Improve error from invalid syntax inside `proc_macro::quote!`

[dtolnay]

Ideally the `$` must be followed by an ident or `$` in `quote!` error message would not look like an ICE.

#![feature(proc_macro)]

extern crate proc_macro;
use proc_macro::{quote, TokenStream};

#[proc_macro]
pub fn p(input: TokenStream) -> TokenStream {
    quote! {
        $()*
    }
}

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.25.0-nightly (b5392f545 2018-01-08) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at '`$` must be followed by an ident or `$` in `quote!`', libproc_macro/quote.rs:105:26

@jseyfried

[alexreg]

progress?

[estebank]

Current output:

error: proc macro panicked
  --> src/lib.rs:9:5
   |
9  | /     quote! {
10 | |         $()*
11 | |     }
   | |_____^
   |
   = help: message: `$` must be followed by an ident or `$` in `quote!`