Fix generateResourceAccessor WASI regression (#3819)

* Fix `generateResourceAccessor` regression
* Add testSwiftWASIBundleAccessor to BuildPlanTests
* Add clarifying comment to `isWASI` branch of `generateResourceAccessor`

Author: MaxDesiatov

Sources/Build/BuildPlan.swift

@@ -658,13 +658,26 @@ public final class SwiftTargetBuildDescription {
         // Do nothing if we're not generating a bundle.
         guard let bundlePath = self.bundlePath else { return }
 
+        let mainPathSubstitution: String
+        if buildParameters.triple.isWASI() {
+            // We prefer compile-time evaluation of the bundle path here for WASI. There's no benefit in evaluating this at runtime, 
+            // especially as Bundle support in WASI Foundation is partial. We expect all resource paths to evaluate to 
+            // `/\(resourceBundleName)/\(resourcePath)`, which allows us to pass this path to JS APIs like `fetch` directly, or to
+            // `<img src=` HTML attributes. The resources are loaded from the server, and we can't hardcode the host part in the URL.
+            // Making URLs relative by starting them with `/\(resourceBundleName)` makes it work in the browser.
+            let mainPath = AbsolutePath(Bundle.main.bundlePath).appending(component: bundlePath.basename).pathString
+            mainPathSubstitution = #""\#(mainPath.asSwiftStringLiteralConstant)""#
+        } else {
+            mainPathSubstitution = #"Bundle.main.bundleURL.appendingPathComponent("\#(bundlePath.basename.asSwiftStringLiteralConstant)").path"#
+        }
+
         let stream = BufferedOutputByteStream()
         stream <<< """
         import class Foundation.Bundle
 
         extension Foundation.Bundle {
             static var module: Bundle = {
-                let mainPath = Bundle.main.bundleURL.appendingPathComponent("\(bundlePath.basename.asSwiftStringLiteralConstant)").path
+                let mainPath = \(mainPathSubstitution)
                 let buildPath = "\(bundlePath.pathString.asSwiftStringLiteralConstant)"
 
                 let preferredBundle = Bundle(path: mainPath)

Tests/BuildTests/BuildPlanTests.swift

@@ -2652,6 +2652,70 @@ final class BuildPlanTests: XCTestCase {
         ])
     }
 
+    func testSwiftWASIBundleAccessor() throws {
+        // This has a Swift and ObjC target in the same package.
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/PkgA/Sources/Foo/Foo.swift",
+            "/PkgA/Sources/Foo/foo.txt",
+            "/PkgA/Sources/Foo/bar.txt",
+            "/PkgA/Sources/Bar/Bar.swift"
+        )
+
+        let observability = ObservabilitySystem.makeForTesting()
+
+        let graph = try loadPackageGraph(
+            fs: fs,
+            manifests: [
+                Manifest.createRootManifest(
+                    name: "PkgA",
+                    path: .init("/PkgA"),
+                    toolsVersion: .v5_2,
+                    targets: [
+                        TargetDescription(
+                            name: "Foo",
+                            resources: [
+                                .init(rule: .copy, path: "foo.txt"),
+                                .init(rule: .process, path: "bar.txt"),
+                            ]
+                        ),
+                        TargetDescription(
+                            name: "Bar"
+                        ),
+                    ]
+                )
+            ],
+            observabilityScope: observability.topScope
+        )
+
+        XCTAssertNoDiagnostics(observability.diagnostics)
+
+        let plan = try BuildPlan(
+            buildParameters: mockBuildParameters(destinationTriple: .wasi),
+            graph: graph,
+            fileSystem: fs,
+            observabilityScope: observability.topScope
+        )
+        let result = BuildPlanResult(plan: plan)
+
+        let fooTarget = try result.target(for: "Foo").swiftTarget()
+        XCTAssertEqual(fooTarget.objects.map{ $0.pathString }, [
+            "/path/to/build/debug/Foo.build/Foo.swift.o",
+            "/path/to/build/debug/Foo.build/resource_bundle_accessor.swift.o"
+        ])
+
+        let resourceAccessor = fooTarget.sources.first{ $0.basename == "resource_bundle_accessor.swift" }!
+        let contents = try fs.readFileContents(resourceAccessor).cString
+        XCTAssertMatch(contents, .contains("extension Foundation.Bundle"))
+        // Assert that `Bundle.main` is executed in the compiled binary (and not during compilation)
+        // See https://bugs.swift.org/browse/SR-14555 and https://github.com/apple/swift-package-manager/pull/2972/files#r623861646
+        XCTAssertMatch(contents, .contains("let mainPath = \""))
+
+        let barTarget = try result.target(for: "Bar").swiftTarget()
+        XCTAssertEqual(barTarget.objects.map{ $0.pathString }, [
+            "/path/to/build/debug/Bar.build/Bar.swift.o",
+        ])
+    }
+
     func testShouldLinkStaticSwiftStdlib() throws {
         let fs = InMemoryFileSystem(emptyFiles:
             "/Pkg/Sources/exe/main.swift",