From: "yahonda (Yasuo Honda) via ruby-core" <ruby-core@...>
Date: 2026-11-13T23:01:49+00:00
Subject: [ruby-core:127476]  [Ruby Bug#22547] `String.new("", encoding: Encoding::CESU_8)` fails the STR_EMBEDDABLE_P assertion	“too big termlen: 6” on debug builds

Issue #22547 has been reported by yahonda (Yasuo Honda).

----------------------------------------
Bug #22547: `String.new("", encoding: Encoding::CESU_8)` fails the STR_EMBEDDABLE_P assertion ���too big termlen: 6��� on debug builds
https://bugs.ruby-lang.org/issues/22547

* Author: yahonda (Yasuo Honda)
* Status: Open
* ruby -v: ruby 4.1.0dev (2026-11-13T22:58:20Z HEAD 346d1f63b7) +PRISM [x86_64-linux]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
The Rails nightly CI against Ruby master (debug build) started aborting in Active Support with `Assertion Failed: STR_EMBEDDABLE_P:termlen <= 4: too big termlen: 6`. It bisects to 346d1f63b7 ("Ignore negative capacity", https://github.com/ruby/ruby/pull/19887).

https://buildkite.com/rails/rails-nightly/builds/4922#01a1d6b6-024b-49f3-95b6-6afe9c6328e8/L2134

The full log of that Active Support job is attached (rails-nightly_build_4922_activesupport-master-debug.log.zip).

It can be reproduced with a single Active Support test:

```
cd activesupport && bin/test test/transliterate_test.rb -n test_transliterate_handles_strings_with_incompatible_encodings
```

## Steps to reproduce

Build Ruby with `cppflags=-DRUBY_DEBUG=1`, then:

```ruby
s = String.new("", encoding: Encoding::CESU_8)
puts "OK #{s.encoding}"
```

## Expected behavior

```
$ ruby -v
ruby 4.1.0dev (2026-11-13T22:47:35Z HEAD 293c4f9246) +PRISM [x86_64-linux]
$ ruby rep.rb
OK CESU-8
```

## Actual behavior

```
$ ruby -v
ruby 4.1.0dev (2026-11-13T22:58:20Z HEAD 346d1f63b7) +PRISM [x86_64-linux]
$ ruby rep.rb
string.c:281: Assertion Failed: STR_EMBEDDABLE_P:termlen <= 4: too big termlen: 6
ruby 4.1.0dev (2026-11-13T22:58:20Z HEAD 346d1f63b7) +PRISM [x86_64-linux]

-- Control frame information -----------------------------------------------
c:0002 p:0016 s:0011 E:002280 l:n b:---- r:(nil) EVAL   rep.rb:1 [FINISH]
c:0001 p:0000 s:0003 E:000f38 l:y b:---- r:(nil) DUMMY  [FINISH]
```

---Files--------------------------------
rails-nightly_build_4922_activesupport-master-debug.log.zip (73.7 KB)


-- 
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