Emacs: stay at the correct position when indenting

When indenting a non-blank line, stay at the same cursor position
relative to the content after indenting.

src/etc/emacs/rust-mode-tests.el

@@ -488,6 +488,26 @@ struct Foo {
 }
 "
       rust-test-region-string rust-test-motion-string
+      rust-test-indent-motion-string
+      "
+fn blank_line(arg:int) -> bool {
+
+}
+
+fn indenting_closing_brace() {
+    if(true) {
+}
+}
+
+fn indenting_middle_of_line() {
+    if(true) {
+ push_me_out();
+} else {
+               pull_me_back_in();
+}
+}
+"
+
       ;; Symbol -> (line column)
       rust-test-positions-alist '((start-of-fn1 (2 0))
                                   (start-of-fn1-middle-of-line (2 15))
@@ -502,7 +522,17 @@ struct Foo {
                                   (middle-of-fn3 (16 4))
                                   (middle-of-struct (21 10))
                                   (before-start-of-struct (19 0))
-                                  (after-end-of-struct (23 0))))
+                                  (after-end-of-struct (23 0))
+                                  (blank-line-indent-start (3 0))
+                                  (blank-line-indent-target (3 4))
+                                  (closing-brace-indent-start (8 1))
+                                  (closing-brace-indent-target (8 5))
+                                  (middle-push-indent-start (13 2))
+                                  (middle-push-indent-target (13 9))
+                                  (after-whitespace-indent-start (13 1))
+                                  (after-whitespace-indent-target (13 8))
+                                  (middle-pull-indent-start (15 19))
+                                  (middle-pull-indent-target (15 12))))
 
 (defun rust-get-buffer-pos (pos-symbol)
   "Get buffer position from POS-SYMBOL.
@@ -664,3 +694,38 @@ All positions are position symbols found in `rust-test-positions-alist'."
    'middle-of-struct
    'before-start-of-struct 'after-end-of-struct
    #'mark-defun))
+
+(ert-deftest indent-line-blank-line-motion ()
+  (rust-test-motion
+   rust-test-indent-motion-string
+   'blank-line-indent-start
+   'blank-line-indent-target
+   #'indent-for-tab-command))
+
+(ert-deftest indent-line-closing-brace-motion ()
+  (rust-test-motion
+   rust-test-indent-motion-string
+   'closing-brace-indent-start
+   'closing-brace-indent-target
+   #'indent-for-tab-command))
+
+(ert-deftest indent-line-middle-push-motion ()
+  (rust-test-motion
+   rust-test-indent-motion-string
+   'middle-push-indent-start
+   'middle-push-indent-target
+   #'indent-for-tab-command))
+
+(ert-deftest indent-line-after-whitespace-motion ()
+  (rust-test-motion
+   rust-test-indent-motion-string
+   'after-whitespace-indent-start
+   'after-whitespace-indent-target
+   #'indent-for-tab-command))
+
+(ert-deftest indent-line-middle-pull-motion ()
+  (rust-test-motion
+   rust-test-indent-motion-string
+   'middle-pull-indent-start
+   'middle-pull-indent-target
+   #'indent-for-tab-command))

src/etc/emacs/rust-mode.el

@@ -114,14 +114,14 @@
 
               ;; Otherwise we're in a column-zero definition
               (t 0))))))
-    (cond
-     ;; If we're to the left of the indentation, reindent and jump to it.
-     ((<= (current-column) indent)
-      (indent-line-to indent))
-
-     ;; We're to the right; if it needs indent, do so but save excursion.
-     ((not (eq (current-indentation) indent))
-      (save-excursion (indent-line-to indent))))))
+    (when (not (eq (current-indentation) indent))
+      ;; If we're at the beginning of the line (before or at the current
+      ;; indentation), jump with the indentation change.  Otherwise, save the
+      ;; excursion so that adding the indentations will leave us at the
+      ;; equivalent position within the line to where we were before.
+      (if (<= (current-column) (current-indentation))
+          (indent-line-to indent)
+        (save-excursion (indent-line-to indent))))))
 
 
 ;; Font-locking definitions and helpers