Revert "Revert "error.c: Let Exception#inspect inspect its message""

This reverts commit b9f030954a.

[Bug #18170]

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"
@@ -1439,8 +1440,15 @@ 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;

internal/string.h

@@ -44,6 +44,7 @@ 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);
 int rb_ascii8bit_appendable_encoding_index(rb_encoding *enc, unsigned int code);
+VALUE rb_str_include(VALUE str, VALUE arg);
 
 static inline bool STR_EMBED_P(VALUE str);
 static inline bool STR_SHARED_P(VALUE str);

string.c

@@ -6456,7 +6456,7 @@ rb_str_reverse_bang(VALUE str)
  *
  */
 
-static VALUE
+VALUE
 rb_str_include(VALUE str, VALUE arg)
 {
     long i;

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