From 52c4dfd8c3f93502245296205298d86169693a6b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ren=C3=A9=20Kijewski?= <rene.kijewski@fu-berlin.de>
Date: Mon, 25 Aug 2025 01:21:09 +0200
Subject: [PATCH] parser: reject `extends` below top-level, too

---
 askama_parser/src/node.rs                     | 45 ++++++++++---------
 .../tests/ui/blocks_below_top_level.stderr    |  4 +-
 2 files changed, 27 insertions(+), 22 deletions(-)

diff --git a/askama_parser/src/node.rs b/askama_parser/src/node.rs
index 27d1797d..37ad661e 100644
--- a/askama_parser/src/node.rs
+++ b/askama_parser/src/node.rs
@@ -38,8 +38,27 @@ pub enum Node<'a> {
 
 impl<'a: 'l, 'l> Node<'a> {
     pub(super) fn parse_template(i: &mut InputStream<'a, 'l>) -> ParseResult<'a, Vec<Box<Self>>> {
-        let mut p = parse_with_unexpected_fallback(Self::many, unexpected_tag);
-        let nodes = p.parse_next(i)?;
+        let mut nodes = vec![];
+        let mut allow_extends = true;
+        while let Some(node) = parse_with_unexpected_fallback(
+            opt(move |i: &mut _| Self::one(i, allow_extends)),
+            unexpected_tag,
+        )
+        .parse_next(i)?
+        {
+            if allow_extends {
+                match &*node {
+                    // Since comments don't impact generated code, we allow them before `extends`.
+                    Node::Comment(_) => {}
+                    // If it only contains whitespace characters, it's fine too.
+                    Node::Lit(lit) if lit.val.is_empty() => {}
+                    // Everything else must not come before an `extends` block.
+                    _ => allow_extends = false,
+                }
+            }
+            nodes.push(node);
+        }
+
         if !i.is_empty() {
             opt(unexpected_tag).parse_next(i)?;
             return cut_error!(
@@ -53,27 +72,13 @@ impl<'a: 'l, 'l> Node<'a> {
     }
 
     fn many(i: &mut InputStream<'a, 'l>) -> ParseResult<'a, Vec<Box<Self>>> {
-        let mut nb_nodes = 0;
-
-        repeat(0.., move |i: &mut _| Self::parse_nodes(i, &mut nb_nodes))
-            .map(|v: Vec<_>| v)
-            .parse_next(i)
+        repeat(0.., |i: &mut _| Self::one(i, false)).parse_next(i)
     }
 
-    fn parse_nodes(
-        i: &mut InputStream<'a, 'l>,
-        nb_nodes: &mut usize,
-    ) -> ParseResult<'a, Box<Self>> {
+    fn one(i: &mut InputStream<'a, 'l>, allow_extends: bool) -> ParseResult<'a, Box<Self>> {
         let node = alt((Lit::parse, Comment::parse, Self::expr, Self::parse)).parse_next(i)?;
-        match *node {
-            Self::Extends(..) if *nb_nodes > 0 => {
-                return cut_error!("`extends` block must come first in a template", node.span());
-            }
-            // Since comments don't impact generated code, we allow them before `extends`.
-            Self::Comment(..) => {}
-            // If it only contains whitespace characters, it's fine too.
-            Self::Lit(ref lit) if lit.val.trim().is_empty() => {}
-            _ => *nb_nodes += 1,
+        if !allow_extends && let Node::Extends(node) = &*node {
+            return cut_error!("`extends` block must come first in a template", node.span());
         }
         Ok(node)
     }
diff --git a/testing/tests/ui/blocks_below_top_level.stderr b/testing/tests/ui/blocks_below_top_level.stderr
index ce1a084f..d7e932c3 100644
--- a/testing/tests/ui/blocks_below_top_level.stderr
+++ b/testing/tests/ui/blocks_below_top_level.stderr
@@ -1,5 +1,5 @@
-error: `extends` blocks are not allowed below top level
- --> MyTemplate1.txt:3:2
+error: `extends` block must come first in a template
+ --> <source attribute>:3:2
        " extends \"bla.txt\" %}\n{% endblock %}\n"
  --> tests/ui/blocks_below_top_level.rs:4:21
   |