Denote empty files in stack traces

[edolstra]

Motivation

Instead of

error: syntax error, unexpected end of file
 at «git+file:///.../nixos?rev=0defd19d32413f757d7273596cd9641cb91af25a&shallow=1&submodules=1»/configuration.nix:1:1:

(note the : followed by nothing at the end of the line), Nix now shows

error: syntax error, unexpected end of file
at «git+file:///home/eelco/Dev/nix-master/nixos?rev=0defd19d32413f757d7273596cd9641cb91af25a&shallow=1&submodules=1»/configuration.nix:1:1:
    1| (empty file)

Context

Summary by CodeRabbit

• Bug Fixes
  • Improved error-message display when source code context is unavailable: shows a clear "(empty file)" placeholder instead of omitting the code line.
  • Refined error position formatting so the "at :" header appears only when code lines are present, ensuring more consistent and readable error reports.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 4ccafad9-5618-4fc1-a884-9b455f8687cc

📥 Commits

Reviewing files that changed from the base of the PR and between 5014f42 and 8a568b3.

📒 Files selected for processing (1)

• src/libutil/error.cc

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📝 Walkthrough

Walkthrough

Error formatting improvements in src/libutil/error.cc handle unavailable line-of-code data by printing an italic "(empty file)" placeholder and adjusting the position header punctuation. printCodeLines now emits the placeholder when no errLineOfCode exists; printPosMaybe caches pos->getCodeLines() and only prints the colon and code lines when available.

Changes

Error Output Formatting

Layer / File(s)	Summary
Missing code line handling and header formatting src/libutil/error.cc	printCodeLines adds an else branch that outputs an italic "(empty file)" placeholder when loc.errLineOfCode is absent. printPosMaybe caches pos->getCodeLines() once and appends : to the "at " header only when code lines are available, and uses the cached value to decide whether to print code lines.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A line that vanished from the file,
I print a gentle, italic smile—
"(empty file)" where nothing shows,
Now errors tell what no code knows.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Denote empty files in stack traces' clearly and concisely summarizes the main change: adding explicit notation for empty files in error stack traces instead of omitting them.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch render-empty-files

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/libutil/error.cc`:
- Around line 102-104: The else branch prints an "(empty file)" line but wrongly
dereferences the optional loc.errLineOfCode causing UB; update the formatting in
the else path (the block that writes to out using prefix, errPos.line and
*loc.errLineOfCode) to not dereference loc.errLineOfCode when has_value() is
false—use loc.errLineOfCode.value_or("") or pass an empty placeholder string (or
adjust the fmt call to omit that argument) so the output uses a safe empty
string instead of *loc.errLineOfCode.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c24a406c-4f9f-4bc2-8a06-04600aa80336

📥 Commits

Reviewing files that changed from the base of the PR and between 5d4c99f and 5014f42.

📒 Files selected for processing (1)

• src/libutil/error.cc

[github-actions]

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