That is, only a single expression or item gets parsed, so if there are
any extra tokens (e.g. the start of another item/expression) the user
should be told, rather than silently dropping them.
An example:
macro_rules! foo {
() => {
println("hi");
println("bye);
}
}
would expand to just `println("hi")`, which is almost certainly not
what the programmer wanted.
Fixes #8012.