Span::source_text panics with multibyte source

[aatifsyed]

    #[test]
    fn proc_macro2_source_text_is_correct_for_single_byte() {
        let source_code = "const FOO: () = ();";
        let ast = syn::parse_str::<syn::ItemConst>(source_code).unwrap();
        assert_eq!("FOO", ast.ident.span().source_text().unwrap())
    }

    #[test]
    #[should_panic = "is not a char boundary"]
    fn proc_macro2_source_text_is_incorrect_for_multibyte() {
        let source_code = "const 𓀕: () = ();";
        let ast = syn::parse_str::<syn::ItemConst>(source_code).unwrap();
        let reported = ast.ident.span().source_text(); // boom
        assert_eq!("𓀕", reported.unwrap())
    }

Note this is just calling the method, not unwrapping the Option:

thread 'tests::proc_macro2_source_text_is_incorrect_for_multibyte' panicked at 'byte index 7 is not a char boundary; it is inside '𓀕' (bytes 6..10) of `const 𓀕: () = ();`', /home/aatif/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/fallback.rs:369:9
...
   6: <alloc::string::String as core::ops::index::Index<core::ops::range::Range<usize>>>::index
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/alloc/src/string.rs:2350:10
   7: proc_macro2::fallback::FileInfo::source_text
             at /home/aatif/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/fallback.rs:369:9
   8: proc_macro2::fallback::Span::source_text::{{closure}}
             at /home/aatif/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/fallback.rs:569:43
   9: std::thread::local::LocalKey<T>::try_with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:270:16
  10: std::thread::local::LocalKey<T>::with
             at /rustc/5680fa18feaa87f3ff04063800aec256c3d4b4be/library/std/src/thread/local.rs:246:9
  11: proc_macro2::fallback::Span::source_text
             at /home/aatif/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/fallback.rs:569:22
  12: proc_macro2::Span::source_text
             at /home/aatif/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.67/src/lib.rs:522:9
  13: synsert::tests::proc_macro2_source_text_is_incorrect_for_multibyte
             at ./src/lib.rs:165:24
  14: synsert::tests::proc_macro2_source_text_is_incorrect_for_multibyte::{{closure}}
             at ./src/lib.rs:162:61
...

proc-macro2 1.0.67 (latest)
Not called from a proc-macro context

[dtolnay]

I have published a fix in proc-macro2 1.0.68.