{"record":{"id":"87d4408f5856a774","repo":"leptos-rs/leptos","slug":"internal-error-entered-unreachable-code-87d440","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/property.rs","lineNumber":66,"sourceCode":"                value.rebuild(&mut state, &key);\n                state\n            } else {\n                value.build(&el, &key)\n            }\n        })\n    }\n\n    fn rebuild(mut self, state: &mut Self::State, key: &str) {\n        let prev_value = state.take_value();\n        let key = key.to_owned();\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, &key);\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\nmacro_rules! property_reactive {\n    ($name:ident, <$($gen:ident),*>, $v:ty, $( $where_clause:tt )*) =>\n    {","sourceCodeStart":48,"sourceCodeEnd":84,"githubUrl":"https://github.com/leptos-rs/leptos/blob/32d20f6c9d517451d77beb68d88c7716823dac41/tachys/src/reactive_graph/property.rs#L48-L84","documentation":"`rebuild()` for a reactive property binding (`value.rebuild(&mut state, &key)`) asserts the effect's previous property state is `Some` so the DOM property can be diffed. The `unreachable!()` panic means the effect re-ran with `prev == None` — the seeded `prev_value` was missing (effect created without a value, async value unresolved, or value already taken).","triggerScenarios":"A signal driving `prop:{name}={...}` updates while the property effect has no stored previous value: rebuild before build/hydrate committed state, async-seeded effect firing early, or a second rebuild after the value was taken by `reset`.","commonSituations":"Property bindings updated during SSR hydration before the initial value is stored; dynamic attribute sets (`{..props}` spread) whose state is reset on branch swap; mixing leptos crate versions in a workspace.","solutions":["Update signals only after the element and its attribute states are built/hydrated.","Seed property effects with `RenderEffect::new_with_value` and an initial value in custom attribute implementations.","Avoid resetting the same property state more than once.","Upgrade to the latest leptos/tachys release where effect seeding is guaranteed."],"exampleFix":"// before\n*state = RenderEffect::new_with_value(move |prev| { /* expects Some */ }, prev_value);\n// after\n*state = RenderEffect::new_with_value(\n    move |prev: Option<PropState>| { /* Some guaranteed */ },\n    prev_value.unwrap_or_else(|| initial_prop_state(&key)),\n);","handlingStrategy":"fallback","validationCode":"// Gate property updates on mounted state\nlet mounted = RwSignal::new(false);\ncreate_effect(move |_| {\n    if !mounted.get() { return; }\n    let _ = prop_signal.get();\n});\n// mounted.set(true) after the element with prop: bindings mounts","typeGuard":"fn prop_prev_present(prev: &Option<PropState>) -> bool { prev.is_some() }","tryCatchPattern":"// Panic; use hook for diagnostics\nstd::panic::set_hook(Box::new(|info| {\n    if info.to_string().contains(\"entered unreachable code\") {\n        leptos::logging::error!(\"property rebuild without prev: {info}\");\n    }\n}));","preventionTips":["Update prop: signals only after hydration/mount","Avoid resetting property states twice during branch swaps","Seed custom property effects with new_with_value","Keep leptos workspace on a single version"],"tags":["panic","reactivity","property-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"}