Use envrc-mode-hook to add back ends; closes #706

Also improve racket-repl-buffer-name-project to be per back end as
well as per project. In other words, if a project subdir happens to
use an envrc with a distinct back end, that has to be a distinct REPL.

racket-back-end.el

@@ -257,6 +257,32 @@ 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.
 
@@ -443,13 +469,13 @@ a possibly slow remote connection."
 (defun racket--back-end-args->command (back-end racket-command-args)
   "Given RACKET-COMMAND-ARGS, prepend path to racket for BACK-END."
   (if (racket--back-end-local-p back-end)
-      `(,(or (plist-get back-end :racket-program)
-             (executable-find racket-program)
-             (user-error
-              "Cannot find Racket executable\nracket-program: %S\nexec-path: %S"
-              racket-program
-              exec-path))
-        ,@racket-command-args)
+      (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"
+                 racket-program
+                 exec-path))
+            racket-command-args)
     (pcase-let ((`(,host ,user ,port ,_name)
                  (racket--file-name->host+user+port+name
                   (plist-get back-end :directory))))

racket-repl-buffer-name.el

@@ -26,7 +26,7 @@ customization."
 
 ;;;###autoload
 (defun racket-repl-buffer-name-shared ()
-  "All `racket-mode' edit buffers share one `racket-repl-mode' buffer per back end.
+  "Share one `racket-repl-mode' buffer per back end.
 
 A value for the variable `racket-repl-buffer-name-function'."
   (interactive)
@@ -45,14 +45,15 @@ A value for the variable `racket-repl-buffer-name-function'."
 
 ;;;###autoload
 (defun racket-repl-buffer-name-project ()
-  "All `racket-mode' buffers in a project share a `racket-repl-mode' buffer.
+  "Share a `racket-repl-mode' buffer per back end and per project.
 
 A value for the variable `racket-repl-buffer-name-function'.
 
 The \"project\" is determined by `racket-project-root'."
   (interactive)
   (setq-local racket-repl-buffer-name
-              (format "*Racket REPL <%s>*"
+              (format "*Racket REPL <%s %s>*"
+                      (racket-back-end-name)
                       (racket--file-name-sans-remote-method
                        (racket-project-root (racket--buffer-file-name))))))