Trailing comments (#7441)

Carbon currently requires a comment to be the only non-whitespace on its
line. A `//` comment that follows other content on a line, called a
_trailing comment_, is a lexer error. This proposal removes that
restriction, allowing a comment to follow other content on a line.
Everything else about comments is unchanged: a comment still begins with
`//`, still requires whitespace after the `//`, and still runs to the
end of the line. Carbon continues to provide only line comments; no
block or intra-line comments are added.

Three observations motivate the change. First, trailing comments are
well suited to short _annotations_ attached to a specific entity or
value on a line. Second, the lexer design now makes it trivial to lex
trailing comments, and in fact requires extra logic and potentially cost
to reject them. Third, C++ code routinely uses trailing comments, so
allowing them lets Carbon carry the layout of migrated code over
directly, rather than reworking each comment to read well in a different
structure.

Implementation notes (beyond the proposal's design):

Keeping trailing comments cheap to lex required a few supporting
changes, all of which keep the cost off the lexer's hot path:

- The lexer already dispatches `//` to comment lexing wherever it
appears, so classifying a comment as trailing is a single O(1) check of
whether the `//` is the line's first non-whitespace (`start + indent`).
The hot comment path is otherwise unchanged.

- That check relies on each line's recorded indentation being its real
leading whitespace. Multi-line string literals previously recorded the
column where the literal opened for the lines they span; they now record
the true (closing-delimiter) indentation instead.

- Parser error recovery (`SkipPastLikelyEnd`) had relied on that
opening-column indentation to keep tokens following a multi-line string
literal attached to the same construct. It now reconstructs that
relationship directly by consulting the line on which the literal
opened, including when other tokens follow the closing delimiter (such
as `''' + "more"`). This is on the cold recovery path.

- `CommentData` records the trailing bit in the high bit of its length
field, keeping it at 8 bytes.

Assisted-by: Claude Code

toolchain/lex/tokenized_buffer.cpp

@@ -413,16 +413,33 @@ auto TokenizedBuffer::GetCommentText(CommentIndex comment_index) const
   return source_->text().substr(comment_data.start, comment_data.length);
 }
 
-auto TokenizedBuffer::AddComment(int32_t indent, int32_t start, int32_t end)
-    -> void {
-  if (comments_.size() > 0) {
+auto TokenizedBuffer::IsTrailingComment(CommentIndex comment_index) const
+    -> bool {
+  return comments_.Get(comment_index).is_trailing;
+}
+
+auto TokenizedBuffer::AddComment(int32_t indent, int32_t start, int32_t end,
+                                 bool is_trailing) -> void {
+  // A comment runs forward from its start, and its length is stored in 31 bits
+  // (the high bit holds `is_trailing`). A non-negative byte offset always fits
+  // in 31 bits because the source size is bounded by `INT32_MAX`.
+  CARBON_DCHECK(start <= end);
+
+  // A block of identical full-line comments is coalesced into a single comment.
+  // A trailing comment is always standalone: it never extends a preceding
+  // comment, nor is it extended by a following one.
+  if (!is_trailing && comments_.size() > 0) {
     auto& comment = comments_.Get(CommentIndex(comments_.size() - 1));
-    if (comment.start + comment.length + indent == start) {
+    if (!comment.is_trailing &&
+        comment.start + comment.length + indent == start) {
+      CARBON_DCHECK(comment.start <= end);
       comment.length = end - comment.start;
       return;
     }
   }
-  comments_.Add({.start = start, .length = end - start});
+  comments_.Add({.start = start,
+                 .length = static_cast<uint32_t>(end - start),
+                 .is_trailing = is_trailing});
 }
 
 auto TokenizedBuffer::CollectMemUsage(MemUsage& mem_usage,