Iteration error on AnnotatedStrings involving multiple codepoints

[serenity4]

I stumbled upon an error while working with something similar to styled"𝟏{red:x}":

julia> styled"𝟏{red:x}"
"Error showing value of type Base.AnnotatedString{String}:
ERROR: StringIndexError: invalid index [4], valid nearby indices [1]=>'𝟏', [5]=>'x'
Stacktrace:
  [1] string_index_err(s::String, i::Int64)
    @ Base ./strings/string.jl:12
  [2] SubString{String}(s::String, i::Int64, j::Int64)
    @ Base ./strings/substring.jl:35
  [3] SubString(s::String, i::Int64, j::Int64)
    @ Base ./strings/substring.jl:49
  [4] SubString(s::String, r::UnitRange{Int64})
    @ Base ./strings/substring.jl:52
  [5] iterate(si::StyledStrings.RegionIterator{String}, i::Int64)
    @ StyledStrings ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/StyledStrings/src/regioniterator.jl:13
  [6] iterate
    @ ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/StyledStrings/src/regioniterator.jl:12 [inlined]
  [7] _ansi_writer(io::IOContext{Base.TTY}, s::Base.AnnotatedString{String}, string_writer::StyledStrings.var"#20#21"{Tuple{}, Tuple{Char, Char}})
    @ StyledStrings ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/StyledStrings/src/io.jl:233
  [8] escape_string
    @ ~/.julia/juliaup/julia-nightly/share/julia/stdlib/v1.12/StyledStrings/src/io.jl:266 [inlined]
  [9] print_quoted
    @ ./strings/io.jl:471 [inlined]
 [10] show
    @ ./strings/io.jl:197 [inlined]
 [11] show(io::IOContext{Base.TTY}, ::MIME{Symbol("text/plain")}, s::Base.AnnotatedString{String})
    @ Base ./strings/annotated.jl:186
  [...]

julia> versioninfo()
Julia Version 1.12.0-DEV.1093
Commit adb323f1b51 (2024-08-25 10:41 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 20 × 12th Gen Intel(R) Core(TM) i7-12700H
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, alderlake)
Threads: 20 default, 0 interactive, 20 GC (on 20 virtual cores)
Environment:
  JULIA_INFO_COLOR = green
  JULIA_PKG_USE_CLI_GIT = true

It worked fine at some point (fairly early after the introduction of StyledStrings), then a change seems to have broken it.

[tecosaur]

Thanks for the report Cedric! I'm unfortunately busy for the next ~week or so, but I'll give this a look soon.

[serenity4]

I had to dig into region iteration for some other stuff, and then had an idea what was up in this issue. Feel free to take a look at #86 :)