Support regex literals

https://github.com/apple/swift-evolution/blob/main/proposals/0354-regex-literals.md

Author: taku0

README.md

@@ -172,6 +172,15 @@ var x = foo
 
 Both are syntactically correct code. We cannot handle this case properly. This is also a future work.
 
+Other example is regex literals and custom operators.  The following example is valid Swift code with regex literals and custom operators.
+
+```swift
+let x = /^/ /^/ /^/
+```
+
+We parse them as regex literals rather than custom operators for now.
+
+
 ## Hacking
 
 To build the package locally, run `make package`.

swift-mode-indent.el

@@ -225,11 +225,15 @@ declaration and its offset is `swift-mode:basic-offset'."
       (swift-mode:indentation (point) 0)))))
 
 (defun swift-mode:calculate-indent-of-multiline-string ()
-  "Return the indentation of the current line inside a multiline string."
+  "Return the indentation of the current line inside a multiline string.
+
+Also used for regexes."
   (back-to-indentation)
   (let ((string-beginning-position
          (save-excursion (swift-mode:beginning-of-string))))
-    (if (looking-at "\"\"\"")
+    (if (and (looking-at "\\(\"\"\"\\|/\\)#*")
+             (equal (get-text-property (1- (match-end 0)) 'syntax-table)
+                    (string-to-syntax "|")))
         ;; The last line.
         (progn
           (goto-char string-beginning-position)
@@ -240,13 +244,13 @@ declaration and its offset is `swift-mode:basic-offset'."
       (swift-mode:goto-non-interpolated-expression-bol)
       (back-to-indentation)
       (if (<= (point) string-beginning-position)
-          ;; The cursor was on the 2nd line of the comment, so aligns with
+          ;; The cursor was on the 2nd line of the string, so aligns with
           ;; that line with offset.
           (progn
             (goto-char string-beginning-position)
             (swift-mode:calculate-indent-of-expression
              swift-mode:multiline-statement-offset))
-        ;; The cursor was on the 3rd or following lines of the comment, so
+        ;; The cursor was on the 3rd or following lines of the string, so
         ;; aligns with a non-empty preceding line.
         (if (and (bolp) (eolp))
             ;; The cursor is on an empty line, so seeks a non-empty-line.