Invalid identifier in proc_macro causes ICE

[CAD97]

lib.proc-macro:

#[proc_macro]
pub fn foo(
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let ident = Ident::new("?", Span::call_site());
    item
}

bin:

foo!{}

build:

thread 'rustc' panicked at '`"?"` is not a valid identifier', src\libsyntax\ext\proc_macro_server.rs:333:13
stack backtrace:
   0:         0x663638fb - mingw_set_invalid_parameter_handler
   1:         0x66390c6b - mingw_set_invalid_parameter_handler
   2:         0x66356a17 - mingw_set_invalid_parameter_handler
   3:         0x663677a7 - mingw_set_invalid_parameter_handler
   4:         0x66367485 - mingw_set_invalid_parameter_handler
   5:         0x6934f60e - mingw_set_invalid_parameter_handler
   6:         0x66367d8c - mingw_set_invalid_parameter_handler
   7:         0x663679dc - mingw_set_invalid_parameter_handler
   8:         0x6636791f - mingw_set_invalid_parameter_handler
   9:         0x6af16bad - mingw_set_invalid_parameter_handler
  10:         0x6b049122 - mingw_set_invalid_parameter_handler
  11:         0x6afbc4d8 - mingw_set_invalid_parameter_handler
  12:         0x663791e9 - mingw_set_invalid_parameter_handler
  13:         0x6af5ca8d - mingw_set_invalid_parameter_handler
  14:         0x6af1d014 - mingw_set_invalid_parameter_handler
  15:         0x67b66578 - mingw_set_invalid_parameter_handler
  16:         0x67b4146d - mingw_set_invalid_parameter_handler
  17:         0x67b42234 - mingw_set_invalid_parameter_handler
  18:         0x67ba3039 - mingw_set_invalid_parameter_handler
  19:         0x67b487fe - mingw_set_invalid_parameter_handler
  20:         0x6af585dd - mingw_set_invalid_parameter_handler
  21:         0x6b06f21c - mingw_set_invalid_parameter_handler
  22:         0x6afdeaa7 - mingw_set_invalid_parameter_handler
  23:         0x6afdaa07 - mingw_set_invalid_parameter_handler
  24:         0x6945de82 - mingw_set_invalid_parameter_handler
  25:         0x694598c7 - mingw_set_invalid_parameter_handler
  26:         0x693f8436 - mingw_set_invalid_parameter_handler
  27:         0x6945ce5b - mingw_set_invalid_parameter_handler
  28:         0x6943669f - mingw_set_invalid_parameter_handler
  29:         0x693fb531 - mingw_set_invalid_parameter_handler
  30:         0x6949fada - mingw_set_invalid_parameter_handler
  31:         0x69315a48 - mingw_set_invalid_parameter_handler
  32:         0x6932c6ff - mingw_set_invalid_parameter_handler
  33:         0x6935193a - mingw_set_invalid_parameter_handler
  34:         0x69372ac1 - mingw_set_invalid_parameter_handler
  35:         0x692eabfd - mingw_set_invalid_parameter_handler
  36:         0x663791e9 - mingw_set_invalid_parameter_handler
  37:         0x69308033 - mingw_set_invalid_parameter_handler
  38:         0x66348176 - mingw_set_invalid_parameter_handler
  39:         0x66376767 - mingw_set_invalid_parameter_handler
  40:     0x7ffdc5967bd4 - mingw_set_invalid_parameter_handler
  41:     0x7ffdc5b6ced1 - mingw_set_invalid_parameter_handler

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 1.39.0-beta.5 (fa5c2f3e5 2019-10-02) running on x86_64-pc-windows-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: proc macro panicked
 --> src\main.rs:3:1
  |
3 | foo!{}
  | ^^^^^^
  |
  = help: message: `"?"` is not a valid identifier

error: aborting due to previous error

error: could not compile `playground`.

toolchain:

active toolchain
----------------

beta-x86_64-pc-windows-gnu (default)
rustc 1.39.0-beta.5 (fa5c2f3e5 2019-10-02)

[jonas-schievink]

Duplicate of #59998