Build settings for warning treating rules (SE-0443, draft)

Sources/Build/BuildDescription/ClangModuleBuildDescription.swift

@@ -363,11 +363,18 @@ public final class ClangModuleBuildDescription {
 
         // suppress warnings if the package is remote
         if self.package.isRemote {
-            args += ["-w"]
-            // `-w` (suppress warnings) and `-Werror` (warnings as errors) flags are mutually exclusive
-            if let index = args.firstIndex(of: "-Werror") {
-                args.remove(at: index)
+            // `-w` (suppress warnings) and the other warning control flags are mutually exclusive
+            args = args.filter { arg in
+                // we consider the following flags:
+                // -Wxxxx
+                // -Wno-xxxx
+                // -Werror
+                // -Werror=xxxx
+                // -Wno-error
+                // -Wno-error=xxxx
+                arg.count <= 2 || !arg.starts(with: "-W")
             }
+            args += ["-w"]
         }
 
         return args

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -621,11 +621,27 @@ public final class SwiftModuleBuildDescription {
 
         // suppress warnings if the package is remote
         if self.package.isRemote {
-            args += ["-suppress-warnings"]
-            // suppress-warnings and warnings-as-errors are mutually exclusive
-            if let index = args.firstIndex(of: "-warnings-as-errors") {
-                args.remove(at: index)
+            // suppress-warnings and the other warning control flags are mutually exclusive
+            var removeNextArg = false
+            args = args.filter { arg in
+                if removeNextArg {
+                    removeNextArg = false
+                    return false
+                }
+                switch arg {
+                case "-warnings-as-errors", "-no-warnings-as-errors":
+                    return false
+                case "-Wwarning", "-Werror":
+                    removeNextArg = true
+                    return false
+                default:
+                    return true
+                }
             }
+            guard !removeNextArg else {
+                throw InternalError("Unexpected '-Wwarning' or '-Werror' at the end of args")
+            }
+            args += ["-suppress-warnings"]
         }
 
         // Pass `-user-module-version` for versioned packages that aren't pre-releases.

Sources/PackageDescription/BuildSettings.swift

@@ -201,6 +201,92 @@ public struct CSetting: Sendable {
     public static func unsafeFlags(_ flags: [String], _ condition: BuildSettingCondition? = nil) -> CSetting {
         return CSetting(name: "unsafeFlags", value: flags, condition: condition)
     }
+
+    /// Controls how all C compiler warnings are treated during compilation.
+    ///
+    /// Use this setting to specify whether all warnings should be treated as warnings (default behavior)
+    /// or as errors. This is equivalent to passing `-Werror` or `-Wno-error`
+    /// to the C compiler.
+    ///
+    /// This setting applies to all warnings emitted by the C compiler. To control specific
+    /// warnings individually, use `treatWarning(name:as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - level: The treatment level for all warnings (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatAllWarnings(
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CSetting {
+        return CSetting(
+            name: "treatAllWarnings", value: [level.rawValue], condition: condition)
+    }
+
+    /// Controls how a specific C compiler warning is treated during compilation.
+    ///
+    /// Use this setting to specify whether a particular warning should be treated as a warning
+    /// (default behavior) or as an error. This is equivalent to passing `-Werror=` or `-Wno-error=`
+    /// followed by the warning name to the C compiler.
+    ///
+    /// This setting allows for fine-grained control over individual warnings. To control all
+    /// warnings at once, use `treatAllWarnings(as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the specific warning to control.
+    ///   - level: The treatment level for the warning (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatWarning(
+      _ name: String,
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CSetting {
+        return CSetting(
+            name: "treatWarning", value: [name, level.rawValue], condition: condition)
+    }
+
+    /// Enable a specific C compiler warning group.
+    ///
+    /// Use this setting to enable a specific warning group. This is equivalent to passing
+    /// `-W` followed by the group name to the C compiler.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the warning group to enable.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func enableWarning(
+      _ name: String,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CSetting {
+        return CSetting(
+            name: "enableWarning", value: [name], condition: condition)
+    }
+
+    /// Disable a specific C compiler warning group.
+    ///
+    /// Use this setting to disable a specific warning group. This is equivalent to passing
+    /// `-Wno-` followed by the group name to the C compiler.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the warning group to disable.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func disableWarning(
+      _ name: String,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CSetting {
+        return CSetting(
+            name: "disableWarning", value: [name], condition: condition)
+    }
 }
 
 /// A CXX-language build setting.
@@ -271,6 +357,92 @@ public struct CXXSetting: Sendable {
     public static func unsafeFlags(_ flags: [String], _ condition: BuildSettingCondition? = nil) -> CXXSetting {
         return CXXSetting(name: "unsafeFlags", value: flags, condition: condition)
     }
+
+    /// Controls how all C++ compiler warnings are treated during compilation.
+    ///
+    /// Use this setting to specify whether all warnings should be treated as warnings (default behavior)
+    /// or as errors. This is equivalent to passing `-Werror` or `-Wno-error`
+    /// to the C++ compiler.
+    ///
+    /// This setting applies to all warnings emitted by the C++ compiler. To control specific
+    /// warnings individually, use `treatWarning(name:as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - level: The treatment level for all warnings (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatAllWarnings(
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CXXSetting {
+        return CXXSetting(
+            name: "treatAllWarnings", value: [level.rawValue], condition: condition)
+    }
+
+    /// Controls how a specific C++ compiler warning is treated during compilation.
+    ///
+    /// Use this setting to specify whether a particular warning should be treated as a warning
+    /// (default behavior) or as an error. This is equivalent to passing `-Werror=` or `-Wno-error=`
+    /// followed by the warning name to the C++ compiler.
+    ///
+    /// This setting allows for fine-grained control over individual warnings. To control all
+    /// warnings at once, use `treatAllWarnings(as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the specific warning to control.
+    ///   - level: The treatment level for the warning (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatWarning(
+      _ name: String,
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CXXSetting {
+        return CXXSetting(
+            name: "treatWarning", value: [name, level.rawValue], condition: condition)
+    }
+
+    /// Enable a specific C++ compiler warning group.
+    ///
+    /// Use this setting to enable a specific warning group. This is equivalent to passing
+    /// `-W` followed by the group name to the C++ compiler.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the warning group to enable.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func enableWarning(
+      _ name: String,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CXXSetting {
+        return CXXSetting(
+            name: "enableWarning", value: [name], condition: condition)
+    }
+
+    /// Disable a specific C++ compiler warning group.
+    ///
+    /// Use this setting to disable a specific warning group. This is equivalent to passing
+    /// `-Wno-` followed by the group name to the C++ compiler.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the warning group to disable.
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func disableWarning(
+      _ name: String,
+      _ condition: BuildSettingCondition? = nil
+    ) -> CXXSetting {
+        return CXXSetting(
+            name: "disableWarning", value: [name], condition: condition)
+    }
 }
 
 /// A Swift language build setting.
@@ -463,6 +635,54 @@ public struct SwiftSetting: Sendable {
             name: "swiftLanguageMode", value: [.init(describing: mode)], condition: condition)
     }
 
+    /// Controls how all Swift compiler warnings are treated during compilation.
+    ///
+    /// Use this setting to specify whether all warnings should be treated as warnings (default behavior)
+    /// or as errors. This is equivalent to passing `-warnings-as-errors` or `-no-warnings-as-errors`
+    /// to the Swift compiler.
+    ///
+    /// This setting applies to all warnings emitted by the Swift compiler. To control specific
+    /// warnings individually, use `treatWarning(name:as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - level: The treatment level for all warnings (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatAllWarnings(
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+            name: "treatAllWarnings", value: [level.rawValue], condition: condition)
+    }
+
+    /// Controls how a specific Swift compiler warning is treated during compilation.
+    ///
+    /// Use this setting to specify whether a particular warning should be treated as a warning
+    /// (default behavior) or as an error. This is equivalent to passing `-Werror` or `-Wwarning`
+    /// followed by the warning name to the Swift compiler.
+    ///
+    /// This setting allows for fine-grained control over individual warnings. To control all
+    /// warnings at once, use `treatAllWarnings(as:_:)` instead.
+    ///
+    /// - Since: First available in PackageDescription 6.2.
+    ///
+    /// - Parameters:
+    ///   - name: The name of the specific warning to control.
+    ///   - level: The treatment level for the warning (`.warning` or `.error`).
+    ///   - condition: A condition that restricts the application of the build setting.
+    @available(_PackageDescription, introduced: 6.2)
+    public static func treatWarning(
+      _ name: String,
+      as level: WarningLevel,
+      _ condition: BuildSettingCondition? = nil
+    ) -> SwiftSetting {
+        return SwiftSetting(
+            name: "treatWarning", value: [name, level.rawValue], condition: condition)
+    }
+
     /// Set the default isolation to the given global actor type.
     ///
     /// - Since: First available in PackageDescription 6.2.

Sources/PackageDescription/CMakeLists.txt

@@ -23,7 +23,8 @@ add_library(PackageDescription
   Target.swift
   Trait.swift
   Version.swift
-  Version+StringLiteralConvertible.swift)
+  Version+StringLiteralConvertible.swift
+  WarningLevel.swift)
 
 target_compile_options(PackageDescription PUBLIC
   $<$<COMPILE_LANGUAGE:Swift>:-package-description-version$<SEMICOLON>999.0>)

Sources/PackageDescription/WarningLevel.swift

@@ -0,0 +1,28 @@
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// The level at which a compiler warning should be treated.
+///
+/// This enum is used with the `SwiftSetting.treatAllWarnings(as:_:)` and 
+/// `SwiftSetting.treatWarning(name:as:_:)` methods to control how warnings
+/// are handled during compilation.
+@available(_PackageDescription, introduced: 6.2)
+public enum WarningLevel: String {
+    /// Treat as a warning.
+    ///
+    /// Warnings will be displayed during compilation but will not cause the build to fail.
+    case warning
+
+    /// Treat as an error.
+    ///
+    /// Warnings will be elevated to errors, causing the build to fail if any such warnings occur.
+    case error
+}

Sources/PackageLoading/ManifestJSONParser.swift

@@ -749,6 +749,45 @@ extension TargetBuildSettingDescription.Kind {
             }
 
             return .swiftLanguageMode(version)
+        case "treatAllWarnings":
+            guard values.count == 1 else {
+                throw InternalError("invalid build settings value")
+            }
+
+            let rawLevel = values[0]
+
+            guard let level = TargetBuildSettingDescription.WarningLevel(rawValue: rawLevel) else {
+                throw InternalError("unknown warning treat level: \(rawLevel)")
+            }
+
+            return .treatAllWarnings(level)
+
+        case "treatWarning":
+            guard values.count == 2 else {
+                throw InternalError("invalid build settings value")
+            }
+
+            let name = values[0]
+            let rawValue = values[1]
+
+            guard let level = TargetBuildSettingDescription.WarningLevel(rawValue: rawValue) else {
+                throw InternalError("unknown warning treat level: \(rawValue)")
+            }
+
+            return .treatWarning(name, level)
+
+        case "enableWarning":
+            guard values.count == 1 else {
+                throw InternalError("invalid build settings value")
+            }
+            return .enableWarning(values[0])
+
+        case "disableWarning":
+            guard values.count == 1 else {
+                throw InternalError("invalid build settings value")
+            }
+            return .disableWarning(values[0])
+
         case "defaultIsolation":
             guard let rawValue = values.first else {
                 throw InternalError("invalid (empty) build settings value")