Add SwiftPM @testable build setting proposal

Author: jakepetroules

proposals/0000-swiftpm-testable-build-setting.md

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+# SwiftPM @testable build setting
+
+* Proposal: [SE-0000](0000-swiftpm-testable-build-setting.md)
+* Authors: [Jake Petroules](https://github.com/jakepetroules)
+* Review Manager: TBD
+* Status: **Awaiting review**
+* Implementation: [swiftlang/swift-package-manager#8004](https://github.com/swiftlang/swift-package-manager/pull/8004)
+* Review: ([pitch](https://forums.swift.org/t/pitch-swiftpm-testable-build-setting/75084))
+
+## Introduction
+
+The current Swift Package Manager build system is currently hardcoded to pass the `-enable-testing` flag to the Swift compiler to enable `@testable import` when building in debug mode. 
+
+Swift-evolution thread: [Pitch: [SwiftPM] @testable build setting](https://forums.swift.org/t/pitch-swiftpm-testable-build-setting/75084)
+
+## Motivation
+
+Not all targets in a given package make use of the `@testable import` feature (or wish to use it at all), but all targets are presently forced to build their code with this support enabled regardless of whether it's needed.
+
+Developers should be able to disable `@testable import` when it's not needed or desired, just as they're able to do so in Xcode's build system.
+
+On Windows in particular, where a shared library is limited to 65k exported symbols, disabling `@testable import` provides developers an option to significantly reduce the exported symbol count of a library by hiding all of the unnecessary internal APIs. It can also improve debug build performance as fewer symbols exported from a binary can result in faster linking.
+
+## Proposed solution
+
+Add a new Swift target setting API to specify whether testing should be enabled for the specified target, falling back to the current behavior by default.
+
+## Detailed design
+
+Add a new `enableTesting` API to `SwiftSetting` limited to manifests >= 6.1:
+
+```swift
+public struct SwiftSetting {
+  // ... other settings
+  
+  @available(_PackageDescription, introduced: 6.1)
+  public static func enableTesting(
+      _ enable: Bool,
+      _ condition: BuildSettingCondition? = nil
+  ) -> SwiftSetting {
+     ...
+  }
+}
+```
+
+The existing `--enable-testable-imports` / `--disable-testable-imports` command line flag to `swift-test` currently defaults to `--enable-testable-imports`. It will be changed to default to "unspecified" (respecting any target settings), and explicitly passing `--enable-testable-imports` or `--disable-testable-imports` will force all targets to enable or disable testing, respectively.
+
+## Security
+
+New language version setting has no implications on security, safety or privacy.
+
+## Impact on existing packages
+
+Since this is a new API, all existing packages will use the default behavior - testing will be enabled when building for the debug configuration, and disabled when building for the release configuration.
+
+## Alternatives considered
+
+None.