[SLG-0004]: Revised Metadata value attributes as CustomStringConvertible

Benchmarks/MaxLogLevelWarning/Benchmarks/MaxLogLevelWarningBenchmarks/MaxLogLevelWarning.swift

@@ -30,4 +30,14 @@ public let benchmarks: @Sendable () -> Void = {
     makeBenchmark(loggerLevel: .warning, logLevel: .warning, "_generic") { logger in
         logger.log(level: .warning, "hello, benchmarking world")
     }
+    makeBenchmark(loggerLevel: .notice, logLevel: .notice, "_attributed_generic") { logger in
+        logger.log(
+            level: .notice,
+            "hello, benchmarking world",
+            metadata: [
+                "public-key": "\("public-value", sensitivity: .public)",
+                "private-key": "\("private-value", sensitivity: .sensitive)",
+            ]
+        )
+    }
 }

Benchmarks/MaxLogLevelWarning/Thresholds/Xcode 26.1/MaxLogLevelWarning.notice_log_with_notice_log_level.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 619,
+  "instructions" : 622,
   "objectAllocCount" : 0
 }

Benchmarks/MaxLogLevelWarning/Thresholds/Xcode 26.1/MaxLogLevelWarning.notice_log_with_notice_log_level_attributed_generic.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 619,
+  "objectAllocCount" : 0
+}

Benchmarks/MaxLogLevelWarning/Thresholds/Xcode 26.1/MaxLogLevelWarning.warning_log_with_warning_log_level.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 1651,
+  "instructions" : 1669,
   "objectAllocCount" : 0
 }

Benchmarks/MaxLogLevelWarning/Thresholds/Xcode 26.1/MaxLogLevelWarning.warning_log_with_warning_log_level_generic.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 1651,
+  "instructions" : 1669,
   "objectAllocCount" : 0
 }

Benchmarks/NoTraits/Benchmarks/NoTraitsBenchmarks/NoTraits.swift

@@ -24,4 +24,40 @@ public let benchmarks: @Sendable () -> Void = {
     makeBenchmark(loggerLevel: .error, logLevel: .debug, "_generic") { logger in
         logger.log(level: .debug, "hello, benchmarking world")
     }
+    makeBenchmark(loggerLevel: .error, logLevel: .error, "_1_attribute") { logger in
+        logger.log(
+            level: .error,
+            "hello, benchmarking world",
+            metadata: [
+                "public-key": "\("public-value", sensitivity: .public)"
+            ]
+        )
+    }
+    makeBenchmark(loggerLevel: .error, logLevel: .debug, "_1_attribute") { logger in
+        logger.log(
+            level: .debug,
+            "hello, benchmarking world",
+            metadata: [
+                "public-key": "\("public-value", sensitivity: .public)"
+            ]
+        )
+    }
+    makeBenchmark(loggerLevel: .error, logLevel: .error, "_2_attributes") { logger in
+        logger.log(
+            level: .error,
+            "hello, benchmarking world",
+            metadata: [
+                "public-key": "\("public-value", attributes: [BenchmarkSensitivity.public, BenchmarkColor.red])"
+            ]
+        )
+    }
+    makeBenchmark(loggerLevel: .error, logLevel: .debug, "_2_attributes") { logger in
+        logger.log(
+            level: .debug,
+            "hello, benchmarking world",
+            metadata: [
+                "public-key": "\("public-value", attributes: [BenchmarkSensitivity.public, BenchmarkColor.red])"
+            ]
+        )
+    }
 }

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.debug_log_with_error_log_level_1_attribute.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 985,
+  "objectAllocCount" : 0
+}

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.debug_log_with_error_log_level_2_attributes.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 985,
+  "objectAllocCount" : 0
+}

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.debug_log_with_error_log_level_generic.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 984,
+  "instructions" : 981,
   "objectAllocCount" : 0
 }

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.error_log_with_error_log_level_1_attribute.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 5443,
+  "objectAllocCount" : 2
+}

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.error_log_with_error_log_level_2_attributes.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 6599,
+  "objectAllocCount" : 3
+}

Benchmarks/NoTraits/Thresholds/Xcode 16.4/NoTraits.error_log_with_error_log_level_generic.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 1368,
+  "instructions" : 1386,
   "objectAllocCount" : 0
 }

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.debug_log_with_error_log_level_1_attribute.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 897,
+  "objectAllocCount" : 0
+}

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.debug_log_with_error_log_level_2_attributes.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 897,
+  "objectAllocCount" : 0
+}

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.debug_log_with_error_log_level_generic.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 893,
+  "instructions" : 896,
   "objectAllocCount" : 0
 }

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.error_log_with_error_log_level_1_attribute.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 5339,
+  "objectAllocCount" : 2
+}

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.error_log_with_error_log_level_2_attributes.p90.json

@@ -0,0 +1,4 @@
+{
+  "instructions" : 6515,
+  "objectAllocCount" : 3
+}

Benchmarks/NoTraits/Thresholds/Xcode 26.1/NoTraits.error_log_with_error_log_level_generic.p90.json

@@ -1,4 +1,4 @@
 {
-  "instructions" : 1649,
+  "instructions" : 1667,
   "objectAllocCount" : 0
 }

Benchmarks/Sources/BenchmarksFactory/NoOpLogHandler.swift

@@ -14,6 +14,40 @@
 
 import Logging
 
+// MARK: - Benchmark attributes
+
+public enum BenchmarkSensitivity: Int64, Logger.MetadataAttributeKey, Sendable {
+    case sensitive = 1
+    case `public` = 2
+}
+
+extension Logger.MetadataValue.StringInterpolation {
+    @inlinable
+    public mutating func appendInterpolation<T: CustomStringConvertible & Sendable>(
+        _ value: T,
+        sensitivity: BenchmarkSensitivity
+    ) {
+        self.appendInterpolation(value, attributes: { $0[BenchmarkSensitivity.self] = sensitivity })
+    }
+}
+
+public enum BenchmarkColor: Int64, Logger.MetadataAttributeKey, Sendable {
+    case red = 1
+    case blue = 2
+}
+
+extension Logger.MetadataValue.StringInterpolation {
+    @inlinable
+    public mutating func appendInterpolation<T: CustomStringConvertible & Sendable>(
+        _ value: T,
+        color: BenchmarkColor
+    ) {
+        self.appendInterpolation(value, attributes: { $0[BenchmarkColor.self] = color })
+    }
+}
+
+// MARK: - NoOpLogHandler
+
 struct NoOpLogHandler: LogHandler {
     let label: String
     public var metadataProvider: Logger.MetadataProvider?
@@ -31,6 +65,13 @@ struct NoOpLogHandler: LogHandler {
     func log(
         event: LogEvent
     ) {
+        // Access metadata attributes
+        if let metadata = event.metadata {
+            for (_, value) in metadata {
+                let _ = value.attributes
+            }
+        }
+
         // Do nothing
     }
 
@@ -43,6 +84,13 @@ struct NoOpLogHandler: LogHandler {
         function: String,
         line: UInt
     ) {
+        // Access metadata attributes
+        if let metadata = explicitMetadata {
+            for (_, value) in metadata {
+                let _ = value.attributes
+            }
+        }
+
         // Do nothing
     }
 

Package.swift

@@ -6,7 +6,9 @@ let package = Package(
     name: "swift-log",
     products: [
         .library(name: "Logging", targets: ["Logging"]),
+        .library(name: "LoggingAttributes", targets: ["LoggingAttributes"]),
         .library(name: "InMemoryLogging", targets: ["InMemoryLogging"]),
+        .library(name: "OSLogHandler", targets: ["OSLogHandler"]),
     ],
     traits: [
         .trait(name: "MaxLogLevelDebug", description: "Debug and above available (compiles out trace)"),
@@ -31,18 +33,34 @@ let package = Package(
             name: "Logging",
             dependencies: []
         ),
+        .target(
+            name: "LoggingAttributes",
+            dependencies: ["Logging"]
+        ),
         .target(
             name: "InMemoryLogging",
             dependencies: ["Logging"]
         ),
+        .target(
+            name: "OSLogHandler",
+            dependencies: ["Logging", "LoggingAttributes"]
+        ),
         .testTarget(
             name: "LoggingTests",
             dependencies: ["Logging"]
         ),
+        .testTarget(
+            name: "LoggingAttributesTests",
+            dependencies: ["LoggingAttributes", "Logging"]
+        ),
         .testTarget(
             name: "InMemoryLoggingTests",
             dependencies: ["InMemoryLogging", "Logging"]
         ),
+        .testTarget(
+            name: "OSLogHandlerTests",
+            dependencies: ["OSLogHandler", "Logging", "LoggingAttributes"]
+        ),
     ]
 )
 

Package@swift-6.1.swift

@@ -6,7 +6,9 @@ let package = Package(
     name: "swift-log",
     products: [
         .library(name: "Logging", targets: ["Logging"]),
+        .library(name: "LoggingAttributes", targets: ["LoggingAttributes"]),
         .library(name: "InMemoryLogging", targets: ["InMemoryLogging"]),
+        .library(name: "OSLogHandler", targets: ["OSLogHandler"]),
     ],
     traits: [
         .trait(name: "MaxLogLevelDebug", description: "Debug and above available (compiles out trace)"),
@@ -31,18 +33,34 @@ let package = Package(
             name: "Logging",
             dependencies: []
         ),
+        .target(
+            name: "LoggingAttributes",
+            dependencies: ["Logging"]
+        ),
         .target(
             name: "InMemoryLogging",
             dependencies: ["Logging"]
         ),
+        .target(
+            name: "OSLogHandler",
+            dependencies: ["Logging", "LoggingAttributes"]
+        ),
         .testTarget(
             name: "LoggingTests",
             dependencies: ["Logging"]
         ),
+        .testTarget(
+            name: "LoggingAttributesTests",
+            dependencies: ["LoggingAttributes", "Logging"]
+        ),
         .testTarget(
             name: "InMemoryLoggingTests",
             dependencies: ["InMemoryLogging", "Logging"]
         ),
+        .testTarget(
+            name: "OSLogHandlerTests",
+            dependencies: ["OSLogHandler", "Logging", "LoggingAttributes"]
+        ),
     ]
 )
 

Sources/InMemoryLogging/InMemoryLogHandler.swift

@@ -57,18 +57,23 @@ public struct InMemoryLogHandler: LogHandler {
             self.metadata = metadata
         }
 
-        public init(level: Logger.Level, message: Logger.Message, error: (any Error)?, metadata: Logger.Metadata) {
+        public init(
+            level: Logger.Level,
+            message: Logger.Message,
+            error: (any Error)?,
+            metadata: Logger.Metadata
+        ) {
             self.level = level
             self.message = message
             self.error = error
             self.metadata = metadata
         }
 
-        public static func == (lhs: InMemoryLogHandler.Entry, rhs: InMemoryLogHandler.Entry) -> Bool {
+        public static func == (lhs: Entry, rhs: Entry) -> Bool {
             lhs.level == rhs.level
                 && lhs.message == rhs.message
-                && errorsEqual(lhs.error, rhs.error)
                 && lhs.metadata == rhs.metadata
+                && errorsEqual(lhs.error, rhs.error)
         }
 
         private static func errorsEqual(_ lhs: (any Error)?, _ rhs: (any Error)?) -> Bool {
@@ -126,14 +131,23 @@ public struct InMemoryLogHandler: LogHandler {
     }
 
     public func log(event: LogEvent) {
-        // Start with the metadata provider..
-        var mergedMetadata: Logger.Metadata = self.metadataProvider?.get() ?? [:]
-        // ..merge in self.metadata, overwriting existing keys
-        mergedMetadata = mergedMetadata.merging(self.metadata) { $1 }
-        // ..merge in metadata from this log call, overwriting existing keys
-        mergedMetadata = mergedMetadata.merging(event.metadata ?? [:]) { $1 }
-
-        self.logStore.append(level: event.level, message: event.message, error: event.error, metadata: mergedMetadata)
+        var merged = self.metadata
+
+        if let provider = self.metadataProvider {
+            let provided = provider.get()
+            merged.merge(provided, uniquingKeysWith: { _, rhs in rhs })
+        }
+
+        if let eventMetadata = event.metadata {
+            merged.merge(eventMetadata, uniquingKeysWith: { _, rhs in rhs })
+        }
+
+        self.logStore.append(
+            level: event.level,
+            message: event.message,
+            error: event.error,
+            metadata: merged
+        )
     }
 
     @available(*, deprecated, renamed: "log(event:)")