macos: lint Swift files using SwiftLint

SwiftLint <https://realm.github.io/SwiftLint/> is both a linter and
formatting. It's a popular way to spot issues and enforce a consistent
style.

Our SwiftLint configuration lives in macos/.swiftlint.yml, where is is
automatically discovered. It's very configurable, and I made an initial
pass as some basic, weakly-opinionated rules. The "TODO" section lists
rules that currently have violations but can be easily (auto)fixed in
follow-up commits.

Our integration is CLI-based. Similar to our other support tools, we
expect developers to install `swiftlint` via nix or e.g. Homebrew.
This is documented in HACKING.md.

We also have an optional Xcode integration, for in-editor feedback. When
`swiftlint` is available, it's run as a script-based Build Phase.

SwiftLint supports an auto-fix mode (--fix). Agents are aware of this
via AGENTS.md.

The rules are enforced using a (nix-based) CI job.

.github/workflows/test.yml

@@ -36,6 +36,7 @@ jobs:
       - test-macos
       - pinact
       - prettier
+      - swiftlint
       - alejandra
       - typos
       - shellcheck
@@ -927,6 +928,27 @@ jobs:
       - name: prettier check
         run: nix develop -c prettier --check .
 
+  swiftlint:
+    if: github.repository == 'ghostty-org/ghostty'
+    runs-on: namespace-profile-ghostty-macos-tahoe
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      # TODO(tahoe): https://github.com/NixOS/nix/issues/13342
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          determinate: true
+      - uses: cachix/cachix-action@3ba601ff5bbb07c7220846facfa2cd81eeee15a1 # v16
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+          skipPush: true
+          useDaemon: false # sometimes fails on short jobs
+
+      - name: swiftlint check
+        run: nix develop -c swiftlint lint --strict macos
+
   alejandra:
     if: github.repository == 'ghostty-org/ghostty'
     runs-on: namespace-profile-ghostty-xsm

AGENTS.md

@@ -8,6 +8,7 @@ A file for [guiding coding agents](https://agents.md/).
 - **Test (Zig):** `zig build test`
 - **Test filter (Zig)**: `zig build test -Dtest-filter=<test name>`
 - **Formatting (Zig)**: `zig fmt .`
+- **Formatting (Swift)**: `swiftlint lint --fix macos`
 - **Formatting (other)**: `prettier -w .`
 
 ## Directory Structure

HACKING.md

@@ -186,6 +186,31 @@ shellcheck \
     $(find . \( -name "*.sh" -o -name "*.bash" \) -type f ! -path "./zig-out/*" ! -path "./macos/build/*" ! -path "./.git/*" | sort)
 ```
 
+### SwiftLint
+
+Swift code is linted using [SwiftLint](https://github.com/realm/SwiftLint). A
+SwiftLint CI check will fail builds with improper formatting. Therefore, if you
+are modifying Swift code, you may want to install it locally and run this from
+the repo root before you commit:
+
+```
+swiftlint lint --fix macos
+```
+
+Make sure your SwiftLint version matches the version in [devShell.nix](https://github.com/ghostty-org/ghostty/blob/main/nix/devShell.nix).
+
+Nix users can use the following command to format with SwiftLint:
+
+```
+nix develop -c swiftlint lint --fix macos
+```
+
+To check for violations without auto-fixing:
+
+```
+nix develop -c swiftlint lint --strict macos
+```
+
 ### Updating the Zig Cache Fixed-Output Derivation Hash
 
 The Nix package depends on a [fixed-output

macos/.swiftlint.yml

@@ -0,0 +1,64 @@
+# SwiftLint <https://realm.github.io/SwiftLint/>
+#
+check_for_updates: false
+
+disabled_rules:
+  - cyclomatic_complexity
+  - file_length
+  - function_body_length
+  - nesting
+  - todo
+  - trailing_comma
+  - trailing_newline
+  - type_body_length
+
+  # TODO
+  - colon
+  - comma
+  - comment_spacing
+  - control_statement
+  - deployment_target
+  - empty_enum_arguments
+  - empty_parentheses_with_trailing_closure
+  - for_where
+  - force_cast
+  - implicit_getter
+  - implicit_optional_initialization
+  - legacy_constant
+  - legacy_constructor
+  - line_length
+  - mark
+  - multiple_closures_with_trailing_closure
+  - no_fallthrough_only
+  - opening_brace
+  - orphaned_doc_comment
+  - private_over_fileprivate
+  - shorthand_operator
+  - switch_case_alignment
+  - syntactic_sugar
+  - trailing_semicolon
+  - trailing_whitespace
+  - unneeded_synthesized_initializer
+  - unused_closure_parameter
+  - unused_enumerated
+  - unused_optional_binding
+  - vertical_parameter_alignment
+  - vertical_whitespace
+
+identifier_name:
+  min_length: 1
+  allowed_symbols: ["_"]
+  excluded:
+    - Core.*
+
+type_name:
+  min_length: 2
+  allowed_symbols: ["_"]
+  excluded:
+    - iOS_.*
+
+function_parameter_count:
+  warning: 6
+
+large_tuple:
+  warning: 3

macos/Ghostty.xcodeproj/project.pbxproj

@@ -355,6 +355,7 @@
 			isa = PBXNativeTarget;
 			buildConfigurationList = A5B30540299BEAAB0047F10C /* Build configuration list for PBXNativeTarget "Ghostty" */;
 			buildPhases = (
+				FC501E0B2F46B410007AE49D /* Run SwiftLint */,
 				A5B3052D299BEAAA0047F10C /* Sources */,
 				A5B3052E299BEAAA0047F10C /* Frameworks */,
 				A5B3052F299BEAAA0047F10C /* Resources */,
@@ -490,6 +491,29 @@
 		};
 /* End PBXResourcesBuildPhase section */
 
+/* Begin PBXShellScriptBuildPhase section */
+		FC501E0B2F46B410007AE49D /* Run SwiftLint */ = {
+			isa = PBXShellScriptBuildPhase;
+			alwaysOutOfDate = 1;
+			buildActionMask = 2147483647;
+			files = (
+			);
+			inputFileListPaths = (
+			);
+			inputPaths = (
+			);
+			name = "Run SwiftLint";
+			outputFileListPaths = (
+			);
+			outputPaths = (
+			);
+			runOnlyForDeploymentPostprocessing = 0;
+			shellPath = /bin/sh;
+			shellScript = "[[ -z \"$GITHUB_ACTIONS\" ]] || exit 0;\n\nSWIFTLINT=\"\"\nif command -v swiftlint >/dev/null 2>&1; then\n    SWIFTLINT=\"$(command -v swiftlint)\"\nelif [[ -f \"/opt/homebrew/bin/swiftlint\" ]]; then\n    SWIFTLINT=\"/opt/homebrew/bin/swiftlint\"\nfi\n\nif [[ -n \"$SWIFTLINT\" ]]; then\n    \"$SWIFTLINT\" lint --quiet \"$SRCROOT\"\nfi\n";
+			showEnvVarsInLog = 0;
+		};
+/* End PBXShellScriptBuildPhase section */
+
 /* Begin PBXSourcesBuildPhase section */
 		810ACC9B2E9D3301004F8F92 /* Sources */ = {
 			isa = PBXSourcesBuildPhase;

nix/devShell.nix

@@ -66,6 +66,7 @@
   poop,
   typos,
   shellcheck,
+  swiftlint,
   uv,
   wayland,
   wayland-scanner,
@@ -198,6 +199,9 @@ in
 
         # for benchmarking
         poop
+      ]
+      ++ lib.optionals stdenv.hostPlatform.isDarwin [
+        swiftlint
       ];
 
     # This should be set onto the rpath of the ghostty binary if you want