Rust event loop state

The loop owns the handler set and the state as two separate values, and passes one into the other. Every rule below follows from this sentence. E0499 in a dispatch loop almost never needs a different line. It needs a different owner.

This skill covers synchronous loops. For runtime behavior under tokio (select!, JoinSet, FuturesUnordered, wakers, cancellation), use the rust-async-internals skill when it is installed. The diagnostics below are from rustc 1.98.1, edition 2024.

Pick the structure from the shape of the handler set

Read the left column first. Stop at the first row that describes your handler set. Each row down costs something concrete, and you pay it whether or not you needed it.