From a6ee7ae4c453b590b9fefd6a46bf69cd8e321694 Mon Sep 17 00:00:00 2001
From: st0012 <stan001212@gmail.com>
Date: Sun, 22 Mar 2026 14:29:27 +0000
Subject: [PATCH] Fix encoding error when C parser reads external source files
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When a C file references another source file via `/* in file.c */`,
the parser read it with bare `File.read` which uses
`Encoding.default_external`. On systems where this is US-ASCII
(e.g. Debian CI), non-ASCII bytes in the source file cause
`ArgumentError: invalid byte sequence in US-ASCII` in String#scan.

Use `RDoc::Encoding.read_file` instead, which reads in binary mode
and properly handles encoding detection and transcoding.

This was triggered by Ruby commit a2531ba293 which added UTF-8
right arrows (→) in comments in class.c, which is referenced from
object.c via `/* in class.c */`.
---
 lib/rdoc/parser/c.rb       |  2 +-
 test/rdoc/parser/c_test.rb | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/lib/rdoc/parser/c.rb b/lib/rdoc/parser/c.rb
index b13e7cad8a..a31b36404d 100644
--- a/lib/rdoc/parser/c.rb
+++ b/lib/rdoc/parser/c.rb
@@ -1016,7 +1016,7 @@ def handle_method(type, var_name, meth_name, function, param_count,
         file_name = File.join @file_dir, source_file
 
         if File.exist? file_name then
-          file_content = File.read file_name
+          file_content = RDoc::Encoding.read_file file_name, @options.encoding
         else
           @options.warn "unknown source #{source_file} for #{meth_name} in #{@file_name}"
         end
diff --git a/test/rdoc/parser/c_test.rb b/test/rdoc/parser/c_test.rb
index 17237c2400..00b57d5d7d 100644
--- a/test/rdoc/parser/c_test.rb
+++ b/test/rdoc/parser/c_test.rb
@@ -2292,6 +2292,42 @@ def test_reparse_c_file_no_duplicates
     assert_include method_names, 'baz'
   end
 
+  def test_handle_method_source_file_with_non_ascii
+    # Regression test: when the C parser reads an external source file
+    # (via "/* in file.c */"), it must use RDoc::Encoding.read_file instead
+    # of File.read. On systems where Encoding.default_external is US-ASCII,
+    # bare File.read produces a US-ASCII string that raises ArgumentError
+    # on String#scan when the file contains non-ASCII bytes.
+    source_path = File.join(File.dirname(@fn), 'greet.c')
+    File.binwrite source_path, <<~C.encode('UTF-8')
+      /*
+       * Returns a greeting \u2014 "h\u00e9llo w\u00f6rld"
+       */
+      VALUE
+      rb_greet(VALUE obj) {
+        return rb_str_new2("hello");
+      }
+    C
+
+    parser = util_parser <<~C
+      void Init_Foo(void) {
+        VALUE cFoo = rb_define_class("Foo", rb_cObject);
+        rb_define_method(cFoo, "greet", rb_greet, 0); /* in greet.c */
+      }
+    C
+
+    parser.scan
+
+    foo = @top_level.find_module_named 'Foo'
+    assert foo, 'Foo class should be found'
+
+    greet = foo.method_list.first
+    assert greet, 'greet method should be found'
+    assert_equal 'greet', greet.name
+  ensure
+    File.delete source_path if source_path && File.exist?(source_path)
+  end
+
   def util_get_class(content, name = nil)
     @parser = util_parser content
     @parser.scan