wrapi_text: fix handling of successive parts that exceed width

wrapi_text() splits a string into pieces and then builds it back up,
inserting a newline before the next piece if it would lead to the
string exceeding the specified width.  The result is stored in the
`start` variable, which gets extended each time the width is exceeded.

To extend `start`, wrapi_text() appends the value of

  {pieces that did not exceed width}{sep}{newline}{piece that exceeded width}

wrapi_text() incorrectly assumes that there is at least one piece that
did _not_ exceed the width since the last time `start` was extended,
leading to a malformed result with the last piece repeated in an
incorrect position.

Update the logic to account for the fact that successive pieces can
exceed the specified width.

Author: kyleam

R/util.R

@@ -191,8 +191,14 @@ wrapi_text <- function(
       if(max_line_char(s) <= width){
         next
       }else{
-        pieces_start <- pieces[shift:(i - 1)]
-        add <- paste(paste0(pieces_start, collapse = wrap_chr), pieces[i], sep = newline_sep)
+        if (shift == i) {
+          # `start` includes everything aside from the current piece.
+          add <- paste0("\n", pieces[i])
+        } else {
+          pieces_start <- pieces[shift:(i - 1)]
+          add <- paste(paste0(pieces_start, collapse = wrap_chr), pieces[i], sep = newline_sep)
+        }
+
         if (i == npieces) {
           s <- cat_start(start = start, add = add, sep = wrap_chr)
         }else{

tests/testthat/test-util.R

@@ -61,4 +61,9 @@ test_that("wrap_text(): past offenders", {
     ),
     "test\n-foo\n.R\ntest\n-foo\n_bar\n.R"
   )
+
+  expect_identical(
+    wrap_text("foo/bar/baz", width = 4),
+    "foo/\nbar/\nbaz"
+  )
 })