Small improvements for Ruby 4.0 support (#659)

* fix: ensure CString lifetime in memory test

Previously, the CString was being created inline and immediately dropped,
causing undefined behavior when rb_str_new_cstr tried to read from the
dangling pointer. Now we keep the CString alive until after the FFI call.

* chore: allow unpredictable_function_pointer_comparisons lint

Suppress the new Rust lint warning for function pointer comparisons
in generated bindings.

* chore: update Nix flake for Ruby 4.0 support

- Add nixpkgs-ruby input for better Ruby version management
- Configure Ruby 4.0.0-preview2 via .ruby-version file
- Refactor devShell to use explicit pkgs prefix
- Remove bundler (included with Ruby)
- Update flake.lock with new dependencies

* chore: add direnv and Ruby version configuration

Add .envrc for automatic Nix flake loading and .ruby-version
to specify Ruby 4.0.0-preview2.

* ci: allow truffleruby failures until 2025-01-01

Add continue-on-error for truffleruby matrix entries due to known
compatibility issues. Remove allow_failure after 2025-01-01.

* Call ruby_sysinit and ruby_init_stack

Update tests to invoke crate::ruby_sysinit (platform-agnostic wrapper for rb_w32_sysinit)
and add a stack marker (using std::ptr::addr_of_mut) passed to crate::ruby_init_stack
before calling ruby_setup. Ensures proper stdio/VM initialization and correct GC stack
scanning, especially on Windows.

* Relax Gemfile version constraints and add tsort

Unpin several gems in Gemfile (rake, minitest, rake-compiler,
rake-compiler-dock, racc, base64, standard) to allow flexible versions.
Remove the Ruby-version guard around the standard gem so it's always
included. Add tsort as a dependency.

* fix: correct ensure block syntax in list_platforms

The ensure block was inside the each iterator without a begin block,
which is invalid Ruby syntax and causes a parse error with older Ruby
parsers.

* Fix ternary precedence and small style issues

- Fix conditional grouping in cargo_test_task so CI/--verbose check
  evaluates correctly when deciding test args.
- Add parentheses around ternary and comparisons to avoid ambiguity.
- Use super(<<~MSG) to pass heredoc safely.
- Simplify string interpolation in mkmf and normalize require_relative paths.

Author: ianks

.envrc

@@ -0,0 +1 @@
+use flake

.github/workflows/ci.yml

@@ -70,11 +70,14 @@ jobs:
           - os: windows-2025
             rust_toolchain: stable
         include:
+          # TODO: Remove allow_failure after 2025-01-01 (known compatibility issues)
           - ruby_version: "truffleruby"
+            allow_failure: true
             sys:
               os: ubuntu-latest
               rust_toolchain: stable
           - ruby_version: "truffleruby"
+            allow_failure: true
             sys:
               os: macos-15
               rust_toolchain: stable
@@ -86,6 +89,12 @@ jobs:
             sys:
               os: ubuntu-latest
               rust_toolchain: stable
+          # TODO: Remove allow_failure after 2025-01-01 (known Windows compatibility issues)
+          - ruby_version: "3.4"
+            allow_failure: true
+            sys:
+              os: windows-2025
+              rust_toolchain: stable
         exclude:
           - ruby_version: "3.1"
             sys:
@@ -99,15 +108,14 @@ jobs:
             sys:
               os: windows-2022
               rust_toolchain: stable
-          - ruby_version: "3.4"
-            sys:
-              os: windows-2025
-              rust_toolchain: stable
+          # Ruby 4.0 not available on Windows yet
           - ruby_version: "4.0.0-preview2"
             sys:
               os: windows-2025
               rust_toolchain: stable
     runs-on: ${{ matrix.sys.os }}
+    # Allow truffleruby to fail until 2025-01-01 (known compatibility issues)
+    continue-on-error: ${{ matrix.allow_failure || false }}
     steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
 

.ruby-version

@@ -0,0 +1 @@
+ruby-4.0.0-preview2

Gemfile

@@ -5,15 +5,13 @@ source "https://rubygems.org"
 gemspec path: "gem"
 gemspec path: "examples/rust_reverse"
 
-gem "rake", "~> 13.0"
-gem "minitest", "5.15.0"
-gem "rake-compiler", "~> 1.3.0" # Small bug in 1.2.4 that breaks Ruby 2.5
-gem "rake-compiler-dock", "1.10.0" # This should match the versions used in docker/Dockerfile.*
-gem "racc", "~> 1.7"
-gem "base64", "~> 0.3.0"
+gem "rake"
+gem "minitest"
+gem "rake-compiler"
+gem "rake-compiler-dock"
+gem "racc"
+gem "base64"
 gem "yard"
 gem "mutex_m"
-
-if RUBY_VERSION >= "2.7.0"
-  gem "standard", "~> 1.12.1"
-end
+gem "standard"
+gem "tsort"

Rakefile

@@ -20,8 +20,8 @@ def cargo_test_task(name, *args, crate: name)
       next
     end
 
-    default_args = ENV["CI"] || extra_args.include?("--verbose") ? [] : ["--quiet"]
-    test_args = ENV["CI"] || extra_args.include?("--verbose") ? ["--", "--nocapture"] : []
+    default_args = (ENV["CI"] || extra_args.include?("--verbose")) ? [] : ["--quiet"]
+    test_args = (ENV["CI"] || extra_args.include?("--verbose")) ? ["--", "--nocapture"] : []
     sh "cargo", "test", *default_args, *extra_args, *args, "-p", crate, *test_args
     puts "=" * 80
   end

crates/rb-sys-tests/src/memory_test.rs

@@ -25,15 +25,19 @@ fn test_rb_gc_guarded_ptr_vec() {
         unsafe {
             let mut vec_of_values: Vec<VALUE> = Default::default();
 
-            let s1 = rb_str_new_cstr(format!("hello world{i}\0").as_ptr() as _);
+            // Keep the CStrings alive until after rb_str_new_cstr uses them
+            let cstr1 = std::ffi::CString::new(format!("hello world{i}")).unwrap();
+            let s1 = rb_str_new_cstr(cstr1.as_ptr());
             let s1 = rb_gc_guard!(s1);
             vec_of_values.push(s1);
 
-            let s2 = rb_str_new_cstr(format!("hello world{i}\0").as_ptr() as _);
+            let cstr2 = std::ffi::CString::new(format!("hello world{i}")).unwrap();
+            let s2 = rb_str_new_cstr(cstr2.as_ptr());
             let s2 = rb_gc_guard!(s2);
             vec_of_values.push(s2);
 
-            let s3 = rb_str_new_cstr(format!("hello world{i}\0").as_ptr() as _);
+            let cstr3 = std::ffi::CString::new(format!("hello world{i}")).unwrap();
+            let s3 = rb_str_new_cstr(cstr3.as_ptr());
             let s3 = rb_gc_guard!(s3);
             vec_of_values.push(s3);
 

crates/rb-sys/src/bindings.rs

@@ -14,6 +14,7 @@
 #![allow(rustdoc::invalid_html_tags)]
 #![allow(deprecated)]
 #![allow(dead_code)]
+#![allow(unpredictable_function_pointer_comparisons)]
 
 include!(env!("RB_SYS_BINDINGS_PATH"));
 

crates/rb-sys/src/utils.rs

@@ -64,19 +64,24 @@ macro_rules! debug_ruby_assert_type {
 mod tests {
     use super::*;
     use rusty_fork::rusty_fork_test;
+    use std::ptr::addr_of_mut;
 
     rusty_fork_test! {
         #[test]
         fn test_is_ruby_vm_started() {
             assert!(!unsafe { is_ruby_vm_started() });
 
-            #[cfg(windows)]
-            {
-                let mut argc = 0;
-                let mut argv: [*mut std::os::raw::c_char; 0] = [];
-                let mut argv = argv.as_mut_ptr();
-                unsafe { rb_sys::rb_w32_sysinit(&mut argc, &mut argv) };
-            }
+            // Call ruby_sysinit which handles platform-specific initialization
+            // (rb_w32_sysinit on Windows) and sets up standard file descriptors
+            let mut argc = 0;
+            let mut argv: [*mut std::os::raw::c_char; 0] = [];
+            let mut argv_ptr = argv.as_mut_ptr();
+            unsafe { crate::ruby_sysinit(&mut argc, &mut argv_ptr) };
+
+            // ruby_init_stack must be called before ruby_setup, especially on
+            // Windows where it's required for proper GC stack scanning
+            let mut stack_marker: crate::VALUE = 0;
+            unsafe { crate::ruby_init_stack(addr_of_mut!(stack_marker) as *mut _) };
 
             match unsafe { crate::ruby_setup() } {
                 0 => {}

flake.lock

@@ -2,29 +2,36 @@
   description = "Dev environment for rb-sys";
 
   inputs = {
-    nixpkgs.url      = "github:NixOS/nixpkgs/nixos-unstable";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
     rust-overlay.url = "github:oxalica/rust-overlay";
-    flake-utils.url  = "github:numtide/flake-utils";
+    flake-utils.url = "github:numtide/flake-utils";
+    nixpkgs-ruby.url = "github:bobvanderlinden/nixpkgs-ruby";
+    nixpkgs-ruby.inputs.nixpkgs.follows = "nixpkgs";
   };
 
-  outputs = { self, nixpkgs, rust-overlay, flake-utils, ... }:
+  outputs = { self, nixpkgs, rust-overlay, flake-utils, nixpkgs-ruby, ... }:
     flake-utils.lib.eachDefaultSystem (system:
       let
         overlays = [ (import rust-overlay) ];
         pkgs = import nixpkgs {
           inherit system overlays;
         };
+        ruby = nixpkgs-ruby.lib.packageFromRubyVersionFile {
+          file = ./.ruby-version;
+          inherit system;
+        };
+        rustToolchain = pkgs.rust-bin.stable.latest.default;
       in
-      with pkgs;
       {
-        devShells.default = mkShell {
+        packages.ruby = ruby;
+
+        devShells.default = pkgs.mkShell {
           buildInputs = [
-            fastmod
-            ruby_3_3.devEnv
-            rust-bin.stable.latest.default
-            bundler
-            zsh
-            nodejs
+            pkgs.fastmod
+            ruby
+            rustToolchain
+            pkgs.zsh
+            pkgs.nodejs
           ];
 
           # Make is so nix develop --impure uses zsh config