`object_usage_linter()` tries too many assignments

[AshesITR]

Seen in our own package when running lint_package():

..........Warning in formals(fun) : argument is not a function
Warning in body(fun) : argument is not a function
Warning in formals(fun) : argument is not a function
Warning in body(fun) : argument is not a function
...................................................Warning in formals(fun) : argument is not a function
Warning in body(fun) : argument is not a function
................................................................................................................................................

The reason is the XPath for object_usage_linter() finds e.g.

assign(
    envir = cache,
    x = digest_content(linters, filename),
    value = lints,
    inherits = FALSE
  )

which is not a function assignment.

[AshesITR]

For the future: Debugging this wasn't too easy because options(warn = 3) didn't throw, since the bug was hidden behind a try.
To locate it, a custom warning handler was needed. I used

withCallingHandlers(lint_package(), warning = function(w) print(sys.calls()))