`TokenStream` assumes that its subtree's delimeter is `None`, and this
should be encoded in the type system instead of having a delimiter field
that is mostly ignored.
`tt::Subtree` is just `pub delimiter: Option<Delimiter>, pub
token_trees: Vec<TokenTree>`, so a Subtree that is statically guaranteed
not to have a delimiter is just Vec<TokenTree>.
TokenStream holds a `tt::Subtree` but assumes its `delimiter` is always
`None`. In particular, the iterator implementation iterates over the
inner `token_trees` and ignores the `delimiter`.
However, `TokenStream::from_str` violated this assumption when the input
consists of a single Group by producing a Subtree with an outer
delimiter, which was ignored as seen by a procedural macro.
In this case, wrap an extra level of Subtree around it.
Fixes#7810Fixes#7875
It's very useful when `pub` is equivalent to "this is crate's public
API", let's enforce this!
Ideally, we should enforce it for local `cargo test`, and only during
CI, but that needs https://github.com/rust-lang/cargo/issues/5034.
