slint-ui/slint · error

Component should cleanly upgrade here

Error message

Component should cleanly upgrade here

What it means

This panic occurs in object_tree.rs:4968 during the component snapshotting/restore logic. The code maps entries of `use_component` weak references back to strong `Component` references and asserts the `Weak::upgrade` succeeds — the referenced component must still be alive at this point in the pipeline. If it was already dropped, the compiler's ownership invariant around component snapshots is violated.

Solutions

1. Retry compilation with the latest stable Slint compiler version to rule out a known regression.
2. Simplify the .slint import graph — remove cyclic or redundant imports that may interact with the snapshot mechanism.
3. For compiler contributors: verify `use_component` entries are kept alive (strong refs held) until this restore point runs.
4. File a minimal reproducer as a compiler bug.

// Prefer flat, acyclic import graphs
fn validate_imports(files: &[String]) -> Result<(), String> {
    // reject duplicate and cyclic imports before invoking the compiler
    // e.g. build a graph and check with toposort
    Ok(())
}

When it happens

Trigger: Running the compiler pass that restores recorded component usage (rebuilding a map from en -> Either<Weak<Component>, ...>) when a recorded weak component reference has been freed — e.g. components dropped between snapshot and restore due to a pass-ordering bug or re-entrant compilation of the same document.

Common situations: Compiler development touching the snapshot/restore mechanism (e.g. around `compile_with_builtin_libraries` or dependency tracking); nightly compiler builds; unusual import graphs that cause components to be dropped early.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• constraint must have binding
• EmbedTextures builds the shared font collection
• internal compiler error: more than one strong reference…
• LibGlobal not found
• named struct declarations have symbol counters

AI-assisted analysis of slint-ui/slint@3a7e700487 (2026-09-16). Data as JSON: /api/errors/4fba559ccc4434a1. Report an issue: GitHub.

Appendix: source

Thrown at internal/compiler/object_tree.rs:5028

    }

    pub fn retain(
        &mut self,
        func: impl FnMut(&mut (ExportedName, Either<Rc<Component>, Type>)) -> bool,
    ) {
        self.components_or_types.retain_mut(func)
    }

    pub(crate) fn snapshot(&self, snapshotter: &mut crate::typeloader::Snapshotter) -> Self {
        let components_or_types = self
            .components_or_types
            .iter()
            .map(|(en, either)| {
                let en = en.clone();
                let either = match either {
                    itertools::Either::Left(l) => itertools::Either::Left({
                        Weak::upgrade(&snapshotter.use_component(l))
                            .expect("Component should cleanly upgrade here")
                    }),
                    itertools::Either::Right(r) => itertools::Either::Right(r.clone()),
                };
                (en, either)
            })
            .collect();

        Self { components_or_types }
    }
}

impl std::iter::IntoIterator for Exports {
    type Item = (ExportedName, Either<Rc<Component>, Type>);

    type IntoIter = std::vec::IntoIter<Self::Item>;

    fn into_iter(self) -> Self::IntoIter {
        self.components_or_types.into_iter()

View on GitHub (pinned to 3a7e700487)