aab6c28450
An error that only reaches the echo area is gone the moment you type, and the location was the useful half of it. So the client draws an overlay at the `:loc` the daemon sent, with the message beside the code, and clears it the next time that buffer's evaluation is accepted — a marker left behind after a fix is a lie about the running program. Two things had to be right first, and neither was. The column in a `:loc` is a *byte* offset: lib/reader.ml walks the source a byte at a time and OCaml strings are bytes. The old code did `forward-char` with it, which is the same mistake as counting a frame's length in characters, in a different place — one accented character earlier on the line puts the marker as many columns to the right. It goes through `byte-to-position` from the line's start now, and is clamped to the end of the line, which the old code also needed: a column past a short line walked into the next one and pointed at innocent code. And the daemon numbers lines from the start of what it was *sent*, so `C-c C-c` on a defn halfway down a buffer came back saying line 1. Every overlay would have sat on the file's first line. The fix is leading newlines: the reader skips them, and the reply's line numbers are then the buffer's own. No protocol change, and nothing the daemon has to know. Marking the error must not itself signal — the error the caller is owed is the daemon's, and losing it to a bad location would report the wrong thing.