the wasm dump_ast.