jsc: preserve tabs in runtime error caret padding

[Ersaoktaviannn]

Summary

• preserve tabs from source lines when positioning runtime error carets
• account for supplementary-plane codepoints when building caret padding
• add regression coverage for tab-indented runtime errors

Root cause

Runtime error source lines retain literal tabs, but the caret prefix was rendered entirely as spaces. Terminals therefore expanded the source and caret prefixes differently, placing the caret several columns to the left.

The updated padding logic walks the displayed UTF-8 source by codepoint, emits tabs at their original logical positions, and emits two padding cells for supplementary-plane codepoints.

Testing

• cargo fmt --all --check
• git diff --check
• verified both regression cases fail against Bun 1.3.14 with the previous all-space padding

Fixes #10857

[claude]

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Reviewing files that changed from the base of the PR and between 9a543cc and 6cbbe8a.

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• src/jsc/VirtualMachine.rs
• test/js/bun/test/stack.test.ts

---

Walkthrough

The error renderer now aligns carets with tabs and supplementary Unicode characters by using UTF-16 columns. Regression tests verify caret placement and process exit behavior.

Changes

Source error caret alignment

Layer / File(s)	Summary
UTF-16-aware caret rendering src/jsc/VirtualMachine.rs, test/js/bun/test/stack.test.ts	write_caret_padding preserves tabs and accounts for supplementary Unicode characters. Error rendering uses the helper, and regression tests verify caret alignment, empty stdout, and exit status 1.

Suggested reviewers: robobun, cirospaciari, dylan-conway

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the primary change: preserving tabs in runtime error caret padding.
Description check	✅ Passed	The description explains the changes, root cause, verification steps, and linked issue.
Linked Issues check	✅ Passed	The changes address issue #10857 by preserving tab spacing and aligning runtime error carets correctly.
Out of Scope Changes check	✅ Passed	The implementation and regression tests remain within the scope of the linked issue and stated objectives.

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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 `@test/js/bun/test/stack.test.ts`:
- Around line 88-107: Extend the test named “runtime error caret preserves tabs
before the error column (`#10857`)” with a supplementary-plane Unicode character
and a tab before error(). Assert the resulting source and caret lines, including
the expected UTF-16-width column alignment, so the cursor.c > 0xFFFF branch is
covered while preserving the existing stdout and exit behavior.

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• src/jsc/VirtualMachine.rs
• test/js/bun/test/stack.test.ts

[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/jsc/VirtualMachine.rs`:
- Around line 6032-6039: Update the padding loop near codepoints_written and
codepoint_width so codepoints_written advances by codepoint_width rather than a
fixed 1, preserving UTF-16 column accounting for supplementary code points while
retaining the existing tab and space output behavior.

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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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Reviewing files that changed from the base of the PR and between 048b02f and 7af8423.

📒 Files selected for processing (2)

• src/jsc/VirtualMachine.rs
• test/js/bun/test/stack.test.ts

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