fix(parsing): reject a comment with no preceding whitespace

A '#' starts a comment only after a space or tab, or at the start of a
line. A '#' adjacent to the previous token is not a comment: 'key: "v"#c',
'[ a ]#c', '[ a,#c', and a '>#c' block-scalar header are malformed, but
the port (like libyaml) accepted them.

Validate the character immediately before the '#' (a plain scalar already
consumes its own trailing blanks, so an adjacent '#' the next-token scan
sees genuinely had no separator). Applied in scan_to_next_token and the
block-scalar header tail.

Surpasses libyaml on 9JBA, CVW2, SU5Z, X4QW; whole-suite conformance
388/402. Canonical event stream stays 308/308.

CHANGELOG.md

@@ -22,6 +22,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Reject a comment that is not preceded by whitespace. A `#` only begins a comment
+  after a space, a tab, or at the start of a line; a `#` adjacent to the previous
+  token (`key: "value"# c`, `[ a ]#c`, `[ a,#c`, a `>#c` block scalar header) is
+  malformed. The check uses the character immediately before the `#`, since a plain
+  scalar consumes its own trailing blanks. libyaml accepts the adjacent form.
+  Surpasses libyaml on 9JBA, CVW2, SU5Z, and X4QW. Whole-suite conformance is now
+  388/402.
 - Reject a directive that follows a document without an explicit `...` end marker.
   Directives apply to the document that follows, so the previous document must be
   closed with `...` before them; after an implicit end a `%YAML`/`%TAG`/reserved

Sources/Parsing/LibYAML.Scanner.BlockScalar.swift

@@ -96,6 +96,11 @@ extension PureYAML.Parsing.LibYAML.Scanner {
             reader.skip()
         }
         if reader.peek() == "#" {
+            // A comment needs a preceding space, so `>#` (a `#` adjacent to the
+            // block scalar indicator) is malformed, not a comment. libyaml accepts it.
+            if reader.column > 0, let previous = reader.peek(-1), !Classify.isBlank(previous) {
+                throw blockError("found a comment without a preceding space", startMark)
+            }
             while !Classify.isBreakZ(reader.peek()) {
                 reader.skip()
             }

Sources/Parsing/LibYAML.Scanner.Fetch.swift

@@ -19,7 +19,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
     /// the block context. A tab is valid in-line separation, so it is skipped
     /// here; `tabbedSeparation` records when one preceded the token so a block
     /// indicator or implicit key (where the tab would be indentation) can reject.
-    private mutating func scanToNextToken() {
+    private mutating func scanToNextToken() throws {
         while true {
             tabbedSeparation = false
             if reader.column == 0, reader.peek() == "\u{FEFF}" {
@@ -41,9 +41,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
                 }
             }
             if reader.peek() == "#" {
-                while !Classify.isBreakZ(reader.peek()) {
-                    reader.skip()
-                }
+                try consumeComment()
             }
             if Classify.isBreak(reader.peek()) {
                 reader.skipLine()
@@ -56,6 +54,24 @@ extension PureYAML.Parsing.LibYAML.Scanner {
         }
     }
 
+    /// Consume a comment to the end of the line. A comment must be preceded by
+    /// whitespace or start the line; a `#` adjacent to the previous token (e.g.
+    /// `"x"#`, `]#`, `,#`) is not a comment. The plain-scalar scanner consumes its
+    /// own trailing blanks, so the character just before the `#` is the reliable
+    /// signal. libyaml accepts the adjacent form.
+    private mutating func consumeComment() throws {
+        if reader.column > 0, let previous = reader.peek(-1), !Classify.isBlank(previous) {
+            throw ScannerError(
+                context: "while scanning for the next token",
+                problem: "found a comment without a preceding space",
+                mark: reader.mark,
+            )
+        }
+        while !Classify.isBreakZ(reader.peek()) {
+            reader.skip()
+        }
+    }
+
     /// Port of `yaml_parser_fetch_next_token`: emit STREAM-START first, otherwise
     /// skip to the next token, expire stale simple keys, unroll the indentation to
     /// the current column, and dispatch on the leading character. Constructs not
@@ -66,7 +82,7 @@ extension PureYAML.Parsing.LibYAML.Scanner {
             return
         }
 
-        scanToNextToken()
+        try scanToNextToken()
         mark = reader.mark
         try staleSimpleKeys()
         indentEngine.unrollIndent(column: reader.column, mark: reader.mark)

Tests/Fixtures/yaml-test-suite-manifest.yaml

@@ -10,8 +10,8 @@ source:
 # oracle:   json if in.json present, else none
 # status:   passing (PureYAML conformant today) | gap (not yet conformant)
 total: 402
-passing: 384
-gap: 18
+passing: 388
+gap: 14
 cases:
   - id: "229Q"
     name: "Spec Example 2.4. Sequence of Mappings"
@@ -1034,7 +1034,7 @@ cases:
     localPath: yaml-test-suite/9JBA/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 25
     lineCount: 2
   - id: "9KAX"
@@ -1354,7 +1354,7 @@ cases:
     localPath: yaml-test-suite/CVW2/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 25
     lineCount: 3
   - id: "CXX2"
@@ -2722,7 +2722,7 @@ cases:
     localPath: yaml-test-suite/SU5Z/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 30
     lineCount: 1
   - id: "SU74"
@@ -3002,7 +3002,7 @@ cases:
     localPath: yaml-test-suite/X4QW/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 27
     lineCount: 2
   - id: "X8DW"