Allow empty arrays of unsafe flags (#3586)

This is currently only disallowed using assertions, and those are compiled out for release builds.  There is no reason to disallow this, and it's useful when programmatically building up lists of unsafe flags.

Author: abertelrud

Sources/PackageModel/Manifest/TargetBuildSettingDescription.swift

@@ -61,7 +61,6 @@ public enum TargetBuildSettingDescription {
                 assert(value.count == 1, "\(tool) \(name) \(value)")
                 break
             case .unsafeFlags:
-                assert(value.count >= 1, "\(tool) \(name) \(value)")
                 break
             }
 

Tests/PackageLoadingTests/PackageBuilderTests.swift

@@ -1974,6 +1974,82 @@ class PackageBuilderTests: XCTestCase {
         }
     }
 
+    func testEmptyUnsafeFlagsAreAllowed() throws {
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/Sources/foo/foo.swift",
+            "/Sources/bar/bar.cpp",
+            "/Sources/bar/bar.c",
+            "/Sources/bar/include/bar.h"
+        )
+
+        let manifest = Manifest.createManifest(
+            name: "pkg",
+            v: .v5,
+            targets: [
+                try TargetDescription(
+                    name: "foo",
+                    settings: [
+                        .init(tool: .c, name: .unsafeFlags, value: []),
+                        .init(tool: .cxx, name: .unsafeFlags, value: []),
+                        .init(tool: .cxx, name: .unsafeFlags, value: [], condition: .init(config: "release")),
+                        .init(tool: .linker, name: .unsafeFlags, value: []),
+                    ]
+                ),
+                try TargetDescription(
+                    name: "bar",
+                    settings: [
+                        .init(tool: .swift, name: .unsafeFlags, value: [], condition: .init(platformNames: ["macos"], config: "debug")),
+                        .init(tool: .linker, name: .unsafeFlags, value: []),
+                        .init(tool: .linker, name: .unsafeFlags, value: [], condition: .init(platformNames: ["linux"])),
+                    ]
+                ),
+            ]
+        )
+
+        PackageBuilderTester(manifest, in: fs) { package, _ in
+            package.checkModule("foo") { package in
+                let macosDebugScope = BuildSettings.Scope(
+                    package.target.buildSettings,
+                    environment: BuildEnvironment(platform: .macOS, configuration: .debug)
+                )
+                XCTAssertEqual(macosDebugScope.evaluate(.OTHER_CFLAGS), [])
+                XCTAssertEqual(macosDebugScope.evaluate(.OTHER_CPLUSPLUSFLAGS), [])
+                XCTAssertEqual(macosDebugScope.evaluate(.OTHER_LDFLAGS), [])
+
+                let macosReleaseScope = BuildSettings.Scope(
+                    package.target.buildSettings,
+                    environment: BuildEnvironment(platform: .macOS, configuration: .release)
+                )
+                XCTAssertEqual(macosReleaseScope.evaluate(.OTHER_CFLAGS), [])
+                XCTAssertEqual(macosReleaseScope.evaluate(.OTHER_CPLUSPLUSFLAGS), [])
+                XCTAssertEqual(macosReleaseScope.evaluate(.OTHER_LDFLAGS), [])
+            }
+
+            package.checkModule("bar") { package in
+                let linuxDebugScope = BuildSettings.Scope(
+                    package.target.buildSettings,
+                    environment: BuildEnvironment(platform: .linux, configuration: .debug)
+                )
+                XCTAssertEqual(linuxDebugScope.evaluate(.OTHER_SWIFT_FLAGS), [])
+                XCTAssertEqual(linuxDebugScope.evaluate(.OTHER_LDFLAGS), [])
+
+                let linuxReleaseScope = BuildSettings.Scope(
+                    package.target.buildSettings,
+                    environment: BuildEnvironment(platform: .linux, configuration: .release)
+                )
+                XCTAssertEqual(linuxReleaseScope.evaluate(.OTHER_SWIFT_FLAGS), [])
+                XCTAssertEqual(linuxReleaseScope.evaluate(.OTHER_LDFLAGS), [])
+
+                let macosDebugScope = BuildSettings.Scope(
+                    package.target.buildSettings,
+                    environment: BuildEnvironment(platform: .macOS, configuration: .debug)
+                )
+                XCTAssertEqual(macosDebugScope.evaluate(.OTHER_SWIFT_FLAGS), [])
+                XCTAssertEqual(macosDebugScope.evaluate(.OTHER_LDFLAGS), [])
+            }
+        }
+    }
+
     func testInvalidHeaderSearchPath() throws {
         let fs = InMemoryFileSystem(emptyFiles:
             "/pkg/Sources/exe/main.swift"