fix(parsing): end a plain scalar at a tab before a comment

A comment opens after white space, a tab as well as a space, but the plain
scalar terminator checked only for a space before `#`. So `c: d\t#X` kept the
tab and hash as content, giving the value `d\t#X` rather than `d` with a
trailing comment.

Treat a tab before `#` as a space there, in both the lookahead from a white
space character and the check on the accumulated value's last character. A tab
inside a plain scalar with no `#` after it stays content, and a `#` glued to
content with no white space is still content.

Raises yaml-test-suite value-level conformance to 187/194 (96.4%) with the
parse-level and should-error rejection rates unchanged. 448 tests pass.

CHANGELOG.md

@@ -84,6 +84,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A tab before a `#` now ends a plain scalar and begins a trailing comment, as a space already did,
+  since a comment opens after any white space. `c: d\t#X` is the value `d` with a comment, not the
+  literal `d\t#X`. A tab inside a plain scalar with no `#` after it stays part of the value, and a `#`
+  glued to content with no white space is still content. Raises yaml-test-suite value-level
+  conformance to 187/194 (96.4%), with the parse and should-error rates unchanged.
+
 - A blank line next to a more-indented line in a folded block scalar (`>`) now keeps both line
   breaks. Between two non-more-indented lines a blank run of N folds to N breaks (the fold is
   absorbed into the first), but next to a more-indented line nothing folds, so the absorbed break

Sources/Parsing/Scanner.Helpers.swift

@@ -279,10 +279,12 @@ extension PureYAML.Parsing.Scanner {
         if isLineBreak(character) {
             return true
         }
-        if character == " ", state.reader.peek(offset: 1) == "#" {
+        // A comment opens after white space, a tab as well as a space, so a `#` preceded by either
+        // ends the plain scalar: `d\t#X` is the value `d` with a trailing comment.
+        if character == " " || character == "\t", state.reader.peek(offset: 1) == "#" {
             return true
         }
-        if character == "#", accumulated.isEmpty || accumulated.last == " " {
+        if character == "#", accumulated.isEmpty || accumulated.last == " " || accumulated.last == "\t" {
             return true
         }
         if character == ":", shouldEndPlainScalarAtMappingValue(state: state) {

Tests/PlainScalarTabCommentTests.swift

@@ -0,0 +1,36 @@
+@testable import PureYAML
+import Testing
+
+/// A comment opens after white space, a tab as well as a space, so a `#` preceded by a tab ends a
+/// plain scalar and begins a trailing comment. A tab inside a plain scalar, with no `#` after it,
+/// stays part of the value, and a `#` glued directly to content (no white space) is content.
+@Suite("Plain scalar tab comment")
+struct PlainScalarTabCommentTests {
+    @Test("A tab before a hash ends the scalar and starts a comment")
+    func test_tabBeforeComment() throws {
+        #expect(try PureYAML.parse("c: d\t#X\n") == .mapping(.init([
+            .init(key: "c", value: .string("d")),
+        ])))
+    }
+
+    @Test("A tab inside a plain scalar value is kept")
+    func test_internalTabKept() throws {
+        #expect(try PureYAML.parse("x: a\tb\n") == .mapping(.init([
+            .init(key: "x", value: .string("a\tb")),
+        ])))
+    }
+
+    @Test("A hash glued to content with no white space stays content")
+    func test_hashWithoutWhitespaceIsContent() throws {
+        #expect(try PureYAML.parse("c: d#X\n") == .mapping(.init([
+            .init(key: "c", value: .string("d#X")),
+        ])))
+    }
+
+    @Test("A space before a hash still ends the scalar")
+    func test_spaceBeforeCommentUnchanged() throws {
+        #expect(try PureYAML.parse("c: d #X\n") == .mapping(.init([
+            .init(key: "c", value: .string("d")),
+        ])))
+    }
+}