[Harsh23Kashyap]

Closes #7359

parse() skips a tab at the start of a line like any other whitespace, so the line is read at the wrong depth. parse("a:\n\tb: 1") returns { a: null, b: 1 } instead of throwing.

This records when a tab shows up before any content on a line, and throws a YamlSyntaxError if that line starts a block mapping or sequence entry. Tabs after a colon or dash, in comments, and on blank lines still parse as before.

Before, on main:

"a:\n\tb: 1"       -> {"a":null,"b":1}
"a:\n  b:\n\tc: 2" -> {"a":{"b":null},"c":2}
"a:\n \tb: 1"      -> {"a":{"b":1}}

After:

"a:\n\tb: 1"       THROWS Tab characters must not be used in indentation at line 2, column 2
"a:\n  b:\n\tc: 2" THROWS Tab characters must not be used in indentation at line 3, column 2
"a:\n \tb: 1"      THROWS Tab characters must not be used in indentation at line 2, column 3

js-yaml and the yaml npm package reject the same three inputs. One correction to the issue: "- a\n-\tb" is accepted by both of them, because the tab there separates the dash from the value, so I left that case alone and added a test that it still parses.

deno test yaml/ front_matter/ passes, and the new test fails without the change. deno fmt, deno lint and deno check are clean for yaml/.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.04%. Comparing base (f834d02) to head (b19e862).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #7366    +/-   ##
========================================
  Coverage   95.04%   95.04%            
========================================
  Files         619      618     -1     
  Lines       52012    51773   -239     
  Branches     9450     9407    -43     
========================================
- Hits        49434    49209   -225     
+ Misses       2031     2022     -9     
+ Partials      547      542     -5     

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