Raw identifiers are not supported

[pwoolcoc]

It appears that the Getters derive panics when encountering a raw identifier. The error was reproducible on master as well as the most recently published crates.io version:

± cargo build
   Compiling getset-bug v0.1.0 (/Users/pwoolcock/code/getset-bug)
thread 'rustc' panicked at '`"r#type"` is not a valid identifier', src/librustc_expand/proc_macro_server.rs:329:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.44.0-nightly (f4c675c47 2020-03-19) running on x86_64-apple-darwin

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

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

error: proc-macro derive panicked
 --> src/lib.rs:3:10
  |
3 | #[derive(Getters)]
  |          ^^^^^^^
  |
  = help: message: `"r#type"` is not a valid identifier

error: aborting due to previous error

error: could not compile `getset-bug`.

To learn more, run the command again with --verbose.

and here's the code I used to generate this error:

use getset::Getters;

#[derive(Getters)]
pub struct Foo {
    r#type: String,
}