Panic With `let a = @(())` / `let a = $(( () ))`

Created by: xTibor

Reproduction:

let a = @((b))

Actual behavior: A panic with this message: thread 'main' panicked at 'attempt to subtract with overflow', src/parser/arguments.rs:87:25 Backtrace

Build information: rustc -V: rustc 1.23.0-nightly (d6b06c63a 2017-11-09) git rev-parse HEAD: 2f176ebe System: KDE neon 5.11 (Ubuntu 16.04 LTS)

Misc: The panic is only reproducible in debug mode. In release mode these overflow checks are disabled by the compiler.