Fix block scalar chomping edge case for EOF without newline

[jonasfj]

The YAML 1.2.2 specification (Section 8.1.1.2: Block Chomping Indicator) defines the rules for handling final line breaks in block scalars. The production rule [165] b-chomped-last(t) dictates that when a block scalar terminates at the end of a file (<end-of-input>), the EOF itself acts identically to a physical line break (b-as-line-feed). Therefore, under the default "clip" chomping behavior, this implicit final line break must be retained as part of the string content.

Detailed specification references

The YAML 1.2.2 specification says about this in Section 8.1.1.2: Block Chomping Indicator.

The specification explicitly addresses how the final line break of a block scalar is handled, even if the file abruptly ends.

The Chomping Indicators

The spec defines the three possible chomping behaviors (production [164]):

[164]
c-chomping-indicator(STRIP) ::= '-'
c-chomping-indicator(KEEP)  ::= '+'
c-chomping-indicator(CLIP)  ::= ""

If you write |-, it's STRIP. If you write |+, it's KEEP. If you just write | (an empty string for the indicator), it defaults to CLIP.

The Final Line Break Rule

The spec explains how the very last character of the block scalar is evaluated (production [165]):

"The interpretation of the final line break of a block scalar is controlled by the chomping indicator specified in the block scalar header."

[165]
b-chomped-last(STRIP) ::= b-non-content  | <end-of-input>
b-chomped-last(CLIP)  ::= b-as-line-feed | <end-of-input>
b-chomped-last(KEEP)  ::= b-as-line-feed | <end-of-input>

Explanation of Rule [165]

The grammar for the default CLIP chomping is:
b-chomped-last(CLIP) ::= b-as-line-feed | <end-of-input>

This means that to successfully complete a block scalar under CLIP chomping, the parser must encounter EITHER:

1. b-as-line-feed (a physical newline character like \n).
2. <end-of-input> (the physical end of the file/stream).

Because they are both valid, parallel branches of the exact same production rule (b-chomped-last(CLIP)), the parser is mathematically required to treat <end-of-input> as functionally identical to a physical line feed for the purpose of concluding the string.

Implications for users

If a user writes:

foo: |
  text

And they omit the final trailing newline, the parser hits <end-of-input>. Because <end-of-input> fulfills the b-chomped-last(CLIP) requirement in the exact same way a physical line feed would, the parser must inject/retain the \n in the final parsed JSON (resulting in "text\n"), pretending the line feed was there.

---

Fix

• Block scalar chomping at EOF: Fixed a bug where block scalars ending at EOF without a physical trailing newline failed to include the implicit trailing newline mandated by the YAML specification.
• Implementation: leadingBreak now explicitly defaults to \n when the scanner hits EOF, ensuring the spec-mandated implicit final newline is preserved under default clip chomping rules.
• Test Suite: Removes L24T-1 from _expectedFailures as it now passes.

Implications

• Block scalars at the very end of files without a physical trailing newline will now correctly include their implicit final newline character.
• Validation: Tested against 1,051,993 YAML files from pub.dev. Exactly 310 cases parsed differently (correctly gaining the previously missing implicit newline) with 0 new parsing failures.

View diffs for the 310 changed YAML files.

[github-actions]

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Package	Version	Status	Publish tag (post-merge)
package:api_summary	0.1.0-wip	WIP (no publish necessary)
package:bazel_worker	1.1.5	already published at pub.dev
package:benchmark_harness	2.4.0	already published at pub.dev
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package:yaml_edit	2.2.4	already published at pub.dev

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[github-actions]

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[jonasfj]

From reading the changed YAML files on pub.dev, I'd suggest we have:

• build.yaml, ffigen.yaml where preamble gets an extra newline
• many .github/ files which we wouldn't be primary readers of anyway
• a few test files that were written in YAML, which depending on how they are used could
  cause problems.
• Some scripts and similar written in YAML where script: "dart pub get && dart test\n" gets what is hopefully a harmless newline.

[jonasfj]

Added skip-leaking-check because there are no public API changes.

[lrhn]

Does comment still apply?