Support macros when cross-compiling (#7118)

### Motivation:

Not supporting macros in cross-compilation is a major limitation,
especially for libraries like https://github.com/apple/swift-mmio.

### Modifications:

Added `enum BuildTriple { case tools, destination }` and `var
buildTriple: BuildTriple` on `ResolvedTarget` and `ResolvedProduct`.
We're not using "host" and "target" triple terminology in this enum, as
that clashes with build system "targets" and can lead to confusion in
this context.

Corresponding value is assigned to this property depending on target and
product type: `tools` for macros, plugins, and their dependencies,
`destination` for everything else (the default). Based on this property
we can choose between `buildParameters.hostTriple` and
`buildParameters.targetTriple` during build plan construction.

Additionally, the resolved products and resolved targets graph is now
constructed in a way that allows certain targets and products to be
built twice: once for host triple, once for target triple if needed.
This required modifying build description and build manifest generation
to distinguish these products and targets from each other that are built
twice.

Artifacts built for the host now have `-tools` suffix appended to their
names. This cascaded into making changes throughout the code base for
build tool plugins and package plugins handling, which constructed their
paths in an ad-hoc manner without accounting for possible changes to
these names.

Also added `CrossCompilationPackageGraphTests` and
`CrossCompilationBuildPlanTests` to verify the changes made are applied
correctly.

### Result:

Resolves #6950
Resolves rdar://105991372

Author: MaxDesiatov

CHANGELOG.md

@@ -1,18 +1,29 @@
 Note: This is in reverse chronological order, so newer entries are added to the top.
 
-Swift Next
+Swift 6.0
 -----------
 
 * [#7202]
 
   Package manifests can now access information about the Git repository the given package is in via the context object's 
   `gitInformation` property. This allows to determine the current tag (if any), the current commit and whether or not there are uncommited changes.
 
+* [#7201]
+
+  `// swift-tools-version:` can now be specified on subsequent lines of `Package.swift`, for example when first few lines are required to contain a licensing comment header.
+
+* [#7118]
+
+  Macros cross-compiled by SwiftPM with Swift SDKs are now correctly built, loaded, and evaluated for the host triple.
+
+Swift 5.10
+-----------
+
 * [#7010]
 
   On macOS, `swift build` and `swift run` now produce binaries that allow backtraces in debug builds. Pass `SWIFT_BACKTRACE=enable=yes` environment variable to enable backtraces on such binaries when running them.
 
-* [7101]
+* [#7101]
 
    Binary artifacts are now cached along side repository checkouts so they do not need to be re-downloaded across projects.
 
@@ -387,4 +398,8 @@ Swift 3.0
 [#6276]: https://github.com/apple/swift-package-manager/pull/6276
 [#6540]: https://github.com/apple/swift-package-manager/pull/6540
 [#6663]: https://github.com/apple/swift-package-manager/pull/6663
+[#7010]: https://github.com/apple/swift-package-manager/pull/7010
 [#7101]: https://github.com/apple/swift-package-manager/pull/7101
+[#7118]: https://github.com/apple/swift-package-manager/pull/7118
+[#7201]: https://github.com/apple/swift-package-manager/pull/7201
+[#7202]: https://github.com/apple/swift-package-manager/pull/7202

Sources/Basics/Collections/IdentifiableSet.swift

@@ -45,13 +45,22 @@ public struct IdentifiableSet<Element: Identifiable>: Collection {
     }
 
     public subscript(id: Element.ID) -> Element? {
-        self.storage[id]
+        get {
+            self.storage[id]
+        }
+        set {
+            self.storage[id] = newValue
+        }
     }
 
     public func index(after i: Index) -> Index {
         Index(storageIndex: self.storage.index(after: i.storageIndex))
     }
 
+    public mutating func insert(_ element: Element) {
+        self.storage[element.id] = element
+    }
+
     public func union(_ otherSequence: some Sequence<Element>) -> Self {
         var result = self
         for element in otherSequence {

Sources/Build/BuildDescription/ClangTargetBuildDescription.swift

@@ -19,6 +19,9 @@ import struct SPMBuildCore.BuildParameters
 import struct SPMBuildCore.BuildToolPluginInvocationResult
 import struct SPMBuildCore.PrebuildCommandResult
 
+@_spi(SwiftPMInternal)
+import SPMBuildCore
+
 import enum TSCBasic.ProcessEnv
 
 /// Target description for a Clang target i.e. C language family target.
@@ -49,7 +52,7 @@ public final class ClangTargetBuildDescription {
 
     /// Path to the bundle generated for this module (if any).
     var bundlePath: AbsolutePath? {
-        guard !resources.isEmpty else {
+        guard !self.resources.isEmpty else {
             return .none
         }
 
@@ -127,7 +130,7 @@ public final class ClangTargetBuildDescription {
         self.target = target
         self.toolsVersion = toolsVersion
         self.buildParameters = buildParameters
-        self.tempsPath = buildParameters.buildPath.appending(component: target.c99name + ".build")
+        self.tempsPath = target.tempsPath(buildParameters)
         self.derivedSources = Sources(paths: [], root: tempsPath.appending("DerivedSources"))
 
         // We did not use to apply package plugins to C-family targets in prior tools-versions, this preserves the behavior.
@@ -219,7 +222,7 @@ public final class ClangTargetBuildDescription {
         if self.buildParameters.triple.isDarwin() {
             args += ["-fobjc-arc"]
         }
-        args += try buildParameters.targetTripleArgs(for: target)
+        args += try self.buildParameters.tripleArgs(for: target)
 
         args += optimizationArguments
         args += activeCompilationConditions

Sources/Build/BuildDescription/ProductBuildDescription.swift

@@ -311,7 +311,7 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
         // setting is the package-level right now. We might need to figure out a better
         // answer for libraries if/when we support specifying deployment target at the
         // target-level.
-        args += try self.buildParameters.targetTripleArgs(for: self.product.targets[self.product.targets.startIndex])
+        args += try self.buildParameters.tripleArgs(for: self.product.targets[self.product.targets.startIndex])
 
         // Add arguments from declared build settings.
         args += self.buildSettingsFlags
@@ -346,7 +346,7 @@ public final class ProductBuildDescription: SPMBuildCore.ProductBuildDescription
         // Library search path for the toolchain's copy of SwiftSyntax.
         #if BUILD_MACROS_AS_DYLIBS
         if product.type == .macro {
-            args += try ["-L", buildParameters.toolchain.hostLibDir.pathString]
+            args += try ["-L", defaultBuildParameters.toolchain.hostLibDir.pathString]
         }
         #endif
 

Sources/Build/BuildDescription/ResolvedTarget+BuildDescription.swift

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2015-2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import struct Basics.AbsolutePath
+import struct PackageGraph.ResolvedTarget
+
+@_spi(SwiftPMInternal)
+import SPMBuildCore
+
+extension ResolvedTarget {
+    func tempsPath(_ buildParameters: BuildParameters) -> AbsolutePath {
+        buildParameters.buildPath.appending(component: self.c99name + "\(self.buildTriple.suffix).build")
+    }
+}