userdiff: add support for Swift

Add a built-in userdiff driver for the Swift programming language so that
diff hunk headers and word diffs work out of the box for ".swift" files.

The funcname pattern is built for Swift's own declaration grammar: an
optional run of attributes ("@objc", "@available(iOS 13, *)", ...),
followed by an optional run of lowercase modifiers ("public", "static",
"final", ...), followed by a declaration keyword (func, class, struct,
enum, protocol, extension, actor, init, deinit, subscript). The keyword
is followed by a boundary that allows whitespace, "(" (init/subscript),
"?" or "!" (failable init), or "<" (generics), while still acting as a
word boundary so e.g. "initialize(" does not match.

The word regex recognizes Swift identifiers, hexadecimal, octal, binary,
integer and floating-point literals, and the language's operators.

Signed-off-by: Shlok Kulshreshtha <diy2903@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: shkuls

Documentation/gitattributes.adoc

@@ -914,6 +914,8 @@ patterns are available:
 - `scheme` suitable for source code in most Lisp dialects,
   including Scheme, Emacs Lisp, Common Lisp, and Clojure.
 
+- `swift` suitable for source code in the Swift language.
+
 - `tex` suitable for source code for LaTeX documents.
 
 

t/t4018/swift-actor

@@ -0,0 +1,5 @@
+actor RIGHT {
+    let a = 1
+    // a comment
+    let b = ChangeMe
+}

t/t4018/swift-attribute-with-args

@@ -0,0 +1,7 @@
+struct View {
+    @available(iOS 13, *) public func RIGHT() {
+        let a = 1
+        // a comment
+        print(ChangeMe)
+    }
+}

t/t4018/swift-class

@@ -0,0 +1,5 @@
+class RIGHT {
+    let a = 1
+    // a comment
+    let b = ChangeMe
+}

t/t4018/swift-enum

@@ -0,0 +1,5 @@
+enum RIGHT {
+    case first
+    // a comment
+    case ChangeMe
+}

t/t4018/swift-extension

@@ -0,0 +1,5 @@
+extension RIGHT {
+    static let a = 1
+    // a comment
+    static let b = ChangeMe
+}

t/t4018/swift-failable-init

@@ -0,0 +1,7 @@
+class Bar {
+    init?(RIGHT: Int) {
+        let x = 0
+        // a comment
+        print(ChangeMe)
+    }
+}

t/t4018/swift-func

@@ -0,0 +1,5 @@
+func RIGHT(x: Int) -> Int {
+    let y = x
+    // a comment
+    return ChangeMe
+}

t/t4018/swift-generic-subscript

@@ -0,0 +1,7 @@
+struct Container {
+    subscript<RIGHT>(index: Int) -> Int {
+        let a = 0
+        // a comment
+        return ChangeMe
+    }
+}

t/t4018/swift-init

@@ -0,0 +1,7 @@
+class Foo {
+    init(RIGHT: Int) {
+        let x = 0
+        // a comment
+        print(ChangeMe)
+    }
+}