* lisp/env.el (env--substitute-vars-regexp): New const.

(substitute-env-vars): Use it.  Add `only-defined' arg.
* lisp/net/tramp.el (tramp-replace-environment-variables): Use it.

lisp/ChangeLog

 2012-11-08  Stefan Monnier  <monnier@iro.umontreal.ca>
+	* env.el (env--substitute-vars-regexp): New const.
+	(substitute-env-vars): Use it.  Add `only-defined' arg.
+	* net/tramp.el (tramp-replace-environment-variables): Use it.
+
 	* emacs-lisp/bytecomp.el (byte-compile-initial-macro-environment):
 	Byte-compile *before* eval in eval-and-compile.
 	(byte-compile-log-warning): Remove redundant inhibit-read-only.

lisp/env.el

@@ -57,31 +57,31 @@ If it is also not t, RET does not exit if it does non-null completion."
 ;; History list for VALUE argument to setenv.
 (defvar setenv-history nil)
 
+(defconst env--substitute-vars-regexp
+  (rx "$"
+      (or (submatch-n 1 (1+ (regexp "[[:alnum:]_]")))
+          (and "{" (submatch-n 1 (minimal-match (0+ anything))) "}")
+          "$")))
 
-(defun substitute-env-vars (string)
+(defun substitute-env-vars (string &optional only-defined)
   "Substitute environment variables referred to in STRING.
 `$FOO' where FOO is an environment variable name means to substitute
 the value of that variable.  The variable name should be terminated
 with a character not a letter, digit or underscore; otherwise, enclose
 the entire variable name in braces.  For instance, in `ab$cd-x',
 `$cd' is treated as an environment variable.
+If ONLY-DEFINED is nil, references to undefined environment variables
+are replaced by the empty string; if it is non-nil, they are left unchanged.
 
 Use `$$' to insert a single dollar sign."
   (let ((start 0))
-    (while (string-match
-	    (eval-when-compile
-	      (rx (or (and "$" (submatch (1+ (regexp "[[:alnum:]_]"))))
-		      (and "${" (submatch (minimal-match (0+ anything))) "}")
-		      "$$")))
-	    string start)
+    (while (string-match env--substitute-vars-regexp string start)
       (cond ((match-beginning 1)
 	     (let ((value (getenv (match-string 1 string))))
+               (if (and (null value) only-defined)
+                   (setq start (match-end 0))
 	       (setq string (replace-match (or value "") t t string)
-		     start (+ (match-beginning 0) (length value)))))
-	    ((match-beginning 2)
-	     (let ((value (getenv (match-string 2 string))))
-	       (setq string (replace-match (or value "") t t string)
-		     start (+ (match-beginning 0) (length value)))))
+                       start (+ (match-beginning 0) (length value))))))
 	    (t
 	     (setq string (replace-match "$" t t string)
 		   start (+ (match-beginning 0) 1)))))
@@ -185,7 +185,7 @@ VARIABLE should be a string.  Value is nil if VARIABLE is undefined in
 the environment.  Otherwise, value is a string.
 
 If optional parameter FRAME is non-nil, then it should be a
-frame.  This function will look up VARIABLE in its 'environment
+frame.  This function will look up VARIABLE in its `environment'
 parameter.
 
 Otherwise, this function searches `process-environment' for

lisp/minibuffer.el

@@ -51,6 +51,9 @@
 
 ;;; Todo:
 
+;; - Make *Completions* readable even if some of the completion
+;;   entries have LF chars or spaces in them (including at
+;;   beginning/end) or are very long.
 ;; - for M-x, cycle-sort commands that have no key binding first.
 ;; - Make things like icomplete-mode or lightning-completion work with
 ;;   completion-in-region-mode.
@@ -74,6 +77,9 @@
 ;;   - whether the user wants completion to pay attention to case.
 ;;   e.g. we may want to make it possible for the user to say "first try
 ;;   completion case-sensitively, and if that fails, try to ignore case".
+;;   Maybe the trick is that we should distinguish completion-ignore-case in
+;;   try/all-completions (obey user's preference) from its use in
+;;   test-completion (obey the underlying object's semantics).
 
 ;; - add support for ** to pcm.
 ;; - Add vc-file-name-completion-table to read-file-name-internal.
@@ -2048,6 +2054,8 @@ This is only used when the minibuffer area has no active minibuffer.")
           process-environment))
 
 (defconst completion--embedded-envvar-re
+  ;; We can't reuse env--substitute-vars-regexp because we need to match only
+  ;; potentially-unfinished envvars at end of string.
   (concat "\\(?:^\\|[^$]\\(?:\\$\\$\\)*\\)"
           "$\\([[:alnum:]_]*\\|{\\([^}]*\\)\\)\\'"))
 

lisp/net/tramp.el

@@ -1748,20 +1748,26 @@ value of `default-file-modes', without execute permissions."
   (or (file-modes filename)
       (logand (default-file-modes) (tramp-compat-octal-to-decimal "0666"))))
 
-(defun tramp-replace-environment-variables (filename)
-  "Replace environment variables in FILENAME.
+(defalias 'tramp-replace-environment-variables
+  (if (ignore-errors
+        (equal "${ tramp?}" (substitute-env-vars "${ tramp?}" 'only-defined)))
+      (lambda (filename)
+        "Like `substitute-env-vars' with `only-defined' non-nil."
+        (substitute-env-vars filename 'only-defined))
+    (lambda (filename)
+      "Replace environment variables in FILENAME.
 Return the string with the replaced variables."
-  (save-match-data
-    (let ((idx (string-match "$\\(\\w+\\)" filename)))
-      ;; `$' is coded as `$$'.
-      (when (and idx
-		 (or (zerop idx) (not (eq ?$ (aref filename (1- idx)))))
-		 (getenv (match-string 1 filename)))
-	(setq filename
-	      (replace-match
-	       (substitute-in-file-name (match-string 0 filename))
-	       t nil filename)))
-      filename)))
+      (save-match-data
+        (let ((idx (string-match "$\\(\\w+\\)" filename)))
+          ;; `$' is coded as `$$'.
+          (when (and idx
+                     (or (zerop idx) (not (eq ?$ (aref filename (1- idx)))))
+                     (getenv (match-string 1 filename)))
+            (setq filename
+                  (replace-match
+                   (substitute-in-file-name (match-string 0 filename))
+                   t nil filename)))
+          filename)))))
 
 ;; In XEmacs, electricity is implemented via a key map for ?/ and ?~,
 ;; which calls corresponding functions (see minibuf.el).