Warn if R used by mistake (#1850)

Author: hadley

NEWS.md

@@ -32,6 +32,7 @@
   * `` `Rd expr` `` inline code now generates `\Sexpr[stage=render,results=rd]{expr}`, providing a convenient syntax for evaluating R code at documentation render time (#1214).
   * Indented bullet lists in `@param` and other two-part tags are no longer incorrectly nested (#1102).
   * Horizontal rules (e.g. `----`) now generate a clear warning instead of an internal error about an unknown `thematic_break` xml node (#1707).
+  * Inline code with uppercase `` `R ` `` now warns that you should use lowercase `` `r ` `` instead, since knitr only supports lowercase.
   * Inline R code (`` `r expr` ``) in non-indented list continuation lines no longer causes an error (#1651).
   * Link text now supports non-code markup like bold and italic, e.g., `[*italic text*][func]` generates `\link[=func]{\emph{italic text}}`, matching R's support for markup in `\link` text in R 4.5.0.
   * Links now do a better job of resolving package names: the process is cached for better performance (#1724); it works with infix operators (e.g. `[%in%]`) (#1728); no longer changes the link text (#1662); and includes base packages when reporting ambiguous functions (#1725).

R/markdown.R

@@ -335,6 +335,10 @@ mdxml_break <- function(state) {
 mdxml_code <- function(xml, tag) {
   code <- xml_text(xml)
 
+  if (grepl("^R ", code)) {
+    warn_roxy_tag(tag$tag, "use ` r`, not `R `, for inline code")
+  }
+
   if (grepl("^Rd ", code)) {
     paste0(
       "\\Sexpr[stage=render,results=rd]{",

tests/testthat/_snaps/markdown-code.md

@@ -1,3 +1,10 @@
+# uppercase R in inline code gives warning
+
+    Code
+      . <- roc_proc_text(rd_roclet(), text)
+    Message
+      x <text>:2: @title use ` r`, not `R `, for inline code.
+
 # multi-line inline code gives useful warning
 
     Code

tests/testthat/test-markdown-code.R

@@ -30,6 +30,15 @@ test_that("can eval inline code", {
   expect_equal(out1$get_value("description"), "Description 4")
 })
 
+test_that("uppercase R in inline code gives warning", {
+  text <- "
+    #' @title Title `R 1 + 1`
+    #' @md
+    foo <- function() NULL
+  "
+  expect_snapshot(. <- roc_proc_text(rd_roclet(), text))
+})
+
 test_that("can eval fenced code", {
   out1 <- roc_proc_text(
     rd_roclet(),