tracel-ai/burn · error

Cannot add multiple attributes when single item is set.

Error message

Cannot add multiple attributes when single item is set.

What it means

Builder-state invariant violation in the module display system: a `Content` can hold either named attributes (added via `add`) or a single unnamed item (via `add_single`), but not both. This panic fires from `add` when `add_single` was already called on the same builder, i.e. the caller mixed the two mutually exclusive construction styles.

Source

Thrown at crates/burn-core/src/module/display.rs:422

            single_item: None,
            display_settings,
            top_level_type: None,
        }
    }

    /// Adds an attribute to the format settings. The value will be formatted and stored as a string.
    ///
    /// # Arguments
    ///
    /// * `name` - Name of the attribute.
    /// * `value` - Value of the attribute.
    ///
    /// # Returns
    ///
    /// Updated `Content` instance with the new attribute added.
    pub fn add<T: ModuleDisplay + ?Sized>(mut self, name: &str, value: &T) -> Self {
        if self.single_item.is_some() {
            panic!("Cannot add multiple attributes when single item is set.");
        }

        let attribute = Attribute {
            name: name.to_owned(),
            value: value.format(self.display_settings.clone()), // TODO level + 1
            ty: any::type_name::<T>().to_string(),
        };
        self.attributes.push(attribute);
        self
    }

    /// Adds an attribute using its `Debug` representation.
    ///
    /// This is intended for fields that do not implement [`ModuleDisplay`].
    ///
    /// # Arguments
    ///
    /// * `name` - Name of the attribute.

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Build the Content either with `add` for all named attributes or with `add_single` for the one rendered item, never both.
2. Create a fresh `Content` builder for the single-item case instead of reusing one that already had `add_single` called.
3. Refactor `custom_content` so each content block has one consistent construction path.

When it happens

Trigger: Thrown at crates/burn-core/src/module/display.rs:422 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/3ba16f08e4fc88d4. Report an issue: GitHub.