1. Add new flag -debug-compilation-dir

2. Forward the flag and its associated variable to frontend
3. Add a test case to validate that the flag is forwarded to frontend correctly

Author: pofat

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -255,6 +255,8 @@ extension Driver {
       if shouldSupportAnonymousContextMangledNames {
         commandLine.appendFlag(.enableAnonymousContextMangledNames)
       }
+
+      try commandLine.appendAll(.debugCompilationDir, from: &parsedOptions)
     }
 
     // Pass through any subsystem flags.

Sources/SwiftOptions/Options.swift

@@ -65,6 +65,7 @@ extension Option {
   public static let c: Option = Option("-c", .flag, alias: Option.emitObject, attributes: [.frontend, .noInteractive], group: .modes)
   public static let debugAssertAfterParse: Option = Option("-debug-assert-after-parse", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Force an assertion failure after parsing", group: .debugCrash)
   public static let debugAssertImmediately: Option = Option("-debug-assert-immediately", .flag, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Force an assertion failure immediately", group: .debugCrash)
+  public static let debugCompilationDir: Option = Option("-debug-compilation-dir", .separate, attributes: [.frontend, .argumentIsPath], metaVar: "<path>", helpText: "The compilation directory to embed in the debug info.")
   public static let debugConstraintsAttempt: Option = Option("-debug-constraints-attempt", .separate, attributes: [.helpHidden, .frontend, .noDriver], helpText: "Debug the constraint solver at a given attempt")
   public static let debugConstraintsOnLineEQ: Option = Option("-debug-constraints-on-line=", .joined, alias: Option.debugConstraintsOnLine, attributes: [.helpHidden, .frontend, .noDriver])
   public static let debugConstraintsOnLine: Option = Option("-debug-constraints-on-line", .separate, attributes: [.helpHidden, .frontend, .noDriver], metaVar: "<line>", helpText: "Debug the constraint solver for expressions on <line>")
@@ -700,6 +701,7 @@ extension Option {
       Option.c,
       Option.debugAssertAfterParse,
       Option.debugAssertImmediately,
+      Option.debugCompilationDir,
       Option.debugConstraintsAttempt,
       Option.debugConstraintsOnLineEQ,
       Option.debugConstraintsOnLine,

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -489,6 +489,17 @@ final class SwiftDriverTests: XCTestCase {
     try assertDriverDiagnostics(args: "swiftc", "foo.swift", "-emit-module", "-gdwarf-types", "-debug-info-format=codeview") {
       $1.expect(.error("argument '-debug-info-format=codeview' is not allowed with '-gdwarf-types'"))
     }
+
+    try assertNoDriverDiagnostics(args: "swiftc", "foo.swift", "-g", "-c", "-debug-compilation-dir", ".") { driver in
+      let jobs = try driver.planBuild()
+      XCTAssertTrue(jobs[0].commandLine.contains(.flag("-debug-compilation-dir")))
+      XCTAssertTrue(jobs[0].commandLine.contains(.path(.relative(.init(".")))))
+    }
+
+    try assertNoDriverDiagnostics(args: "swiftc", "foo.swift", "-c", "-debug-compilation-dir", ".") { driver in
+      let jobs = try driver.planBuild()
+      XCTAssertFalse(jobs[0].commandLine.contains(.flag("-debug-compilation-dir")))
+    }
   }
 
   func testCoverageSettings() throws {