implement `string unescape` to complement `string escape`

[kbauer]

The command string escape allows bypassing limitations of the scripting language where needed, e.g. to store a list in a single string (allowing, but discouraging, nested list semantics) with

set var (printf "%s " (string escape $PATH))

As of 2.4.0, however, there is no safe way to unescape such values. While

set list_var (eval "printf '%s\n' $var")

unpacks the quoted list into an actual list, but will happily evaluate any commands smuggled into a manipulated $var as well. A string unescape subcommand would fix this.

---

• Have you checked if problem occurs with fish 2.4.0?
• Tried fish without third-party customizations (check sh -c 'env HOME=$(mktemp -d) fish')?

fish version installed (fish --version): 2.4.0-111-g7356987

OS/terminal used: Windows 10, Mintty

[faho]

There have been times I've wanted this as well. Note that there's a nice little hack that you can use with commandline.

# Assuming same $var as before
commandline -r -- $var
set list_var (commandline -opc)
commandline -r -- ""

Though really that just has some novelty value to it and shouldn't be actually used.

[floam]

Makes sense to me to add this.