Fix segfault when Ruby coverage is enabled with Rails 8.1 ERB templates

[george-bit]

Summary

Fixes segmentation fault when running tests with code coverage enabled (e.g., SimpleCov) on Rails 8.1+ with Ruby 3.4+.

Problem

v4.2.0 introduced negative lineno values (-1) in class_eval to fix incorrect line numbers in stack traces for Rails 8.1 ERB templates. However, this causes a segmentation fault when Ruby's Coverage module is enabled.

Root Cause

Rails 8.1 change: Rails added annotation comments to compiled ERB output (rails/rails#53731), which added an extra line and caused stack trace line numbers to be off by 1.

v4.2.0 fix: Used lineno = -1 in class_eval to compensate for the extra line.

Ruby bug: Negative line numbers in eval/class_eval cause segmentation faults when Ruby's Coverage module is running (bugs.ruby-lang.org/issues/19363). This bug exists in Ruby 3.4 and has been known since 2022.

Result: Rails 8.1 + Ruby 3.4 + view_component 4.2.0 + SimpleCov = crash.

Solution

Detect whether coverage is running via Coverage.running? and adapt:

Coverage Status	lineno	Stack Traces	Crash?
OFF (normal use)	-1	✅ Correct	No
ON (CI/SimpleCov)	1	~2 lines off	No

Why not 0? Testing showed lineno=0 also causes the segfault. 1 is the minimum safe value.

Trade-off

This is the best possible fix given the Ruby bug. Most users (development, production) get correct line numbers. Only CI/coverage users see slightly off line numbers—but their tests actually run instead of crashing.

Fixes #2540

[george-bit]

Question: Is there a cleaner solution?

While this fix works, I wanted to raise a question about the underlying approach.

The -1 lineno fix in v4.2.0 appears to compensate for extra lines added by Rails' annotate_rendered_view_with_filenames feature (the <!-- BEGIN ... --> / <!-- END ... --> comments). This creates a chain of workarounds:

1. Rails 8.1 adds annotations → line numbers off by 1
2. v4.2.0 uses lineno = -1 → line numbers correct, but segfaults with coverage
3. This PR detects coverage → avoids segfault, but line numbers off when coverage is on

Alternative approaches worth considering:

1. Could users just disable annotations? Setting config.action_view.annotate_rendered_view_with_filenames = false might avoid the issue entirely.

2. Could view_component handle annotations differently? Perhaps strip them from compiled source, or account for them in a way that doesn't require negative line numbers.

3. Is there something in the Rails 8.1 ERB compilation (beyond annotations) that requires the -1 offset?

I'm not deeply familiar with the history here, so I wanted to ask: is the current approach (adjusting lineno) the right long-term solution, or is there a cleaner way to handle Rails 8.1 compatibility that doesn't require these workarounds?

[joelhawksley]

Thanks for taking a crack at fixing this issue! I added a couple of comments, but one key thing is missing: a test to reproduce the issue to prevent regression in the future. Please add a failing test that this PR fixes ❤️

[joelhawksley]

Looks good to me, mind looking at the CI errors?