Add UnusedPipeVariableLinter

[ryangreenberg]

This PR introduces UnusedPipeVariableLinter to identify bugs where the pipe variable $$ is missing from a pipe operation.

I recently encountered a bug in a pipe operation where the right-hand side re-used the initial variable, discarding the intended input in $$:

$output = htmlspecialchars($html) |> format($html);

Note that the docs for "Expressions And Operators: Pipe" say:

The binary pipe operator, |>, evaluates the result of a left-hand expression and stores the result in $$, the pre-defined pipe variable. The right-hand expression must contain at least one occurrence of $$.

If it's a bug that the typechecker does not produce an error for a missing $$, we can close this PR and file an issue in https://github.com/facebook/hhvm.

[lexidor]

If it's a bug that the typechecker does not produce an error for a missing $$, we can close this PR and file an issue in https://github.com/facebook/hhvm.

facebook/hhvm#8698

There is already an issue tracking this 😉.
Please don't let this convince you to withdraw this linter. Pipes without $$ are fishy it should be flagged. A lint rule could be deployed far quicker than a language change. I would be surprised if www would typecheck after 2.5 years since the unintentional (?) removal of the check. Strongly in favor of shipping this linter, even if it becomes a language level error later.

[ryangreenberg]

Two of the test suites are failing because they could not run hhvm and two are failing with this error:

 Server launched with the following command:
  	'/usr/local/Cellar/hhvm-nightly/2022.05.03/bin/hh_server' '-d' '/Users/runner/work/hhast/hhast' '--waiting-client' '8'
  Running in daemon mode
  Server connection took over 2.0 seconds. Refreshing...
  Typing[4297] Was expecting an array or collection but type is unknown [1]
  -> It is unknown because the type of the lambda parameter could not be determined. Please add a type hint to the parameter [2]
  
  src/__Private/XHProf.hack:32:53
        30 |     invariant(self::$enabled === true, "Can't disable twice");
        31 |     self::$enabled = false;
  [1,2] 32 |     $raw = Dict\map(\xhprof_disable(), $v ==> (int) $v['wt']);
        33 | 
        34 |     $inclusive = dict[];
  
  1 error found.
  Error: Process completed with exit code 2.

This seems like an unrelated

[lexidor]

Yes, I ran into that too. You can add a dict<arraykey, mixed> before $v and explicitly reformat. This makes the tests pass again without conflict with my branches.

6c00e36 is the diff you can apply.

[Atry]

Retrigger the CI

[ryangreenberg]

Retriggered CI again to get the build to pass on macOS. I think this PR is now ready for review/discussion.

[Atry]

Looks good to me