[ruby-core:116677] [Ruby master Feature#16828] Introduce find patterns

Issue #16828 has been updated by Eregon (Benoit Daloze).


An easy way to understand the "backtracking" of find pattern is it works like `each_slice(n)` and doesn't actually backtrack.
So its performance is linear to the number of elements of the array being matched.
https://github.com/ruby/ruby/commit/ddded1157a90d21cb54b9f07de35ab9b4cc472e1#diff-a5ba41b51e3655f9f244362a616282b5119d3e15dd6c52ee999bbdfcc5b86a77 has nice Ruby pseudo-code describing how it works.

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Feature #16828: Introduce find patterns
https://bugs.ruby-lang.org/issues/16828#change-106690

* Author: ktsj (Kazuki Tsujimoto)
* Status: Closed
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
I propose to add find pattern to pattern matching.

# Specification
```
find_pattern: Constant(*var, pat, ..., *var)
            | Constant[*var, pat, ..., *var]
            | [*var, pat, ..., *var]
```

This pattern is similar to array pattern, except that it finds the first match values from the given object.

```ruby
case ["a", 1, "b", "c", 2, "d", "e", "f", 3]
in [*pre, String => x, String => y, *post]
  p pre  #=> ["a", 1]
  p x    #=> "b"
  p y    #=> "c"
  p post #=> [2, "d", "e", "f", 3]
end
```

Note that, to avoid complexity, the proposed feature doesn't support backtracking. Thus, the following code raises a NoMatchingPatternError.

```ruby
case [0, 1, 2]
in [*, a, *] if a == 1
end
```

# Implementation
* https://github.com/k-tsj/ruby/tree/find-pattern-prototype




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