Implemented the BuildProjectTask task in the library target.

Library/Sources/Public/Tasks/BuildProjectTask.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+public struct BuildProjectTask {
+    
+    // MARK: Properties
+    
+    private let terminalService: TerminalServicing
+    
+    // MARK: Initialisers
+    
+    public init(terminalService: TerminalServicing) {
+        self.terminalService = terminalService
+    }
+    
+    // MARK: Functions
+    
+    public func callAsFunction(at location: URL? = nil) async throws (TerminalServiceError) {
+        let executableURL = URL(at: "/usr/bin/swift")
+
+        var arguments: [String] = ["build"]
+        
+        if let location {
+            arguments.append(contentsOf: ["--package-path", location.pathString])
+        }
+        
+        try await terminalService.run(executableURL, arguments: arguments)
+    }
+    
+}

Test/Sources/Cases/Public/Tasks/BuildProjectTaskTests.swift

@@ -0,0 +1,42 @@
+import Foundation
+import Testing
+
+@testable import ColibriLibrary
+
+struct BuildProjectTaskTests {
+
+    @Test(arguments: [nil, URL.someCurrentFolder])
+    func task(at location: URL?) async throws {
+        // GIVEN
+        let terminalService = TerminalServiceSpy()
+        let task = BuildProjectTask(terminalService: terminalService)
+
+        // WHEN
+        try await task(at: location)
+
+        // THEN
+        let executableURL = URL(at: "/usr/bin/swift")
+        let arguments = if let location {
+            ["build", "--package-path", location.pathString]
+        } else {
+            ["build"]
+        }
+        
+        #expect(terminalService.actions.count == 1)
+        #expect(terminalService.actions[0] == .ran(executableURL, arguments))
+    }
+    
+    @Test(arguments: [nil, URL.someCurrentFolder], [TerminalServiceError.unexpected, .output(""), .captured("")])
+    func task(at location: URL?, throws error: TerminalServiceError) async throws {
+        // GIVEN
+        let terminalService = TerminalServiceMock(action: .error(error))
+        let task = BuildProjectTask(terminalService: terminalService)
+
+        // WHEN
+        // THEN
+        await #expect(throws: error) {
+            try await task(at: location)
+        }
+    }
+
+}

Test/Sources/Helpers/Mocks/TemplateServiceMock.swift

@@ -19,6 +19,14 @@ final class TemplateServiceMock {
         self.spy = spy
     }
     
+    init(
+        actions: [Action],
+        spy: TemplateServiceSpy? = nil
+    ) {
+        self.actions = actions
+        self.spy = spy
+    }
+    
 }
 
 // MARK: - TemplateServicing

Test/Sources/Helpers/Mocks/TerminalServiceMock.swift

@@ -19,6 +19,14 @@ final class TerminalServiceMock {
         self.spy = spy
     }
     
+    init(
+        actions: [Action],
+        spy: TerminalServiceSpy? = nil
+    ) {
+        self.actions = actions
+        self.spy = spy
+    }
+    
 }
 
 // MARK: - TerminalServicing