Destructuring

[Rich-Harris]

This updates the existing destructuredContexts mechanism to support arbitrary destructuring forms, so that you can do this sort of thing:

{#each things as { x: [y, z] } }
  <p>{z}</p>
{/each}

More realistically, you could use it for simple object pattern destructuring, but I didn't see any reason to arbitrarily restrict the patterns you could use. For now it doesn't support assignment patterns — we'll see if those turn out to be necessary. (They're a little finicky to parse.)

TODO

• sparse array patterns
• two-way binding

[codecov-io]

Codecov Report

Merging #1385 into master will increase coverage by 0.02%.
The diff coverage is 92.75%.

@@            Coverage Diff             @@
##           master    #1385      +/-   ##
==========================================
+ Coverage   91.21%   91.24%   +0.02%     
==========================================
  Files         120      122       +2     
  Lines        4429     4454      +25     
  Branches     1362     1367       +5     
==========================================
+ Hits         4040     4064      +24     
- Misses        154      156       +2     
+ Partials      235      234       -1

Impacted Files	Coverage Δ
src/compile/nodes/Fragment.ts	100% <ø> (ø)	⬆️
src/compile/dom/Block.ts	95.78% <100%> (-0.05%)	⬇️
src/parse/state/mustache.ts	88.88% <100%> (+0.91%)	⬆️
src/validate/html/index.ts	97.91% <100%> (-0.05%)	⬇️
src/compile/nodes/EachBlock.ts	98.65% <100%> (+0.61%)	⬆️
src/compile/nodes/Binding.ts	96% <100%> (-0.04%)	⬇️
src/compile/nodes/Component.ts	95.65% <100%> (-0.02%)	⬇️
src/utils/unpackDestructuring.ts	88.88% <88.88%> (ø)
src/parse/read/context.ts	89.47% <89.47%> (ø)
... and 1 more

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