3 files changed, 58 insertions, 3 deletions

diff --git a/src/construct/heading_setext.rs b/src/construct/heading_setext.rs
index 2078338..cb426a9 100644
--- a/src/construct/heading_setext.rs
+++ b/src/construct/heading_setext.rs
@@ -167,7 +167,10 @@ fn inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind) -> StateFnResult {
             tokenizer.consume(code);
             (State::Fn(Box::new(move |t, c| inside(t, c, kind))), None)
         }
-        _ => tokenizer.attempt_opt(space_or_tab(), after)(tokenizer, code),
+        _ => {
+            tokenizer.exit(Token::HeadingSetextUnderline);
+            tokenizer.attempt_opt(space_or_tab(), after)(tokenizer, code)
+        }
     }
 }
 
@@ -180,7 +183,6 @@ fn inside(tokenizer: &mut Tokenizer, code: Code, kind: Kind) -> StateFnResult {
 fn after(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
     match code {
         Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
-            tokenizer.exit(Token::HeadingSetextUnderline);
             // Feel free to interrupt.
             tokenizer.interrupt = false;
             tokenizer.register_resolver("heading_setext".to_string(), Box::new(resolve));
diff --git a/src/token.rs b/src/token.rs
index 8169657..2e5e2df 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -1877,3 +1877,47 @@ pub enum Token {
     /// ```
     ThematicBreakSequence,
 }
+
+/// List of void tokens, used to make sure everything is working good.
+pub const VOID_TOKENS: [Token; 40] = [
+    Token::AttentionSequence,
+    Token::AutolinkEmail,
+    Token::AutolinkMarker,
+    Token::AutolinkProtocol,
+    Token::BlankLineEnding,
+    Token::BlockQuoteMarker,
+    Token::CharacterEscapeMarker,
+    Token::CharacterEscapeValue,
+    Token::CharacterReferenceMarker,
+    Token::CharacterReferenceMarkerHexadecimal,
+    Token::CharacterReferenceMarkerNumeric,
+    Token::CharacterReferenceMarkerSemi,
+    Token::CharacterReferenceValue,
+    Token::CodeFencedFenceSequence,
+    Token::CodeFlowChunk,
+    Token::CodeTextData,
+    Token::CodeTextLineEnding,
+    Token::CodeTextSequence,
+    Token::Data,
+    Token::DefinitionDestinationLiteralMarker,
+    Token::DefinitionLabelMarker,
+    Token::DefinitionMarker,
+    Token::DefinitionTitleMarker,
+    Token::EmphasisSequence,
+    Token::HardBreakEscapeMarker,
+    Token::HardBreakTrailingSpace,
+    Token::HeadingAtxSequence,
+    Token::HeadingSetextUnderline,
+    Token::HtmlFlowData,
+    Token::HtmlTextData,
+    Token::LabelImageMarker,
+    Token::LabelMarker,
+    Token::LineEnding,
+    Token::ListItemMarker,
+    Token::ListItemValue,
+    Token::ReferenceMarker,
+    Token::ResourceMarker,
+    Token::ResourceTitleMarker,
+    Token::StrongSequence,
+    Token::ThematicBreakSequence,
+];
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 10c877f..9b39e28 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -12,7 +12,7 @@
 //! [`check`]: Tokenizer::check
 
 use crate::parser::ParseState;
-use crate::token::Token;
+use crate::token::{Token, VOID_TOKENS};
 use std::collections::HashMap;
 
 /// Embedded content type.
@@ -385,6 +385,15 @@ impl<'a> Tokenizer<'a> {
             "expected non-empty token"
         );
 
+        if VOID_TOKENS.iter().any(|d| d == &token_type) {
+            assert!(
+                current_token == previous.token_type,
+                "expected token to be void (`{:?}`), instead of including `{:?}`",
+                current_token,
+                previous.token_type
+            );
+        }
+
         // A bit weird, but if we exit right after a line ending, we *don’t* want to consider
         // potential skips.
         if matches!(