fix(emacs): clear diagnostics properly in Flymake

When our Flymake code set diagnostics, it set them for the whole
buffer with :range. However, the :range was incorrect; the begin/end
points given were for the *stdout* temporary buffer, not for the
user's code buffer. This meant that Flymake diagnostics markers were
often not removed because they were not inside the range. (stdout is
often much smaller than the JavaScript source being linted.)

Fix this by using :range with points derived from the correct buffer.

docs/CHANGELOG.md

@@ -50,6 +50,8 @@ Semantic Versioning.
   in the head of a `for` loop. For example, quick-lint-js no longer warns about
   `let x; for (let x = 0;;);`.
 * Emacs: .el files are now installed in the correct place on Arch Linux, btw.
+* Emacs: The Flymake plugin now reliably clears out diagnostics after issues are
+  fixed. Sticky diagnostics are no more.
 * TypeScript support (still experimental):
   * A newline after `public`, `protected`, `private`, or `readonly` inside a
     class is now interpreted correctly.

plugin/emacs/flymake-quicklintjs.el

@@ -96,12 +96,13 @@ REPORT-FN is Flymake's callback."
                                          (point-min) (point-max))))
                        (if (not (string-empty-p stderr-data))
                            (flymake-log :warning "%S" stderr-data))))
-                   (with-current-buffer stdout-buf
-                     (let ((diags (flymake-quicklintjs--make-diagnostics
-                                  src-buf
-                                  (car (read-from-string
-                                        (buffer-substring-no-properties
-                                         (point-min) (point-max)))))))
+                   (let ((diags (flymake-quicklintjs--make-diagnostics
+                                 src-buf
+                                 (car (read-from-string
+                                       (with-current-buffer stdout-buf
+                                         (buffer-substring-no-properties
+                                          (point-min) (point-max))))))))
+                     (with-current-buffer src-buf
                        (if (or diags (zerop (process-exit-status p)))
                            (funcall report-fn diags
                                     :region (cons (point-min) (point-max)))

plugin/emacs/test/quicklintjs-test.el

@@ -110,6 +110,30 @@ foobar\")((16 . 22) 2 \"E0057\" \"use of undeclared variable: foobar\")(\
                      (ert-fail "Test timed out waiting for diagnostics."))
         ;; TODO(strager): Assert specific diagnostics
         (while (not (flymake-diagnostics))
+          (accept-process-output nil 0.01)))))
+
+  ;; This is a regression test. Buffers were mixed up causing
+  ;; diagnostics after a certain point (usually a few bytes in) to not
+  ;; be cleared.
+  (ert-deftest quicklintjs-flymake-fixing-error-clears-diagnostics ()
+    (with-temp-buffer
+      (javascript-mode)
+      (insert "/*xxx*/ consol")
+      (flymake-mode 1)
+      (add-hook 'flymake-diagnostic-functions #'flymake-quicklintjs nil t)
+      (flymake-start)
+
+      (with-timeout (5
+                     (ert-fail "Test timed out waiting for diagnostics."))
+        (while (not (flymake-diagnostics))
+          (accept-process-output nil 0.01)))
+
+      (insert "e")  ;; Buffer content: /*xxx*/ console
+      (flymake-start)
+
+      (with-timeout (5
+                     (ert-fail "Test timed out waiting for diagnostics to be removed."))
+        (while (flymake-diagnostics)
           (accept-process-output nil 0.01))))))
 
 (defun def-eglot-tests ()