[Bug] single digit unicode char causes panic

[0rphon]

🐛 Bug Report

The RFC states We also allow Unicode escapes of the form '\u{X}', where X is a sequence of one to six hex digits yet using a char with a single unicode digit causes a rust panic

possibly related to #993 but seems to have a different cause

Code snippet to reproduce

function main() {
    // error in leo, no error in rust
    let a = '\u1';
}

Stack trace & error message

D:\Work\leo-playground>D:\Work\leo_repos\leo\target\release\leo clean && D:\Work\leo_repos\leo\target\release\leo run
      Done Finished in 0 milliseconds 

     Build Starting...
     Build Compiling main program... ("D:\\Work\\leo-playground\\src/main.leo")
thread 'main' panicked at 'begin <= end (3 <= 2) when slicing `\u1`', parser\src\tokenizer\lexer.rs:116:35
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Your Environment

• leo commit eaadb29
• rustc version 1.53.0-nightly
• Windows 10.0.19042 (Windows 10 Pro) [64-bit]

[damirka]

Hey @0rphon, here and in #993.
Unicode escapes require curly braces, so correct writing should be:

let a = '\u{1}';

I agree that parser message could be more descriptive. That we can improve.

[damirka]

I've summarized all bugs in #995.