Summary

Add stable API method for freezing Ruby objects, equivalent to the C macro RB_OBJ_FREEZE.

Motivation

Object freezing is a common operation for:

• Creating immutable values
• Thread-safe data structures (frozen objects are Ractor-shareable)
• Performance optimization (frozen string literals)

Currently, users must call rb_obj_freeze() which is a function call. The inline version can be more efficient for simple cases.

Ruby Source Reference

// include/ruby/internal/fl_type.h

#define RB_OBJ_FREEZE rb_obj_freeze_inline

static inline void
RB_OBJ_FREEZE_RAW(VALUE obj)
{
    rb_obj_freeze_inline(obj);
}

// rb_obj_freeze_inline is declared as RUBY_SYMBOL_EXPORT
void rb_obj_freeze_inline(VALUE obj);

Note: The actual implementation calls rb_obj_freeze_inline() which handles:

• Setting the RUBY_FL_FREEZE flag
• Freezing singleton class if present
• Other internal bookkeeping

Proposed API

/// Freeze an object, making it immutable (akin to `RB_OBJ_FREEZE`).
///
/// After freezing, any attempt to modify the object will raise FrozenError.
/// Frozen objects are also safe to share across Ractors.
///
/// # Safety
/// This function is unsafe because it modifies the object's internal state.
/// The caller must ensure the VALUE is a valid Ruby object.
unsafe fn rb_obj_freeze(&self, obj: VALUE);

Implementation Notes

• For most cases, simply set RUBY_FL_FREEZE flag
• Need to handle singleton class freezing for some object types
• May need to call the C function for correctness
• Reference: include/ruby/internal/fl_type.h:911-925

Checklist

• Add method to StableApiDefinition trait
• Implement for each Ruby version
• Add C fallback in compiled.c
• Add public macro wrapper in macros.rs
• Add tests verifying freeze behavior
• Test interaction with Ractor shareability