headius · Oct 9, 2026 · Oct 7, 2026
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -43,6 +43,18 @@ Other documentation in this repository and in the wiki explain in more detail ho
 
 Other `test` and `spec` suites may also be appropriate depending on what functionality you are working on.
 
+### Linking Java code to CRuby
+
+When a method, class or field implements or ports a CRuby C function or macro, annotate it with `@CRuby` (`org.jruby.anno.CRuby`) rather than writing a `// MRI: rb_foo` comment:
+
+```java
+@CRuby(value = "match_begin", file = "re.c")
+@JRubyMethod
+public IRubyObject begin(ThreadContext context, IRubyObject index) {
+```
+
+`file` is optional. Repeat the annotation when one Java method covers several C functions, and use `note` for qualifiers such as `note = "first half"`.
+
 ## Testing
 
 See [BUILDING.md](BUILDING.md) for more details.

diff --git a/core/src/main/java/org/jruby/RubyMatchData.java b/core/src/main/java/org/jruby/RubyMatchData.java
@@ -41,6 +41,7 @@
 import org.joni.Region;
 import org.joni.exception.JOniException;
 import org.joni.exception.ValueException;
+import org.jruby.anno.CRuby;
 import org.jruby.anno.JRubyClass;
 import org.jruby.anno.JRubyMethod;
 import org.jruby.api.Convert;
@@ -558,7 +559,7 @@ public static int backrefNumber(ThreadContext context, Regex pattern, Region reg
         return toInt(context, obj);
     }
 
-    // MRI: namev_to_backref_number
+    @CRuby(value = "namev_to_backref_number", file = "re.c")
     private int namevToBackrefNumber(ThreadContext context, IRubyObject name) {
         int num = -1;
 
@@ -583,7 +584,7 @@ private int nameToBackrefError(ThreadContext context, String name) {
         throw indexError(context, "undefined group name reference " + name);
     }
 
-    // MRI: match_ary_subseq
+    @CRuby(value = "match_ary_subseq", file = "re.c")
     private IRubyObject matchArySubseq(ThreadContext context, int beg, int len, RubyArray result) {
         assert result != null;
 
@@ -606,7 +607,7 @@ private IRubyObject matchArySubseq(ThreadContext context, int beg, int len, Ruby
         return result;
     }
 
-    // MRI: match_ary_aref
+    @CRuby(value = "match_ary_aref", file = "re.c")
     private IRubyObject matchAryAref(ThreadContext context, IRubyObject index, RubyArray result) {
         int[] begLen = new int[2];
         int numRegs = regs.getNumRegs();
@@ -678,9 +679,7 @@ public IRubyObject size(ThreadContext context) {
         return regs == null ? RubyFixnum.one(context.runtime) : asFixnum(context, regs.getNumRegs());
     }
 
-    /**
-     * MRI: match_begin
-     */
+    @CRuby(value = "match_begin", file = "re.c")
     @JRubyMethod
     public IRubyObject begin(ThreadContext context, IRubyObject index) {
         check(context);

diff --git a/core/src/main/java/org/jruby/anno/CRuby.java b/core/src/main/java/org/jruby/anno/CRuby.java
@@ -0,0 +1,81 @@
+/***** BEGIN LICENSE BLOCK *****
+ * Version: EPL 2.0/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Eclipse Public
+ * License Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of
+ * the License at http://www.eclipse.org/legal/epl-v20.html
+ *
+ * Software distributed under the License is distributed on an "AS
+ * IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or
+ * implied. See the License for the specific language governing
+ * rights and limitations under the License.
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either of the GNU General Public License Version 2 or later (the "GPL"),
+ * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the EPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the EPL, the GPL or the LGPL.
+ ***** END LICENSE BLOCK *****/
+
+package org.jruby.anno;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Repeatable;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Links a Java declaration to the CRuby C function or macro it implements or was ported from.
+ *
+ * <pre>
+ * &#64;CRuby("match_size")
+ * &#64;CRuby(value = "match_begin", file = "re.c")
+ * &#64;CRuby(value = "ary_reject_bang", file = "array.c")
+ * &#64;CRuby(value = "reject_bang_i", file = "array.c")
+ * &#64;CRuby(value = "rb_str_count", note = "first half")
+ * </pre>
+ *
+ * Use it in place of a {@code // MRI: rb_foo} comment or a Javadoc that only names the C function, so tools can
+ * audit the mapping and documentation can link to the C source. It is retained at runtime so that tools working from a
+ * live JRuby (IRB, RI, language servers) can find it by reflecting on the Java method behind a Ruby method.
+ */
+@Documented
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.TYPE, ElementType.FIELD})
+@Repeatable(CRuby.List.class)
+public @interface CRuby {
+    /**
+     * The C function or macro, e.g. "match_integer_at".
+     */
+    String value();
+
+    /**
+     * The C source file defining it, relative to the CRuby root, e.g. "re.c". Optional, mainly useful when the
+     * name is defined in more than one file.
+     */
+    String file() default "";
+
+    /**
+     * Free text qualifying the link, e.g. "first half" or "loop body".
+     */
+    String note() default "";
+
+    /**
+     * Container for repeated {@link CRuby} annotations; not used directly.
+     */
+    @Documented
+    @Retention(RetentionPolicy.RUNTIME)
+    @Target({ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.TYPE, ElementType.FIELD})
+    @interface List {
+        CRuby[] value();
+    }
+}