Tidy up build flags and fix implicit import circular dependency (swiftlang#392)

- Explicitly ask the compiler not to implicitly import _StringProessing. This is to avoid a circular dependency when `-enable-experimental-string-processing` is enabled by default.
- Unify the build flags for modules that are built in the compiler repo into a `stdlibSettings` value.
- Disable implicit _Concurrency import as well since it is how it's built in the compiler repo. This helps us catch errors before we integrate with the compiler repo.
- Remove `-enable-experimental-pairwise-build-block` since SE-0348 has been implemented and enabled.
- Update the minimum toolchain requirement to 2022-04-20.

Author: rxwei

Package.swift

@@ -3,6 +3,20 @@
 
 import PackageDescription
 
+let availabilityDefinition = PackageDescription.SwiftSetting.unsafeFlags([
+    "-Xfrontend",
+    "-define-availability",
+    "-Xfrontend",
+    #"SwiftStdlib 5.7:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999"#,
+])
+
+let stdlibSettings: [PackageDescription.SwiftSetting] = [
+    .unsafeFlags(["-enable-library-evolution"]),
+    .unsafeFlags(["-Xfrontend", "-disable-implicit-concurrency-module-import"]),
+    .unsafeFlags(["-Xfrontend", "-disable-implicit-string-processing-module-import"]),
+    availabilityDefinition
+]
+
 let package = Package(
     name: "swift-experimental-string-processing",
     products: [
@@ -29,9 +43,7 @@ let package = Package(
         .target(
             name: "_RegexParser",
             dependencies: [],
-            swiftSettings: [
-                .unsafeFlags(["-enable-library-evolution"])
-            ]),
+            swiftSettings: stdlibSettings),
         .testTarget(
             name: "MatchingEngineTests",
             dependencies: [
@@ -42,24 +54,19 @@ let package = Package(
         .target(
             name: "_StringProcessing",
             dependencies: ["_RegexParser", "_CUnicode"],
-            swiftSettings: [
-                .unsafeFlags(["-enable-library-evolution"]),
-            ]),
+            swiftSettings: stdlibSettings),
         .target(
             name: "RegexBuilder",
             dependencies: ["_StringProcessing", "_RegexParser"],
-            swiftSettings: [
-                .unsafeFlags(["-enable-library-evolution"]),
-                .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
-            ]),
+            swiftSettings: stdlibSettings),
         .testTarget(
             name: "RegexTests",
             dependencies: ["_StringProcessing"]),
         .testTarget(
             name: "RegexBuilderTests",
             dependencies: ["_StringProcessing", "RegexBuilder"],
             swiftSettings: [
-                .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
+                .unsafeFlags(["-Xfrontend", "-disable-availability-checking"])
             ]),
         .target(
             name: "Prototypes",
@@ -81,11 +88,11 @@ let package = Package(
 
         // MARK: Exercises
         .target(
-          name: "Exercises",
-          dependencies: ["_RegexParser", "Prototypes", "_StringProcessing", "RegexBuilder"],
-          swiftSettings: [
-              .unsafeFlags(["-Xfrontend", "-enable-experimental-pairwise-build-block"])
-          ]),
+            name: "Exercises",
+            dependencies: ["_RegexParser", "_StringProcessing", "RegexBuilder"],
+            swiftSettings: [
+                .unsafeFlags(["-Xfrontend", "-disable-availability-checking"])
+            ]),
         .testTarget(
           name: "ExercisesTests",
           dependencies: ["Exercises"]),

README.md

@@ -8,7 +8,7 @@ See [Declarative String Processing Overview][decl-string]
 
 ## Requirements
 
-- [Swift Trunk Development Snapshot](https://www.swift.org/download/#snapshots) DEVELOPMENT-SNAPSHOT-2022-03-09 or later.
+- [Swift Trunk Development Snapshot](https://www.swift.org/download/#snapshots) DEVELOPMENT-SNAPSHOT-2022-04-20 or later.
 
 ## Integration with Swift