New FP Parsing: Use relevant part of a substring

stdlib/public/core/FloatingPointParsing.swift.gyb

@@ -256,14 +256,13 @@ extension ${Self}: LosslessStringConvertible {
   // Use the all-Swift `parse_float${bits}()` implementation for Float16/32/64
   @available(SwiftStdlib 5.3, *)
   public init?(_ text: Substring) {
-    // TODO: Someday, this whole function should simplify down to just:
-    // ${Self}(text.utf8.span)
   #if _pointerBitWidth(_16)
     // Always fail on 16-bit targets
     return nil
   #else
-    // Work around span availability limits
-    let parsed = unsafe text.base._guts.withFastUTF8 { chars -> ${Self}? in
+    var mutableText = text
+    let parsed: ${Self}?
+    parsed = mutableText.withUTF8 { chars -> ${Self}? in
       unsafe parse_float${bits}(chars.span)
     }
 

test/stdlib/ParseFloat64-bridging.swift

@@ -0,0 +1,39 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -g %s -o %t/a.out -enable-experimental-feature Extern
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out
+
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+// Needed to declare the ABI entry point
+// REQUIRES: swift_feature_Extern
+
+import StdlibUnittest
+import Foundation
+
+let tests = TestSuite("FloatingPointParsing")
+
+tests.test("Bridged - short") {
+  let s1 = "1.02.03.0"
+  let nss1 = NSString(utf8String: s1)!
+  let bridged = String(nss1)
+  let range = bridged.firstIndex(of: "2")!..<bridged.firstIndex(of: "3")!
+  let sub = bridged[range]
+  let parsed = Float64(sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
+tests.test("Bridged - long") {
+  let s1 = "1.02.0000000000000000000000000000000000000000000000000000000000003.04.05.06.07.08.09.010.011.012.013.014.015.0"
+  let nss1 = NSString(utf8String: s1)!
+  let bridged = String(nss1)
+  let range = bridged.firstIndex(of: "2")!..<bridged.firstIndex(of: "3")!
+  let sub = bridged[range]
+  let parsed = Float64(sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
+runAllTests()

test/stdlib/ParseFloat64.swift

@@ -325,6 +325,47 @@ tests.test("Decimal Floats") {
   expectParse("999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999.999999999999999999999999999", Float64.infinity)
 }
 
+tests.test("Substring - short") {
+  let s1 = "1.02.03.0"
+  let s1sub = s1[s1.firstIndex(of: "2")!..<s1.firstIndex(of: "3")!]
+  let parsed = Float64(s1sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
+tests.test("Substring - long") {
+  let s1 = "1.00000000000000000000000000000000002.0000000000000000000000000000000000000000000000000000000003.00000000000000004.0000000000000000"
+  let s1sub = s1[s1.firstIndex(of: "2")!..<s1.firstIndex(of: "3")!]
+  let parsed = Float64(s1sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
+/*
+ // These need Foundation to run, so can't run on Linux?
+tests.test("Bridged - short") {
+  let s1 = "1.02.03.0"
+  let nss1 = NSString(utf8String: s1)!
+  let bridged = String(nss1)
+  let range = bridged.firstIndex(of: "2")!..<bridged.firstIndex(of: "3")!
+  let sub = bridged[range]
+  let parsed = Float64(sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+
+tests.test("Bridged - long") {
+  let s1 = "1.02.0000000000000000000000000000000000000000000000000000000000003.04.05.06.07.08.09.010.011.012.013.014.015.0"
+  let nss1 = NSString(utf8String: s1)!
+  let bridged = String(nss1)
+  let range = bridged.firstIndex(of: "2")!..<bridged.firstIndex(of: "3")!
+  let sub = bridged[range]
+  let parsed = Float64(sub)
+  expectNotNil(parsed)
+  expectEqual(parsed!.bitPattern, (2.0).bitPattern)
+}
+ */
+
 @_extern(c, "_swift_stdlib_strtod_clocale")
 func _swift_stdlib_strtod_clocale(
   _: Optional<UnsafePointer<CChar>>,