Avoid paste0 overhead in is_lint_level (#2360)

* avoid paste0 overhead

* use switch() for further micro-optimization

---------

Co-authored-by: AshesITR <alexander.rosenstock@web.de>

Author: MichaelChirico

R/is_lint_level.R

@@ -19,9 +19,8 @@
 #'
 #' @export
 is_lint_level <- function(source_expression, level = c("expression", "file")) {
-  stopifnot(!missing(level))
   level <- match.arg(level)
-  required_key <- paste0(if (level == "file") "full_", "parsed_content")
+  required_key <- switch(level, file = "full_parsed_content", expression = "parsed_content")
   required_key %in% names(source_expression)
 }