Abandon automagical envrc-mode-hook; instead recommend dir-locals

Although a neat idea it can't reliably run soon enough.

Effectively this commit reverts a chunk of the previous commit
concerning that hook. However it preserves the change where we make
sure REPLs are per-back-end.

doc/racket-mode.texi

@@ -3092,6 +3092,9 @@ The following values will @emph{not} work:
 
 Function to call to browse a URL@.
 
+Defaults to @ref{racket-browse-url-using-temporary-file} on macOS and
+@code{browse-url} on other platforms.
+
 @node racket-xp-after-change-refresh-delay
 @subsection racket-xp-after-change-refresh-delay
 
@@ -3713,7 +3716,7 @@ A value for the variable @ref{racket-show-functions}.
 @code{(racket-repl-buffer-name-shared)}
 
 
-All @ref{racket-mode} edit buffers share one @ref{racket-repl-mode} buffer per back end.
+Share one @ref{racket-repl-mode} buffer per back end.
 
 A value for the variable @ref{racket-repl-buffer-name-function}.
 
@@ -3733,7 +3736,7 @@ A value for the variable @ref{racket-repl-buffer-name-function}.
 @code{(racket-repl-buffer-name-project)}
 
 
-All @ref{racket-mode} buffers in a project share a @ref{racket-repl-mode} buffer.
+Share a @ref{racket-repl-mode} buffer per back end and per project.
 
 A value for the variable @ref{racket-repl-buffer-name-function}.
 
@@ -3932,6 +3935,15 @@ are a few examples.
                          :racket-program "xvfb-run racket")
 @end lisp
 
+Tip: If you use various versions of Racket via .envrc files and
+@code{envrc-global-mode}, then in each such directory with a .envrc
+file you should have a .dir-locals.el file to force the use of a
+distinct back end:
+
+@lisp
+    ((nil . ((eval . (racket-add-back-end default-directory)))))
+@end lisp
+
 @node Running racket and raco commands in a shell or terminal (1)
 @section Running racket and raco commands in a shell or terminal
 

racket-back-end.el

@@ -200,6 +200,15 @@ are a few examples.
     (racket-add-back-end \"/ssh:headless:~/gui-project/\"
                          :racket-program \"xvfb-run racket\")
 #+END_SRC
+
+Tip: If you use various versions of Racket via .envrc files and
+`envrc-global-mode', then in each such directory with a .envrc
+file you should have a .dir-locals.el file to force the use of a
+distinct back end:
+
+#+BEGIN_SRC lisp
+    ((nil . ((eval . (racket-add-back-end default-directory)))))
+#+END_SRC
 "
   (unless (and (stringp directory) (file-name-absolute-p directory))
     (error "racket-add-back-end: directory must be file-name-absolute-p"))
@@ -257,32 +266,6 @@ are a few examples.
   (unless no-refresh-watches-p
     (racket--back-end-refresh-watches)))
 
-;; Avoid user needing to use dir-locals.el to manually create a
-;; distinct back end for each envrc directory; see issue #706.
-(eval-after-load 'envrc
-  '(progn
-     (defvar envrc--status)
-     (declare-function envrc--find-env-dir "ext:envrc" ())
-     (defun racket-back-end-envrc-mode-hook ()
-       "A value for `envrc-mode-hook'.
-
-Automatically configures a distinct back end for the directory of
-the dominating envrc file, if any. As a result, each such back
-end uses appropriate `exec-path' and `process-environment'
-values."
-       (when-let ((env-dir (and (eq 'on envrc--status)
-                                (envrc--find-env-dir))))
-         (unless (cl-find env-dir
-                          racket-back-end-configurations
-                          :test
-                          (lambda (dir back-end)
-                            (equal dir (plist-get back-end :directory))))
-           (when-let ((path-to-racket (executable-find (if racket--winp "Racket.exe" "racket"))))
-             (message "racket-back-end-envrc-mode-hook configuring back end for %S to use %S"
-                      env-dir path-to-racket)
-             (racket-add-back-end env-dir :racket-program path-to-racket)))))
-     (add-hook 'envrc-mode-hook 'racket-back-end-envrc-mode-hook)))
-
 (defun racket-back-end-name (&optional back-end)
   "Return the \"name\" of a back end.
 
@@ -471,8 +454,8 @@ a possibly slow remote connection."
   (if (racket--back-end-local-p back-end)
       (cons (or (executable-find (or (plist-get back-end :racket-program)
                                      racket-program))
-                (user-error
-                 "Cannot find Racket executable\nracket-program: %S\nexec-path: %S"
+                (error
+                 "Cannot executable-find Racket:\n  racket-program: %S\n  exec-path: %S"
                  racket-program
                  exec-path))
             racket-command-args)