From 9d927204e7b86eb00bfd07a060a6383139edf741 Mon Sep 17 00:00:00 2001
From: Yusuke Endoh <mame@ruby-lang.org>
Date: Tue, 30 Nov 2021 16:27:12 +0900
Subject: error.c: Let Exception#inspect inspect its message

... only when the message string has a newline.

`p StandardError.new("foo\nbar")` now prints `#<StandardError: "foo\nbar">'
instead of:

    #<StandardError:
    bar>

[Bug #18170]
---
 error.c                     | 12 ++++++++++--
 internal/string.h           |  1 +
 string.c                    |  2 +-
 test/ruby/test_exception.rb |  6 ++++++
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/error.c b/error.c
index 3f5c05d681..08b0c8da72 100644
--- a/error.c
+++ b/error.c
@@ -34,6 +34,7 @@
 #include "internal/io.h"
 #include "internal/load.h"
 #include "internal/object.h"
+#include "internal/string.h"
 #include "internal/symbol.h"
 #include "internal/thread.h"
 #include "internal/variable.h"
@@ -1422,8 +1423,15 @@ exc_inspect(VALUE exc)
     str = rb_str_buf_new2("#<");
     klass = rb_class_name(klass);
     rb_str_buf_append(str, klass);
-    rb_str_buf_cat(str, ": ", 2);
-    rb_str_buf_append(str, exc);
+
+    if (RTEST(rb_str_include(exc, rb_str_new2("\n")))) {
+        rb_str_catf(str, ":%+"PRIsVALUE, exc);
+    }
+    else {
+        rb_str_buf_cat(str, ": ", 2);
+        rb_str_buf_append(str, exc);
+    }
+
     rb_str_buf_cat(str, ">", 1);
 
     return str;
diff --git a/internal/string.h b/internal/string.h
index 18b01862f7..57a4af49c7 100644
--- a/internal/string.h
+++ b/internal/string.h
@@ -43,6 +43,7 @@ char *rb_str_to_cstr(VALUE str);
 const char *ruby_escaped_char(int c);
 void rb_str_make_independent(VALUE str);
 int rb_enc_str_coderange_scan(VALUE str, rb_encoding *enc);
+VALUE rb_str_include(VALUE str, VALUE arg);
 
 static inline bool STR_EMBED_P(VALUE str);
 static inline bool STR_SHARED_P(VALUE str);
diff --git a/string.c b/string.c
index 83d90ff3a9..9c37d774b3 100644
--- a/string.c
+++ b/string.c
@@ -6376,7 +6376,7 @@ rb_str_reverse_bang(VALUE str)
  *
  */
 
-static VALUE
+VALUE
 rb_str_include(VALUE str, VALUE arg)
 {
     long i;
diff --git a/test/ruby/test_exception.rb b/test/ruby/test_exception.rb
index 0b05ff7c51..acc3228e2f 100644
--- a/test/ruby/test_exception.rb
+++ b/test/ruby/test_exception.rb
@@ -478,6 +478,12 @@ end.join
       def to_s; ""; end
     end
     assert_equal(e.inspect, e.new.inspect)
+
+    # https://bugs.ruby-lang.org/issues/18170#note-13
+    assert_equal('#<Exception:"foo\nbar">', Exception.new("foo\nbar").inspect)
+    assert_equal('#<Exception: foo bar>', Exception.new("foo bar").inspect)
+    assert_equal('#<Exception: foo\bar>', Exception.new("foo\\bar").inspect)
+    assert_equal('#<Exception: "foo\nbar">', Exception.new('"foo\nbar"').inspect)
   end
 
   def test_to_s
-- 
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