Don't allow headings to span multiple lines

src/parse/incremental.rs

@@ -364,7 +364,7 @@ mod tests {
         test("hey #myfriend", 4 .. 4, "\\", 0 .. 14);
         test("hey  #myfriend", 4 .. 4, "\\", 3 .. 6);
         test("= foo\nbar\n - a\n - b", 6 .. 9, "", 0 .. 11);
-        test("= foo\n  bar\n  baz", 6 .. 8, "", 0 .. 15);
+        test("= foo\n  bar\n  baz", 6 .. 8, "", 0 .. 9);
     }
 
     #[test]

src/parse/mod.rs

@@ -161,6 +161,11 @@ fn markup(p: &mut Parser, mut at_start: bool) {
     });
 }
 
+/// Parse a single line of markup.
+fn markup_line(p: &mut Parser) {
+    markup_indented(p, usize::MAX);
+}
+
 /// Parse markup that stays right of the given `column`.
 fn markup_indented(p: &mut Parser, column: usize) {
     p.eat_while(|t| match t {
@@ -272,8 +277,7 @@ fn heading(p: &mut Parser, at_start: bool) {
     while p.eat_if(&NodeKind::Eq) {}
 
     if at_start && p.peek().map_or(true, |kind| kind.is_space()) {
-        let column = p.column(p.prev_end());
-        markup_indented(p, column);
+        markup_line(p);
         marker.end(p, NodeKind::Heading);
     } else {
         let text = p.get(current_start .. p.prev_end()).into();

tests/typ/structure/heading.typ

@@ -26,19 +26,15 @@ No = heading
 \= No heading
 
 ---
-// While indented at least as much as the start, the heading continues.
+// Blocks can continue the heading.
+
+= [This
+is
+multiline.
+]
 
 = This
-  is
-    indented.
-
-=  This
-is not.
-
-// Code blocks continue heading.
-= A {
-  "B"
-}
+  is not.
 
 ---
 // Test styling.