Add trailing newline if missing from messages

Addresses posit-dev/positron#1878

crates/ark/src/modules/public/errors.R

@@ -62,10 +62,31 @@
         return()
     }
 
+    msg <- conditionMessage(cnd)
+
+    # Add trailing newline if there isn't one, including for the length 0
+    # case. See https://github.com/posit-dev/positron/issues/1878.
+    #
+    # This is to accomodate base message which may or may not have an
+    # explicit trailing newline, see the `appendLF` argument of
+    # `message()`. Because other message emitters might conform to the
+    # behaviour of `warning()` and `stop()` which never add newlines, we
+    # have to account for both possibility.
+    #
+    # Because of this we lose support for constructing messages within a
+    # single line piece by piece with `message(appendLF = FALSE)`, but
+    # that's an esoteric feature. If this is reported, we could do more
+    # specialised behaviour depending on whether the message condition is
+    # inherits from `"simpleMessage"` or not.
+    n <- nchar(msg)
+    if (substr(msg, n, n) != "\n") {
+        msg <- paste0(msg, "\n")
+    }
+
     # Output the condition message to the relevant stream (normally
     # stdout). Note that for historical reasons, messages include a
     # trailing newline
-    cat(conditionMessage(cnd), file = default_message_file())
+    cat(msg, file = default_message_file())
 
     # Silence default message handling
     invokeRestart("muffleMessage")