Like `set` and `read` before it, `eval` can be used to set variables,
and so it can't be shadowed by a function without loss of
functionality.
So this forbids it.
Incidentally, this means we will no longer try to autoload an
`eval.fish` file that's left over from an old version, which would
have helped with #8963.
Otherwise this would look weird if you had, say, a tab in there.
See #7716.
(note that this doesn't handle e.g. zero-width-joiners, because those
aren't currently escaped. we might want to add an escape mode for
unprintable characters, but for combining codepoints that's tricky!)
Prior to this fix, fish was rather inconsistent in when $status gets set
in response to an error. For example, a failed expansion like "$foo["
would not modify $status.
This makes the following inter-related changes:
1. String expansion now directly returns the value to set for $status on
error. The value is always used.
2. parser_t::eval() now directly returns the proc_status_t, which cleans
up a lot of call sites.
3. We expose a new function exec_subshell_for_expand() which ignores
$status but returns errors specifically related to subshell expansion.
4. We reify the notion of "expansion breaking" errors. These include
command-not-found, expand syntax errors, and others.
The upshot is we are more consistent about always setting $status on
errors.
It's now good enough to do so.
We don't allow grid-alignment:
```fish
complete -c foo -s b -l barnanana -a '(something)'
complete -c foo -s z -a '(something)'
```
becomes
```fish
complete -c foo -s b -l barnanana -a '(something)'
complete -c foo -s z -a '(something)'
```
It's just more trouble than it is worth.
The one part I'd change:
We align and/or'd parts of an if-condition with the in-block code:
```fish
if true
and false
dosomething
end
```
becomes
```fish
if true
and false
dosomething
end
```
but it's not used terribly much and if we ever fix it we can just
reindent.
I tested this manually (`littlecheck.py -s fish=fish tests/checks/eval.fish`) from the base directory, which means I got
"tests/checks/eval", while the real test gets "checks/eval".
I then reran `make test_fishscript`, but that didn't pull in the
updated test - we should really handle that better.
