This skipped printing a "^" line if the start or length of the error
was longer than the source.
That seems like the correc thing at first glance, however it means
that the caret line isn't skipped *if the file goes on*.
So, for example
```fish
echo "$abc["
```
by itself, in a file or via `fish -c`, would not print an error, but
```fish
echo "$abc["
true
```
would. That's not a great way to print errors.
So instead we just.. imagine the start was at most at the end.
The underlying issue why `echo "$abc["` causes this is that `wcstol`
didn't move the end pointer for the index value (because there is no
number there). I'd fix this, but apparently some of
our recursive variable calls absolutely rely on this position value.
A command like "printf nonewline | sed s/x/y/" does not print a
concluding newline, whereas "printf nnl | string replace x y" does.
This is an edge case -- usually the user input does have a newline at
the end -- but it seems still better for this command to just forward
the user's data.
Teach most string subcommands to check if stdin is missing the trailing
newline, and stop adding one in that case.
This does not apply when input is read from commandline arguments.
* Most subcommands stop adding the final newline, because they don't
really care about newlines, so besides their normal processing,
they just want to preserve user input. They are:
* string collect
* string escape/unescape
* string join¹
* string lower/upper
* string pad
* string replace
* string repeat
* string sub
* string trim
* string match keeps adding the newline, following "grep". Additionally,
for string match --regex, it's important to output capture groups
separated by newlines, resulting in multiple output lines for an
input line. So it is not obvious where to leave out the newline.
* string split/split0 keep adding the newline for the same reason --
they are meant to output multiple elements for a single input line.
¹) string join0 is not changed because it already printed a trailing
zero byte instead of the trailing newline. This is consistent
with other tools like "find -print0".
Closes#3847
This didn't do all the syntax checks, so something like
fish -c 'echo foo; and $status'
complained of a missing command `0` (i.e. $status), and
fish -c 'echo foo | exec grep'
hit an assert!
So we do what read_ni does, parse each command into an ast, run
parse_util_detect_errors on it if it worked and then eval the ast.
It is possible to do this neater by modifying parser::eval, but I
can't find where.
