Code and architecture revision

[ericodx]

Summary

Three distinct bugs were fixed and an architectural refactor consolidated execution dependencies.

Fixes:

• SPM build output was being suppressed — BuildStage now uses launchCapturing instead of launch, making compilation errors visible on stderr
• swift build --build-tests --target X are mutually exclusive in SPM; the correct flag is now chosen based on the presence of --target
• BuildError.compilationFailed was not caught in the SPM path of buildArtifact, crashing the CLI; it is now intercepted and triggers the incremental retry-with-exclusion flow

Performance:

• Incompatible SPM mutants were creating a fresh sandbox per mutant, recompiling SwiftSyntax and all dependencies from scratch up to 30×. The new strategy uses a shared sandbox: 1 full initial build + N incremental builds (only the mutated file recompiles) + swift test --skip-build per mutant

Refactor:

• ExecutionDeps groups ProcessLaunching, CacheStore, ProgressReporter and MutationCounter — eliminates long parameter lists in TestExecutionStage, IncompatibleMutantExecutor and FallbackExecutor
• Discovery pipeline decomposed: MutantIndexingStage (assigns indices and determines schematizability), IncompatibleRewritingStage (rewrites incompatible mutants), SchematizationStage (schematizable mutants only)
• SchematizationResult removed; pipeline stages return values directly

Type of Change

• feat: A new feature has been added.
• fix: A bug has been fixed.
• perf: A code change that improves performance.
• refactor: A code change that neither fixes a bug nor adds a feature.
• test: Addition or correction of tests.
• docs: Changes only to the documentation.
• ci: Changes related to continuous integration and deployment scripts.
• build: Changes that affect the build system or external dependencies.
• deps: Dependency version upgrades with no logic changes.
• chore: Other changes that do not fit into the previous categories.
• revert: Reverts a previous commit.

Invariants Checklist

• Original project is never modified — all mutations happen inside an isolated sandbox
• xcodebuild build-for-testing runs exactly once for all schematizable mutants
• No mutant results are lost or duplicated
• Mutant positions (file, line, column) are accurate in all reported results
• A cancelled task never leaves a simulator slot permanently acquired from the pool
• schematizedContent never contains the __swiftMutationTestingID global declaration
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

Which stages are affected?

• SandboxFactory
• BuildStage
• SimulatorPool
• TestExecutionStage
• IncompatibleMutantExecutor
• PerFileBuildFallback
• CacheStore
• Reporters (Text / JSON / HTML / Sonar)
• CLI / Configuration
• Models / RunnerInput contract
• None

Testing

• Unit tests added or updated
• Tests use mock ProcessLaunching — no real xcodebuild, xcrun simctl, or xcresulttool
• Tests use FileHelpers for any filesystem interaction (temp directories only)
• Snapshot tests added or updated (if reporter output format changed)
• Integration tests added or updated and tagged separately (if pipeline or CLI behavior changed)
• All tests pass locally (swift test)