Fix completion hint for sub/superscripts (JuliaLang#52402)

Fix JuliaLang#52376

The completion hint starting offset was not computed considering the
case where the input and the completion do not share the same prefix,
which happens when completing into a subscript or a superscript. This
fixes that by iteratively going through the characters of the hint until
reaching that which marks the end of the input.

stdlib/REPL/src/LineEdit.jl

@@ -382,23 +382,27 @@ function check_for_hint(s::MIState)
  return clear_hint(st)
  end
  completions, partial, should_complete = complete_line(st.p.complete, st, s.active_module)::Tuple{Vector{String},String,Bool}
+ isempty(completions) && return clear_hint(st)
  # Don't complete for single chars, given e.g. `x` completes to `xor`
  if length(partial) > 1 && should_complete
- if length(completions) == 1
- hint = only(completions)[sizeof(partial)+1:end]
- if !isempty(hint) # completion on a complete name returns itself so check that there's something to hint
+ singlecompletion = length(completions) == 1
+ p = singlecompletion ? completions[1] : common_prefix(completions)
+ if singlecompletion || p in completions # i.e. complete `@time` even though `@time_imports` etc. exists
+ # The completion `p` and the input `partial` may not share the same initial
+ # characters, for instance when completing to subscripts or superscripts.
+ # So, in general, make sure that the hint starts at the correct position by
+ # incrementing its starting position by as many characters as the input.
+ startind = 1 # index of p from which to start providing the hint
+ maxind = ncodeunits(p)
+ for _ in partial
+ startind = nextind(p, startind)
+ startind > maxind && break
+ end
+ if startind ≤ maxind # completion on a complete name returns itself so check that there's something to hint
+ hint = p[startind:end]
  st.hint = hint
  return true
  end
- elseif length(completions) > 1
- p = common_prefix(completions)
- if p in completions # i.e. complete `@time` even though `@time_imports` etc. exists
- hint = p[sizeof(partial)+1:end]
- if !isempty(hint)
- st.hint = hint
- return true
- end
- end
  end
  end
  return clear_hint(st)

stdlib/REPL/test/repl.jl

@@ -1711,6 +1711,17 @@ fake_repl() do stdin_write, stdout_read, repl
  end
  @test LineEdit.state(repl.mistate).hint === nothing
 
+ # issue #52376
+ write(stdin_write, "\x15")
+ write(stdin_write, "\\_ailuj")
+ while LineEdit.state(repl.mistate).hint !== nothing
+ sleep(0.1)
+ end
+ @test LineEdit.state(repl.mistate).hint === nothing
+ s5 = readuntil(stdout_read, "\\_ailuj")
+ write(stdin_write, "\t")
+ s6 = readuntil(stdout_read, "ₐᵢₗᵤⱼ")
+
  write(stdin_write, "\x15\x04")
  Base.wait(repltask)
 end