[ruby-core:107819] [Ruby master Feature#18615] Use -Werror=implicit-function-declaration by default for building C extensions

Issue #18615 has been updated by Eregon (Benoit Daloze).


shyouhei (Shyouhei Urabe) wrote in #note-8:
> There is nothing that a Ruby _user_ can do for this situation.  Gem _authors_ should be aware of their bugs.  Hiding compiler warnings from their eyes is the problem.  `bundle install` is irrelevant here.  I'm not against bundler to remain silent.

Good point, and I think it also makes sense `gem install` does the same (but anyway probably very few use `gem install` directly).
For gem authors, `rake compile` (or manually `cd ext/... && ruby extconf.rb && make`) does show C warnings, so that's fine.

However, gem authors can't predict which functions will be removed in future versions of Ruby (e.g., if the gem is released 2 years before, or if deprecated warnings are hidden by default ...).

IMHO it makes sense to remove deprecated functions after they've been emitting deprecation warnings.

My conclusion is the same: `-Werror=implicit-function-declaration` makes for a better and clearer error for everyone, and there seems no value to let the C extension compile but fail at runtime.

----------------------------------------
Feature #18615: Use -Werror=implicit-function-declaration by default for building C extensions
https://bugs.ruby-lang.org/issues/18615#change-96748

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
----------------------------------------
Currently, if a C extension refers a non-existing function it will continue to compile and only emit a warning.
And compilation warnings are hidden by default for both `gem install` and `bundle install` (`gem install -V somegem` shows them).

A concrete example is the sqlite3 gem, if we use version 1.3.13 it fails only at runtime:
```
$ gem install sqlite3:1.3.13
Fetching sqlite3-1.3.13.gem
Building native extensions. This could take a while...
Successfully installed sqlite3-1.3.13
1 gem installed

$ ruby -rsqlite3 -e 'db = SQLite3::Database.new "test.db"; p db'
ruby: symbol lookup error: /home/eregon/.rubies/ruby-3.0.2/lib/ruby/gems/3.0.0/gems/sqlite3-1.3.13/lib/sqlite3/sqlite3_native.so: undefined symbol: rb_check_safe_obj
```
This is not nice, it should have failed clearly at compile time, saying the function does not exist.

There is a compiler warning, which can only be seen with (and so most users would miss it):
```
$ gem install -V sqlite3:1.3.13
...
database.c: In function ‘initialize’:
database.c:60:3: warning: implicit declaration of function ‘rb_check_safe_obj’; did you mean ‘rb_check_safe_str’? [-Wimplicit-function-declaration]
   60 |   rb_check_safe_obj(file);
      |   ^~~~~~~~~~~~~~~~~
      |   rb_check_safe_str
...
```

Also multiple CRuby releases are broken on macOS which seems to enable `-Werror=implicit-function-declaration` by default (e.g., #17777).
EDIT: `-Werror=implicit-function-declaration` is now default for building CRuby, but not for C extensions.

How about we just always enable `-Werror=implicit-function-declaration` for all C extensions? (builtin or not). 

It:
* shows clear errors early on and shows where the missing function is called (explained just above), instead of delaying them to runtime
* never compiles a call in C with the wrong type due to a missing include

From https://github.com/oracle/truffleruby/issues/2618



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