Expand description
Proc macro implementation of rcstr!.
The implementation deliberately avoids syn and quote. The macro is
invoked thousands of times across the workspace, so per-invocation cost
matters: we pattern-match on proc_macro::TokenTree directly to
identify a single string-literal token, ask the compiler for its
unescaped value via Literal::str_value (gated by the unstable
proc_macro_value feature), and emit the chosen expansion by parsing
a string template via TokenStream::from_str.