lower top-level statements so that the front-end knows their values are unused

[JeffBezanson]

Fixes most false positives in the deprecation for using the value of .=; see #26088 (comment)

This could also be useful in other ways in the future.

[StefanKarpinski]

Does it make sense to test that module M; x .= y; nothing; end doesn't warn or error?

[JeffBezanson]

Ok, I'm adding tests for this. It was bizarrely difficult. First I found that @test_warn and @test_nowarn simply don't work for this; don't know why. @test_deprecated doesn't have any way to test that something is not deprecated (it seems to, since you can pass a pattern, but passing a pattern that matches the empty string doesn't work). Finally I figured out that, apparently, @test_logs f(x) tests that evaluating f(x) does not log anything. Obviously!

[StefanKarpinski]

@c42f: is the @test_logs behavior intentional? Should it be called @test_no_log instead?

[fredrikekre]

The docstring for @test_logs is very good, and have examples for how to test the logs

help?> @test_logs
  @test_logs [log_patterns...] [keywords] expression

  Collect a list of log records generated by expression using collect_test_logs, check that they match the sequence log_patterns, and return the value of expression. The
  keywords provide some simple filtering of log records: the min_level keyword controls the minimum log level which will be collected for the test, the match_mode keyword
  defines how matching will be performed (the default :all checks that all logs and patterns match pairwise; use :any to check that the pattern matches at least once somewhere
  in the sequence.)

  The most useful log pattern is a simple tuple of the form (level,message). A different number of tuple elements may be used to match other log metadata, corresponding to the
  arguments to passed to AbstractLogger via the handle_message function: (level,message,module,group,id,file,line). Elements which are present will be matched pairwise with the
  log record fields using == by default, with the special cases that Symbols may be used for the standard log levels, and Regexs in the pattern will match string or Symbol
  fields using contains.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  Consider a function which logs a warning, and several debug messages:

  function foo(n)
      @info "Doing foo with n=$n"
      for i=1:n
          @debug "Iteration $i"
      end
      42
  end

  We can test the info message using

  @test_logs (:info,"Doing foo with n=2") foo(2)

  If we also wanted to test the debug messages, these need to be enabled with the min_level keyword:

  @test_logs (:info,"Doing foo with n=2") (:debug,"Iteration 1") (:debug,"Iteration 2") min_level=Debug foo(2)

  The macro may be chained with @test to also test the returned value:

  @test (@test_logs (:info,"Doing foo with n=2") foo(2)) == 42

[JeffBezanson]

The docstring is very good; the surprising part is that you pass zero patterns to check for zero log events. We could add a @test_nolog convenience wrapper for that case.