Ruby: suppress -Wdefault-const-init-field-unsafe for clang 21+#3419
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kpumuk merged 1 commit intoApr 26, 2026
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clang 21 introduced -Wdefault-const-init-field-unsafe, which triggers on Ruby 3.2's rstring.h because struct RString contains a const field (klass) via RBasic. Since extconf.rb unconditionally adds -Werror, this new warning causes the native extension build to fail. The warning originates in Ruby headers, not Thrift code. Use append_cflags to suppress it; append_cflags silently ignores unknown flags, so this is safe on all older compiler versions. Client: rb
kpumuk
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Apr 26, 2026
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Problem
clang 21 introduced
-Wdefault-const-init-field-unsafe, which fires on Ruby 3.2'srstring.h:465:Because
extconf.rbunconditionally adds-Werror, this new warning causes the native extension build to fail with:This affects macOS users on Xcode 26 / Nix with LLVM 21 when building against Ruby 3.2.
Root cause
The warning is in Ruby's own headers (
rstring.h), not in Thrift code. Ruby 3.2 did not anticipate this warning being added to clang.Fix
Add
append_cflags("-Wno-default-const-init-field-unsafe")after the existing flags. Sinceappend_cflagssilently ignores flags that the compiler doesn't recognize, this is safe on all older clang versions.Testing
gem build thrift.gemspec gem install thrift-*.gem