{"record":{"id":"704f18676826fc42","repo":"leptos-rs/leptos","slug":"internal-error-entered-unreachable-code-704f18","errorCode":null,"errorMessage":"internal error: entered unreachable code","messagePattern":"internal error: entered unreachable code","errorType":"panic","errorClass":null,"httpStatus":null,"severity":"error","filePath":"tachys/src/reactive_graph/style.rs","lineNumber":171,"sourceCode":"            if let Some(mut state) = prev {\n                value.rebuild(&mut state);\n                state\n            } else {\n                value.build(&el)\n            }\n        })\n    }\n\n    fn rebuild(mut self, state: &mut Self::State) {\n        let prev_value = state.take_value();\n        *state = RenderEffect::new_with_value(\n            move |prev| {\n                let value = self.invoke();\n                if let Some(mut state) = prev {\n                    value.rebuild(&mut state);\n                    state\n                } else {\n                    unreachable!()\n                }\n            },\n            prev_value,\n        );\n    }\n\n    fn into_cloneable(self) -> Self::Cloneable {\n        self.into_shared()\n    }\n\n    fn into_cloneable_owned(self) -> Self::CloneableOwned {\n        self.into_shared()\n    }\n\n    fn dry_resolve(&mut self) {\n        self.invoke();\n    }\n","sourceCodeStart":153,"sourceCodeEnd":189,"githubUrl":"https://github.com/leptos-rs/leptos/blob/32d20f6c9d517451d77beb68d88c7716823dac41/tachys/src/reactive_graph/style.rs#L153-L189","documentation":"`rebuild()` for a reactive style binding (`value.rebuild(&mut state)`) asserts the effect's previous style state is `Some` so changed styles can be diffed against the DOM. The `unreachable!()` panic means the effect ran with `prev == None`: the seed value was absent at creation (non-seeding effect, unresolved async value, or value already taken).","triggerScenarios":"A signal driving `style:` bindings updates while the style effect's value slot is empty — effect re-run before `build`/`hydrate` stored `prev_value`, an async-seeded effect firing early, or a rebuild after `reset` took the value.","commonSituations":"Style bindings reacting to data fetched right after hydration; view branch swaps (`Show`/`Either`) resetting style state and then updating the same signal; SSR-hydrated pages where a reactive style fires during hydration.","solutions":["Defer style-driving signal updates until after the view is fully built/hydrated.","Initialize style effects with `RenderEffect::new_with_value(f, initial_style_state)` in custom code.","Ensure `reset`/`rebuild` ordering: never rebuild a state whose value was taken without reseeding.","Update leptos/tachys to a release with the effect-seeding fix."],"exampleFix":"// before\nRenderEffect::new_with_value(move |prev| { /* expects Some */ }, prev_value /* may be None */);\n// after\nRenderEffect::new_with_value(\n    move |prev: Option<StyleState>| { /* Some guaranteed */ },\n    prev_value.unwrap_or_else(StyleState::initial),\n);","handlingStrategy":"fallback","validationCode":"// Defer style updates until mounted\nlet mounted = RwSignal::new(false);\ncreate_effect(move |_| {\n    if !mounted.get() { return; }\n    let _ = style_signal.get();\n});\n// mounted.set(true) in on_mount","typeGuard":"fn style_prev_present(prev: &Option<StyleState>) -> bool { prev.is_some() }","tryCatchPattern":"// Panic; diagnostics via hook\nstd::panic::set_hook(Box::new(|info| {\n    if info.to_string().contains(\"unreachable\") {\n        leptos::logging::error!(\"style rebuild without prev state: {info}\");\n    }\n}));","preventionTips":["Avoid signal writes to style sources before hydration completes","Never rebuild a style state after reset took its value without reseeding","Test theme switches that fire style signals during load","Use the newest leptos release with the effect seeding fix"],"tags":["panic","reactivity","style-binding","leptos","tachys"],"backgroundTag":"unreachable-effect-previous-state-panicked","analyzedSha":"32d20f6c9d517451d77beb68d88c7716823dac41","analyzedAt":"2026-09-01T18:55:42.580Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-09T01:17:15.007Z"}