leptos-rs/leptos · error

List of slots must not be empty

Error message

List of slots must not be empty

What it means

slot_to_tokens in leptos_macro collects component slot properties into a map and then drains it, taking .last() of each slot's values to get a span. If a slot key exists in the map with an empty values vec, the expect panics with 'List of slots must not be empty'. This is a compile-time panic inside the view! macro expansion.

Source

Thrown at leptos_macro/src/view/slot_helper.rs:171

                }
            } else {
                quote_spanned! {children.span()=>
                    .children({
                        #(#clonables)*

                        ::leptos::children::ToChildren::to_children(move || #children #view_marker)
                    })
                }
            }
        } else {
            quote! {}
        }
    };

    let slots = slots.drain().map(|(slot, mut values)| {
        let span = values
            .last()
            .expect("List of slots must not be empty")
            .span();
        let slot = Ident::new(&slot, span);
        let value = if values.len() > 1 {
            quote! {
                ::std::vec![
                    #(#values)*
                ]
            }
        } else {
            values.remove(0)
        };

        quote! { .#slot(#value) }
    });

    let build = quote_spanned! {node.name().span()=>
        .build()
    };

View on GitHub (pinned to 32d20f6c9d)

Solutions

1. Ensure each slot passed to the component actually contains content, e.g. <SlotName slot:A>{...}</SlotName> with children or attributes.
2. Update leptos_macro/leptos to matching versions (cargo update) — empty slot values may be a fixed macro bug.
3. Reduce the view! usage to a minimal repro and report it upstream if the slot is non-empty but still triggers the panic.

Example fix

// before
<Parent slot:header /> // empty slot
// after
<Parent slot:header><h1>"Title"</h1></Parent>

// check all slot properties have content before macro expansion
fn slot_has_content(inner: &str) -> bool { !inner.trim().is_empty() }

When it happens

Trigger: A slot entry is inserted into the slots map with no accumulated values — an internal invariant violation when processing component children with #[slot] attributes; typically triggered by malformed or duplicated slot handling in the view! macro (e.g. a slot property with zero children tokens).

Common situations: Using view! with a custom component taking #[slot] parameters where slot markup is empty or the macro version has a bug collecting slot values; mixing macro versions between leptos and leptos_macro.

Related errors

• element needs to have a name
• keyword argument repeated: `encoding`
• keyword argument repeated: `endpoint`
• `encoding` and `input` should not both be specified
• keyword argument repeated: `input`

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/7d2fcd3ea127a65b. Report an issue: GitHub.