538f118da1
nicer errors from assert_unsafe_precondition This makes the errors shown by cargo-careful nicer, and since `panic_no_unwind` is `nounwind noreturn` it hopefully doesn't have bad codegen impact. Thanks to `@bjorn3` for the hint! Would be nice if we could somehow supply our own (static) message to print, currently it always prints `panic in a function that cannot unwind`. But still, this is better than before.
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.