Report #38317
[bug\_fix] the trait bound \`T: Trait\` is not satisfied \[E0277\]
Derive or manually implement the required trait for your type, e.g., \`\#\[derive\(Hash, Eq, PartialEq\)\]\` for HashMap keys, or change the generic constraint to match what the type actually implements. If the trait is implemented for references but not owned values, change \`T\` to \`&T\` in your usage. Root cause: Generic functions and data structures declare trait bounds that concrete types must satisfy; the compiler verifies these bounds at monomorphization time and fails if the implementation is missing.
Journey Context:
You're building a registry of event handlers using a \`HashMap>\`. You define \`trait EventHandler: Send \{\}\` and implement it for various structs. When you try to insert a handler into the map inside a \`tokio::spawn\` task, you get E0277: 'the trait bound \`dyn EventHandler: Send\` is not satisfied'. You stare at the error confused because your trait \*has\* \`Send\` as a supertrait. You try \`Box\` and the error shifts to 'trait bound not satisfied' on the concrete type. You realize the concrete type might contain an \`Rc>\` or a raw pointer that isn't Send. You check your struct and find it holds an \`Rc>>\`. You change it to \`Arc>>\` to make it Send. The error persists because you forgot to add \`\+ Send\` to the Box type alias. Finally, you understand that \`dyn Trait\` is unsized and has object safety rules; for Send to propagate through the dyn, you must explicitly write \`Box\`. The root cause is that auto-trait bounds like Send are not automatically applied to trait objects unless explicitly requested. Later, you encounter a similar E0277 when trying to use your struct as a HashMap key without implementing Hash, reinforcing that trait bounds are strict contracts checked at compile time.
⚠ Workarounds are unverified - always check before running. Confirmations show what worked for others, not a safety guarantee.
Lifecycle
2026-06-18T18:47:14.868992+00:00— report_created — created