[stdlib] Availability checking for String.*View.popFirst

Collection's popFirst is only present when the Collection is its own
SubSequence type. String and String's views are no longer their own
SubSequenes, so popFirst is no longer present. Unfortunately, this
breaks code in swift-version 3 and it also gives a terrible diagnostic
to users. This change introduces an implementation for swift-version 3
mode and better diagnostics for Swift 4 code. Tests included.

stdlib/public/core/Substring.swift.gyb

@@ -625,3 +625,39 @@ extension Substring {
   }
 }
 //===----------------------------------------------------------------------===//
+
+// popFirst() is only present when a collection is its own subsequence. This was
+// dropped in Swift 4.
+extension String {
+  @available(swift, obsoleted: 4, message:
+    "Please use 'first', 'dropFirst()', or 'Substring.popFirst()'.")
+  public mutating func popFirst() -> String.Element? {
+    guard !isEmpty else { return nil }
+    let element = first!
+    let nextIdx = self.index(after: self.startIndex)
+    self = String(self[nextIdx...])
+    return element
+  }
+}
+extension String.CharacterView {
+  @available(swift, obsoleted: 4, message:
+    "Please use 'first', 'dropFirst()', or 'Substring.CharacterView.popFirst()'.")
+  public mutating func popFirst() -> String.CharacterView.Element? {
+    guard !isEmpty else { return nil }
+    let element = first!
+    let nextIdx = self.index(after: self.startIndex)
+    self = String(self[nextIdx...]).characters
+    return element
+  }
+}
+extension String.UnicodeScalarView {
+  @available(swift, obsoleted: 4, message:
+    "Please use 'first', 'dropFirst()', or 'Substring.UnicodeScalarView.popFirst()'.")
+  public mutating func popFirst() -> String.UnicodeScalarView.Element? {
+    guard !isEmpty else { return nil }
+    let element = first!
+    let nextIdx = self.index(after: self.startIndex)
+    self = String(self[nextIdx...]).unicodeScalars
+    return element
+  }
+}

test/stdlib/StringCompatibility.swift

@@ -356,5 +356,20 @@ Tests.test("LosslessStringConvertible/force unwrap/\(swift)") {
 }
 #endif
 
+#if !swift(>=4)
+Tests.test("popFirst") {
+  var str = "abcdef"
+  expectEqual("a", str.popFirst()!)
+  expectEqual("bcdef", str)
+  expectEqual("b", str.characters.popFirst()!)
+  expectEqual("cdef", str)
+  expectEqual("c", str.unicodeScalars.popFirst()!)
+  expectEqual("def", str)
+  expectEqual("d", str.popFirst()!)
+  expectEqual("e", str.popFirst()!)
+  expectEqual("f", str.popFirst()!)
+  expectEqual(nil, str.popFirst())
+}
+#endif
 
 runAllTests()

test/stdlib/StringCompatibilityDiagnostics.swift

@@ -0,0 +1,14 @@
+// RUN: %swift -typecheck -swift-version 4 %s -verify
+
+func testPopFirst() {
+  var str = "abc"
+  _ = str.popFirst() // expected-error{{'popFirst()' is unavailable: Please use 'first', 'dropFirst()', or 'Substring.popFirst()'}}
+  _ = str.characters.popFirst() // expected-error{{'popFirst()' is unavailable: Please use 'first', 'dropFirst()', or 'Substring.CharacterView.popFirst()'}}
+  _ = str.unicodeScalars.popFirst() // expected-error{{'popFirst()' is unavailable: Please use 'first', 'dropFirst()', or 'Substring.UnicodeScalarView.popFirst()'}}
+
+  var substr = str[...]
+  _ = substr.popFirst() // ok
+  _ = substr.characters.popFirst() // ok
+  _ = substr.unicodeScalars.popFirst() // ok
+}
+