Implement VariableMap for Vec<AsRef<str>>

[kaspar030]

This implements VariableMap for &[T] slices containing strings (T: AsRef<str>).

In addition to this, the parser is updated to allow a single * as variable name. This is used in above's VariableMap implementation to make $* and ${*} substitute to the concatenation of all variables but `$0, like a shell would do.

My use-case for this is too take an argument list like argv, and then use subst to replace those numbered arguments within a string.

[de-vri-es]

Hey! Thanks for the PR.

The use case makes sense, but I'm not 100% sure if having a Vec<impl AsRef<str>> implicitly use the index as variable "name" makes sense. We could provide this functionality with a thin wrapper around a vec/slice as well.

On the other hand, I can't really think of another interpretation for using a vector as a map, so maybe it's better to implement it directly on Vec. But then we should also implement is for AsRef<str> slices :)

[de-vri-es]

Hmm, this case seems a little odd: You seem to want "$0" to be used sort-of like "$@" is a shell?

This is quite surprising behavior to me though. Normally, "$0" would be the name of the program, not the concatenation of all arguments.

[kaspar030]

ah, well spotted. I did have key == "*" there at some point. My use case doesn't have $0, but I wanted the numbering to start at 1, so I changed to this.

It probably makes sense to treat the Vec like a regular args vec (just returning the index, starting at 0), and using another key.

[kaspar030]

I remember why I went with 0: * is rejected by the parser as invalid variable name.

[kaspar030]

hmok, if making it possible to implement more shell-like behavior ($* returns all numbered args concatenated), I could add a special case for a name that's just `*, to the parser. Let me know!

[kaspar030]

But then we should also implement is for AsRef<str> slices :)

Hm, doesn't Vec<AsRef<str>> DeRef to a slice anyways. Let me try that.

[kaspar030]

Hm, doesn't Vec<AsRef<str>> DeRef to a slice anyways. Let me try that.

So only implementing this for &[AsRef<str>] slices just works for Vecs as well.

[kaspar030]

I've updated the parser and made this more shell-like.

[de-vri-es]

Hey! Sorry for the delay. It's RustWeek in the Netherlands :)

Now, the PR does a few different things:

1. Allow a special single character variable name: '*'
2. Implement VariableMap for [impl AsRef<str>].
3. Assign meaning to $* in that impl, which means: concatenate all other variables.

I think we can merge 2. But I have some reservations on 1 and 3. I think single symbols as variable names are cryptic. I don't particularly like the choice of shells to use them a lot. And regarding 3, it feels to specific to one use case to put it in the VariableMap impl for string slices.

I could see one way how this use case might be supported:

• Allow more single-character variable names in subst, like '!', '@', '#', '%', '^', '*', maybe more. I'm not really a huge fan of this, as I still think they are rather cryptic. But if they do not have special meaning in subst itself, I think I could be convinced to allow them as variable names.
• Implement a custom VariableMap in your project that wraps a string slice or vector to assign special meaning to the variables of your choice, such as: $* concatenates all variables.

[kaspar030]

It's RustWeek in the Netherlands :)

I'm there, too! :) Maybe we bump into each other, are you going to the 10 years of Rust celebration?

I could see one way how this use case might be supported:

* Allow more single-character variable names in `subst`, like `'!'`, `'@'`, `'#'`, `'%'`, `'^'`, `'*'`, maybe more. I'm not really a huge fan of this, as I still think they are rather cryptic. But if they do not have special meaning in `subst` itself, I think I could be convinced to allow them as variable names.

* Implement a custom `VariableMap` in your project that wraps a string slice or vector to assign special meaning to the variables of your choice, such as: `$*` concatenates all variables.

I did a custom VariableMap (basically the version in this PR but over a newtype holding a [AsRef<str>]) in my project already, without the single letter $*/$@. Those I handle before passing to subst now. That kinda works but is a bit clumsy, I'm working on individual args (via shell_words::split()), and only support the single special letter variants for arguments that exactly match $*/$@/${*}/${@}, as replacing them using string replace would ignore escaping ...

I'd appreciate support for those special single letter variable names, to make this behave more like expected (by users used to shell).

If you'll have it, I'll create a new PR with just e68c25d (the commit allowing *), and update that to allow a list of special characters. Maybe behind a feature flag?

[de-vri-es]

As also discussed at RustWeek: let's go ahead and support special characters as variable names: only single character special symbols, since that is what shells also do :)