fn parse_r_macro_input(
input: TokenStream,
) -> Result<(Option<TokenStream>, TokenStream), Error>Expand description
Parse the r!(...) input into an optional env expression and the tail token stream.
Grammar: [env: <Expr> ;] <tail_tokens...>
The env: head is identified by the leading env ident followed by :.
The <Expr> is parsed greedily up to the first top-level ; token
(matching the macro_rules! contract: env: $env:expr; $($code:tt)+ where
$env:expr consumes greedily until the ;).
After the ;, the remaining tokens form the tail.