//! Compile-fail (UI) tests for the macros' diagnostics. The expected output
//! lives in the sibling `tests/ui/*.stderr` files; regenerate them after an
//! intentional message change with `TRYBUILD=overwrite cargo test --test ui`.
//!
//! Note: `.stderr` rendering is rustc-version-sensitive, so these are gated to
//! run only under the toolchain they were captured on (set `RUN_UI_TESTS=1`).