From c668f922ea819e26926b67e748bfd9685ad8d347 Mon Sep 17 00:00:00 2001
From: Robert Connell <robert_connell@apple.com>
Date: Fri, 7 Nov 2025 19:18:37 -0500
Subject: [PATCH 1/3] Avoid expensive string array operations when performing
 isDescendent checks on AbsolutePath

---
 Sources/Basics/FileSystem/AbsolutePath.swift | 28 +++++++++++++++++---
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/Sources/Basics/FileSystem/AbsolutePath.swift b/Sources/Basics/FileSystem/AbsolutePath.swift
index c472e5c0608..260b81d4028 100644
--- a/Sources/Basics/FileSystem/AbsolutePath.swift
+++ b/Sources/Basics/FileSystem/AbsolutePath.swift
@@ -220,7 +220,7 @@ extension AbsolutePath {
     /// This method is strictly syntactic and does not access the file system
     /// in any way.
     public func isAncestor(of descendant: AbsolutePath) -> Bool {
-        self.underlying.isAncestor(of: descendant.underlying)
+        descendant.isDescendant(of: self)
     }
 
     /// Returns true if the path is an ancestor of or equal to the given path.
@@ -228,7 +228,7 @@ extension AbsolutePath {
     /// This method is strictly syntactic and does not access the file system
     /// in any way.
     public func isAncestorOfOrEqual(to descendant: AbsolutePath) -> Bool {
-        self.underlying.isAncestorOfOrEqual(to: descendant.underlying)
+        descendant.isDescendantOfOrEqual(to: self)
     }
 
     /// Returns true if the path is a descendant of the given path.
@@ -236,7 +236,24 @@ extension AbsolutePath {
     /// This method is strictly syntactic and does not access the file system
     /// in any way.
     public func isDescendant(of ancestor: AbsolutePath) -> Bool {
-        self.underlying.isDescendant(of: ancestor.underlying)
+        let selfPath = self.pathString
+        let ancestorPath = ancestor.pathString
+
+        guard selfPath.count > ancestorPath.count else {
+            return false
+        }
+
+        guard selfPath.hasPrefix(ancestorPath) else {
+            return false
+        }
+
+        if ancestorPath.hasSuffix("/") {
+            return true
+        }
+
+        // Handle non-root paths by ensuring the character after the prefix is a separator
+        let indexAfterPrefix = selfPath.index(selfPath.startIndex, offsetBy: ancestorPath.count)
+        return selfPath[indexAfterPrefix] == "/"
     }
 
     /// Returns true if the path is a descendant of or equal to the given path.
@@ -244,7 +261,10 @@ extension AbsolutePath {
     /// This method is strictly syntactic and does not access the file system
     /// in any way.
     public func isDescendantOfOrEqual(to ancestor: AbsolutePath) -> Bool {
-        self.underlying.isDescendantOfOrEqual(to: ancestor.underlying)
+        if self == ancestor {
+            return true
+        }
+        return self.isDescendant(of: ancestor)
     }
 }
 

From 9fff80ae906ed03f17ea4fdc56c42a57367f5c85 Mon Sep 17 00:00:00 2001
From: Robert Connell <robert_connell@apple.com>
Date: Sat, 8 Nov 2025 16:02:26 -0500
Subject: [PATCH 2/3] Augment error description

---
 Sources/PackageLoading/PackageBuilder.swift | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Sources/PackageLoading/PackageBuilder.swift b/Sources/PackageLoading/PackageBuilder.swift
index 77169042aa0..5d544f4324a 100644
--- a/Sources/PackageLoading/PackageBuilder.swift
+++ b/Sources/PackageLoading/PackageBuilder.swift
@@ -992,7 +992,11 @@ public final class PackageBuilder {
         let publicHeaderComponent = manifestTarget.publicHeadersPath ?? ClangModule.defaultPublicHeadersComponent
         let publicHeadersPath = try potentialModule.path.appending(RelativePath(validating: publicHeaderComponent))
         guard publicHeadersPath.isDescendantOfOrEqual(to: potentialModule.path) else {
-            throw ModuleError.invalidPublicHeadersDirectory(potentialModule.name)
+            let publicHeadersPathString = publicHeadersPath.pathString
+            let potentialModulePathString = potentialModule.path.pathString
+            let potentialModuleName = potentialModule.name
+            let errorMessage = "\(publicHeadersPathString) isDescendantOfOrEqual \(potentialModulePathString) - potentialModule.name(\(potentialModuleName))"
+            throw ModuleError.invalidPublicHeadersDirectory(errorMessage)
         }
 
         let sourcesBuilder = TargetSourcesBuilder(

From 68fe3c8614d226a985730a912e61a370ccfca958 Mon Sep 17 00:00:00 2001
From: Robert Connell <robert_connell@apple.com>
Date: Sun, 9 Nov 2025 22:56:49 -0500
Subject: [PATCH 3/3] Check for windows path separators.

---
 Sources/Basics/FileSystem/AbsolutePath.swift | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Sources/Basics/FileSystem/AbsolutePath.swift b/Sources/Basics/FileSystem/AbsolutePath.swift
index 260b81d4028..08c1237f49a 100644
--- a/Sources/Basics/FileSystem/AbsolutePath.swift
+++ b/Sources/Basics/FileSystem/AbsolutePath.swift
@@ -247,13 +247,19 @@ extension AbsolutePath {
             return false
         }
 
-        if ancestorPath.hasSuffix("/") {
+#if os(Windows)
+        let pathSeparator: Character = "\\"
+#else
+        let pathSeparator: Character = "/"
+#endif
+
+        if ancestorPath.hasSuffix(String(pathSeparator)) {
             return true
         }
 
         // Handle non-root paths by ensuring the character after the prefix is a separator
         let indexAfterPrefix = selfPath.index(selfPath.startIndex, offsetBy: ancestorPath.count)
-        return selfPath[indexAfterPrefix] == "/"
+        return selfPath[indexAfterPrefix] == pathSeparator
     }
 
     /// Returns true if the path is a descendant of or equal to the given path.