extend paste_linter to catch usages like paste0(sep="")

NEWS.md

@@ -123,6 +123,7 @@ function calls. (#850, #851, @renkun-ken)
    * `paste_linter()` lint for common mis-use of `paste()` and `paste()`:
      + `paste0()` encouraged instead of `paste(sep = "")`
      + `toString()` or `glue::glue_collapse()` encouraged instead of `paste(x, collapse = ", ")`
+     + `sep=` passed to `paste0()` -- typically a mistake (extension for #998)
    * `nested_ifelse_linter()` Prevent nested calls to `ifelse()` like `ifelse(A, x, ifelse(B, y, z))`, and similar
    * `condition_message_linter` Prevent condition messages from being constructed like `stop(paste(...))` (where just `stop(...)` is preferable)
    * `redundant_ifelse_linter()` Prevent usage like `ifelse(A & B, TRUE, FALSE)` or `ifelse(C, 0, 1)` (the latter is `as.numeric(!C)`)

R/paste_linter.R

@@ -6,6 +6,8 @@
 #'  1. Block usage of [paste()] with `sep = ""`. [paste0()] is a faster, more concise alternative.
 #'  2. Block usage of `paste()` or `paste0()` with `collapse = ", "`. [toString()] is a direct
 #'     wrapper for this, and alternatives like [glue::glue_collapse()] might give better messages for humans.
+#'  3. Block usage of `paste0()` that supplies `sep=` -- this is not a formal argument to `paste0`, and
+#'     is likely to be a mistake.
 #'
 #' @evalRd rd_tags("paste_linter")
 #' @param allow_empty_sep Logical, default `FALSE`. If `TRUE`, usage of
@@ -69,6 +71,19 @@ paste_linter <- function(allow_empty_sep = FALSE, allow_to_string = FALSE) {
       )
     }
 
-    c(empty_sep_lints, to_string_lints)
+    paste0_sep_xpath <- "//expr[
+      expr[SYMBOL_FUNCTION_CALL[text() = 'paste0']]
+      and SYMBOL_SUB[text() = 'sep']
+    ]"
+    paste0_sep_expr <- xml2::xml_find_all(xml, paste0_sep_xpath)
+    paste0_sep_lints <- lapply(
+      paste0_sep_expr,
+      xml_nodes_to_lint,
+      source_file = source_file,
+      lint_message = "sep= is not a formal argument to paste0(); did you mean to use paste(), or collapse=?",
+      type = "warning"
+    )
+
+    c(empty_sep_lints, to_string_lints, paste0_sep_lints)
   })
 }

tests/testthat/test-paste_linter.R

@@ -64,3 +64,11 @@ test_that("paste_linter respects non-default arguments", {
   expect_lint("paste(collapse = ', ', x)", NULL, paste_linter(allow_to_string = TRUE))
   expect_lint("paste0(foo(x), collapse = ', ')", NULL, paste_linter(allow_to_string = TRUE))
 })
+
+test_that("paste_linter catches use of paste0 with sep=", {
+  expect_lint(
+    "paste0(x, y, sep = '')",
+    rex::rex("sep= is not a formal argument to paste0();"),
+    paste_linter()
+  )
+})