IDE support and SourceKit

Active contributors: akyrtzi, ahoppen, bnbarham

Purpose

The compiler ships several IDE-facing capabilities:

• lib/IDE/ -- in-process code completion, signature help, hover info, semantic tokens. Uses Sema with a custom inspection-callbacks factory.
• lib/IDETool/ -- shared tooling for swift-ide-test, refactoring drivers, batch-mode IDE operations.
• lib/Index/ -- builds an index database (the same one consumed by index-import / IndexStore) used for jump-to-definition and project-wide rename.
• lib/Refactoring/ -- the kit of refactoring transformations (extract function, fill in protocol stubs, ...). Drives the swift-refactor CLI.
• tools/SourceKit/ -- the out-of-process IDE service used by Xcode, Apple's sourcekit-lsp, and command-line tools. The lib/IDE/ features are exposed via a JSON RPC-style protocol.

Together these are how IDE integrations get their Swift smarts.

Directory layout

lib/IDE/                               (45 files)
├── CompletionLookup.cpp
├── CodeCompletion.cpp                 # the completion engine
├── CodeCompletionResult.cpp
├── CodeCompletionResultSink.cpp
├── ConformingMethodList.cpp
├── ImportDepth.cpp
├── REPLCodeCompletion.cpp
├── SyntaxHighlighting.cpp             # semantic tokens
└── ...

lib/IDETool/                           # shared tooling helpers
└── ...

lib/Index/                             # index DB writers
├── Index.cpp
├── IndexRecord.cpp                    # writes index records (compatible with IndexStoreDB)
├── IndexSymbol.cpp
└── ...

lib/Refactoring/                       (47 files)
├── Refactoring.cpp                    # entry point
├── RenameRefactoring.cpp
├── ExtractFunction.cpp
└── ... per-refactoring files

tools/SourceKit/                       # the SourceKit service
├── lib/                               # the C++ implementation
├── tools/                             # sourcekitd-test, sourcekitd-repl
└── docs/

Key abstractions

Type	File	Description
swift::ide::CodeCompletionContext	include/swift/IDE/CodeCompletion.h	The active completion request state.
swift::ide::CodeCompletionResult	include/swift/IDE/CodeCompletionResult.h	One completion suggestion (label, type, kind, ...).
swift::ide::IDEInspectionCallbacks	include/swift/IDE/IDEInspectionCallbacks.h	Sema feeds candidates here during constraint solving.
swift::index::IndexUnitWriter	include/swift/Index/IndexUnitWriter.h	Writes per-translation-unit index records.
swift::refactoring::Refactoring	include/swift/Refactoring/Refactoring.h	Base class for transformations.
sourcekitd::Service	tools/SourceKit/lib/SwiftLang/...	The RPC entry point.

How it works

Completion flow

sequenceDiagram
    participant Client as Editor (Xcode / sourcekit-lsp)
    participant SK as sourcekitd
    participant Compiler as in-process Swift compiler
    participant Sema
    Client->>SK: codecomplete request (file, offset)
    SK->>Compiler: setup CompilerInstance + IDEInspectionCallbacks
    Compiler->>Sema: type-check up to offset
    Sema->>Sema: constraint solve hits the inspection point
    Sema-->>Compiler: candidate decls / types
    Compiler-->>SK: CodeCompletionResult list
    SK-->>Client: completions JSON

The trick is that Sema's constraint solver, when it reaches the user-provided "inspection point" (the cursor location), hands control to IDEInspectionCallbacks. The callbacks subclass records candidates without aborting the solve. This produces semantically aware completions (knowing the expected type, in-scope generics, available conformances, etc.).

Indexing

lib/Index/ walks the typed AST after a successful compile and emits index records to a per-TU file (.indexstore-record-format). The records are consumed by IndexStoreDB and index-import, which Xcode and sourcekit-lsp use for cross-file lookups (jump to definition, find call hierarchy, etc.).

Refactoring

Each refactoring (rename, extract function, fill protocol stubs, ...) is a lib/Refactoring/*.cpp file. They share a common RefactoringActionInfo model. Two front-ends:

• swift-refactor (tools/swift-refactor/) -- a CLI that runs a single refactoring.
• SourceKit -- exposes refactoring as sourcekitd requests; Xcode / sourcekit-lsp consume them.

SourceKit architecture

tools/SourceKit/ is its own mini-product. It runs as sourcekitd-inproc.framework (in-process for embedded clients) or as sourcekit (a daemon). It serializes requests as XPC dictionaries on Apple platforms or framed JSON-like blobs elsewhere.

Internally it spins up CompilerInstance objects for each open project, keeps them warm, and routes requests to:

• lib/IDE/ for completion / hover / type info.
• lib/Refactoring/ for refactor actions.
• lib/Index/ for indexing.
• lib/Markup/ for doc-comment rendering.

See tools/SourceKit/docs/ for the request reference.

Integration points

• Sema is the workhorse of every IDE feature. The IDE asks Sema to do partial work via IDEInspectionCallbacks.
• Frontend (lib/Frontend/CompilerInstance.cpp) supports IDE-like sessions where the compilation is not driven to a final artifact (no SILGen, no IRGen).
• External tools -- sourcekit-lsp (separate repo) talks to sourcekitd; Xcode embeds it.
• swift-syntax -- some refactorings use swift-syntax (the Swift-implemented syntax library) directly to manipulate source text.

Entry points for modification

• Adding a refactoring: drop a file in lib/Refactoring/, register it in RefactoringKinds.def, add a swift-refactor test under test/refactoring/.
• Improving a completion case: most logic lives in lib/IDE/CompletionLookup.cpp and lib/IDE/CodeCompletion.cpp. Tests in test/IDE/.
• Adding a SourceKit request: extend the request schema in tools/SourceKit/lib/SwiftLang/, then route it through to lib/IDE/ or lib/Refactoring/.

Related pages

• Sema -- the type-checker that IDE features hijack.
• AST -- the data model.
• Tools -- the SourceKit service binary view.