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Fix block partial rendering #1054

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Fix block partial rendering #1054

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8b98e66
Fix block partial rendering
GuillaumeGomez May 21, 2024
e44b0ca
Add code comment to explain discard condition
GuillaumeGomez May 23, 2024
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5 changes: 5 additions & 0 deletions askama_derive/src/generator.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,6 +90,7 @@ impl<'a> Generator<'a> {
buf.write(CRATE);
buf.writeln("::Result<()> {")?;

buf.discard = self.buf_writable.discard;
// Make sure the compiler understands that the generated code depends on the template files.
for path in self.contexts.keys() {
// Skip the fake path of templates defined in rust source.
Expand All @@ -113,6 +114,7 @@ impl<'a> Generator<'a> {
} else {
self.handle(ctx, ctx.nodes, buf, AstLevel::Top)
}?;
buf.discard = false;

self.flush_ws(Ws(None, None));
buf.write(CRATE);
Expand Down Expand Up @@ -987,6 +989,9 @@ impl<'a> Generator<'a> {
if block_fragment_write {
self.buf_writable.discard = true;
}
if buf.discard != prev_buf_discard {
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This seems surprising to me -- why do we only want to write if buf.discard is not equal to prev_buf_discard vs writing it out if buf.discard is just false?

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Interesting question. In my previous PR about this, if I wrote too early, we had content from sub-block that was rendered before its parent's, which was problematic. So in this case, if buf.discard != prev_buf_discard, then it's like that the parent is actually not something being displayed and so we should flush everything before the discard is true again.

Does my reasoning sound good to you and do you see potential corner-cases?

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Just thought: do you want me to add a code comment to explain this?

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Yes, that would be a good improvement.

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Done!

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This comment is too long IMO and still doesn't make it very clear what case this is trying to fix/prevent. I want a comment here in like 4-5 lines that explains conceptually what case it is trying to handle.

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Updated the comment.

self.write_buf_writable(buf)?;
}
buf.discard = prev_buf_discard;

Ok(size_hint)
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18 changes: 18 additions & 0 deletions testing/tests/block_fragments.rs
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Expand Up @@ -103,3 +103,21 @@ fn test_specific_block() {
let t = RenderInPlace { s1 };
assert_eq!(t.render().unwrap(), "\nSection: [abc]\n");
}

#[derive(Template)]
#[template(
source = r#"{% block empty %}
{% endblock %}

{% if let Some(var) = var %}
{{ var }}
{% endif %}"#,
block = "empty",
ext = "txt"
)]
struct Empty {}

#[test]
fn test_render_only_block() {
assert_eq!(Empty {}.render().unwrap(), "\n");
}