[WIP] Let In Syntax Initial Impl

[oldwomanjosiah]

Description

This is not intended to be final, but I had some time so I thought I would work on this. First pass ended up being easier than I was expecting.

• Implements [FEATURE] Let-in syntax in simplexpr #408
• Allows the usage of true and false within idents, so long as it is not the entire thing.

Usage

(defvar somevar {{ "key": "Some Value" }})
(label
  :text { let it = somevar.value in "Here Is ${it}" end })

Showcase

My current workspace indicator:

Where I have a script that outputs the following on workspace changes:

{
  "1": {
    "dis": -1,
    "ex": false,
    "urg": false
  },
  ...
}

And the let-in syntax to pull out the relevant information for each part

(defwidget workspaceIndicator [num monitor]
  (box :orientation "v"
    (button
      :class {
        let
          ws = active_workspaces["${num}"],
          foc = ws.dis == monitor ? " focused" : "",
          vis = ws.dis >= 0 ? " visible" : "",
          ex = ws.ex ? " exists" : ""
        in
          "indicator " + foc + vis + ex
        end
      }
      :onclick "scripts/workspaces set ${num}"
      (box :orientation "v"
        (box :orientation "h"
          (box :class "inner"))))))

Diffs containing fuller example of usage

Additional Notes

I borrowed the testing macro from #577

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[oldwomanjosiah]

It's unclear to me atm what should be done here. As far as I can tell the function is (transitively) unused, and could be removed, so it might not be a problem.

It might make the most sense to use the values from the let block to map the inner vars, but the variable defs haven't been evaluated yet here.

[oldwomanjosiah]

@elkowar Just following up here to see if you have any feedback on the direction of this? Specifically if try_map_var_refs can/should be removed, or if not what the behavior there should be, but also in general.