Allow to customize Rule severity

In order to customize the severity of rules, I added
the possibility to do so via the configuration files.
If no severity is specified, we use the one pre-determined
by the Rule itself.

Example:
```
{
    "ruleSeverity": {
        "AlwaysUseLowerCamelCase": "warning",
        "AmbiguousTrailingClosureOverload": "error",
    }
}
```

Issue: swiftlang#879

Author: bkolb

Sources/SwiftFormat/API/Configuration+Default.swift

@@ -22,6 +22,7 @@ extension Configuration {
   /// the JSON will be populated from this default configuration.
   public init() {
     self.rules = Self.defaultRuleEnablements
+    self.ruleSeverity = [:]
     self.maximumBlankLines = 1
     self.lineLength = 100
     self.tabWidth = 8

Sources/SwiftFormat/API/Configuration.swift

@@ -24,6 +24,11 @@ internal let highestSupportedConfigurationVersion = 1
 /// Holds the complete set of configured values and defaults.
 public struct Configuration: Codable, Equatable {
 
+  public enum RuleSeverity: String, Codable, CaseIterable, Equatable, Sendable {
+    case warning = "warning"
+    case error = "error"
+  }
+
   private enum CodingKeys: CodingKey {
     case version
     case maximumBlankLines
@@ -42,6 +47,7 @@ public struct Configuration: Codable, Equatable {
     case fileScopedDeclarationPrivacy
     case indentSwitchCaseLabels
     case rules
+    case ruleSeverity
     case spacesAroundRangeFormationOperators
     case noAssignmentInExpressions
     case multiElementCollectionTrailingCommas
@@ -64,6 +70,10 @@ public struct Configuration: Codable, Equatable {
   /// marked as `false`, or if it is missing from the dictionary.
   public var rules: [String: Bool]
 
+  /// The dictionary containing the severities for the rule names that we wish to run on. If a rule
+  /// is not listed here, the default severity is used.
+  public var ruleSeverity: [String: RuleSeverity]
+
   /// The maximum number of consecutive blank lines that may appear in a file.
   public var maximumBlankLines: Int
 
@@ -390,6 +400,9 @@ public struct Configuration: Codable, Equatable {
     self.rules =
       try container.decodeIfPresent([String: Bool].self, forKey: .rules)
       ?? defaults.rules
+
+    self.ruleSeverity =
+    try container.decodeIfPresent([String: RuleSeverity].self, forKey: .ruleSeverity) ?? [:]
   }
 
   public func encode(to encoder: Encoder) throws {

Sources/SwiftFormat/Core/Rule.swift

@@ -86,6 +86,8 @@ extension Rule {
       syntaxLocation = nil
     }
 
+    let severity: Finding.Severity? = severity ?? context.configuration.findingSeverity(for: type(of: self))
+
     let category = RuleBasedFindingCategory(ruleType: type(of: self), severity: severity)
     context.findingEmitter.emit(
       message,
@@ -95,3 +97,13 @@ extension Rule {
     )
   }
 }
+
+extension Configuration {
+  func findingSeverity(for rule: any Rule.Type) -> Finding.Severity? {
+    guard let severity = self.ruleSeverity[rule.ruleName] else { return nil }
+    switch severity {
+      case .warning: return .warning
+      case .error: return .error
+    }
+  }
+}

Sources/SwiftFormat/Core/RuleBasedFindingCategory.swift

@@ -24,6 +24,10 @@ struct RuleBasedFindingCategory: FindingCategorizing {
 
   var severity: Finding.Severity?
 
+  public var defaultSeverity: Finding.Severity {
+    return severity ?? .warning
+  }
+
   /// Creates a finding category that wraps the given rule type.
   init(ruleType: Rule.Type, severity: Finding.Severity? = nil) {
     self.ruleType = ruleType

Sources/_SwiftFormatTestSupport/DiagnosingTestCase.swift

@@ -145,6 +145,18 @@ open class DiagnosingTestCase: XCTestCase {
         line: line
       )
     }
+
+    XCTAssertEqual(
+      matchedFinding.severity,
+      findingSpec.severity,
+      """
+      Finding emitted at marker '\(findingSpec.marker)' \
+      (line:col \(markerLocation.line):\(markerLocation.column), offset \(utf8Offset)) \
+      had the wrong severity
+      """,
+      file: file,
+      line: line
+    )
   }
 
   private func assertAndRemoveNote(

Sources/_SwiftFormatTestSupport/FindingSpec.swift

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+import SwiftFormat
+
 /// A description of a `Finding` that can be asserted during tests.
 public struct FindingSpec {
   /// The marker that identifies the finding.
@@ -21,11 +23,15 @@ public struct FindingSpec {
   /// A description of a `Note` that should be associated with this finding.
   public var notes: [NoteSpec]
 
+  /// A description of a `Note` that should be associated with this finding.
+  public var severity: Finding.Severity
+
   /// Creates a new `FindingSpec` with the given values.
-  public init(_ marker: String = "1️⃣", message: String, notes: [NoteSpec] = []) {
+  public init(_ marker: String = "1️⃣", message: String, notes: [NoteSpec] = [], severity: Finding.Severity = .warning) {
     self.marker = marker
     self.message = message
     self.notes = notes
+    self.severity = severity
   }
 }
 

Tests/SwiftFormatTests/API/ConfigurationTests.swift

@@ -18,6 +18,27 @@ final class ConfigurationTests: XCTestCase {
     XCTAssertEqual(defaultInitConfig, emptyJSONConfig)
   }
 
+  func testSeverityDecoding() {
+    var config = Configuration()
+    config.ruleSeverity["AlwaysUseLowerCamelCase"] = .warning
+    config.ruleSeverity["AmbiguousTrailingClosureOverload"] = .error
+
+    let dictionaryData =
+    """
+    {
+      "ruleSeverity": {
+          "AlwaysUseLowerCamelCase": "warning",
+          "AmbiguousTrailingClosureOverload": "error",
+      }
+    }
+    """.data(using: .utf8)!
+    let jsonDecoder = JSONDecoder()
+    let jsonConfig =
+    try! jsonDecoder.decode(Configuration.self, from: dictionaryData)
+
+    XCTAssertEqual(config, jsonConfig)
+  }
+
   func testMissingConfigurationFile() throws {
     #if os(Windows)
     #if compiler(<6.0.2)

Tests/SwiftFormatTests/Rules/OmitReturnsTests.swift

@@ -16,7 +16,7 @@ final class OmitReturnsTests: LintOrFormatRuleTestCase {
           }
         """,
       findings: [
-        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression")
+        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring)
       ]
     )
   }
@@ -35,7 +35,7 @@ final class OmitReturnsTests: LintOrFormatRuleTestCase {
           }
         """,
       findings: [
-        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression")
+        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring)
       ]
     )
   }
@@ -76,8 +76,8 @@ final class OmitReturnsTests: LintOrFormatRuleTestCase {
           }
         """,
       findings: [
-        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression"),
-        FindingSpec("2️⃣", message: "'return' can be omitted because body consists of a single expression"),
+        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring),
+        FindingSpec("2️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring),
       ]
     )
   }
@@ -114,8 +114,8 @@ final class OmitReturnsTests: LintOrFormatRuleTestCase {
           }
         """,
       findings: [
-        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression"),
-        FindingSpec("2️⃣", message: "'return' can be omitted because body consists of a single expression"),
+        FindingSpec("1️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring),
+        FindingSpec("2️⃣", message: "'return' can be omitted because body consists of a single expression", severity: .refactoring),
       ]
     )
   }

Tests/SwiftFormatTests/Rules/TypeNamesShouldBeCapitalizedTests.swift

@@ -17,11 +17,11 @@ final class TypeNamesShouldBeCapitalizedTests: LintOrFormatRuleTestCase {
       }
       """,
       findings: [
-        FindingSpec("1️⃣", message: "rename the struct 'a' using UpperCamelCase; for example, 'A'"),
-        FindingSpec("2️⃣", message: "rename the class 'klassName' using UpperCamelCase; for example, 'KlassName'"),
-        FindingSpec("3️⃣", message: "rename the struct 'subType' using UpperCamelCase; for example, 'SubType'"),
-        FindingSpec("4️⃣", message: "rename the protocol 'myProtocol' using UpperCamelCase; for example, 'MyProtocol'"),
-        FindingSpec("5️⃣", message: "rename the struct 'innerType' using UpperCamelCase; for example, 'InnerType'"),
+        FindingSpec("1️⃣", message: "rename the struct 'a' using UpperCamelCase; for example, 'A'", severity: .convention),
+        FindingSpec("2️⃣", message: "rename the class 'klassName' using UpperCamelCase; for example, 'KlassName'", severity: .convention),
+        FindingSpec("3️⃣", message: "rename the struct 'subType' using UpperCamelCase; for example, 'SubType'", severity: .convention),
+        FindingSpec("4️⃣", message: "rename the protocol 'myProtocol' using UpperCamelCase; for example, 'MyProtocol'", severity: .convention),
+        FindingSpec("5️⃣", message: "rename the struct 'innerType' using UpperCamelCase; for example, 'InnerType'", severity: .convention),
       ]
     )
   }
@@ -36,8 +36,8 @@ final class TypeNamesShouldBeCapitalizedTests: LintOrFormatRuleTestCase {
       distributed actor DistGreeter {}
       """,
       findings: [
-        FindingSpec("1️⃣", message: "rename the actor 'myActor' using UpperCamelCase; for example, 'MyActor'"),
-        FindingSpec("2️⃣", message: "rename the actor 'greeter' using UpperCamelCase; for example, 'Greeter'"),
+        FindingSpec("1️⃣", message: "rename the actor 'myActor' using UpperCamelCase; for example, 'MyActor'", severity: .convention),
+        FindingSpec("2️⃣", message: "rename the actor 'greeter' using UpperCamelCase; for example, 'Greeter'", severity: .convention),
       ]
     )
   }
@@ -63,9 +63,9 @@ final class TypeNamesShouldBeCapitalizedTests: LintOrFormatRuleTestCase {
       }
       """,
       findings: [
-        FindingSpec("1️⃣", message: "rename the associated type 'kind' using UpperCamelCase; for example, 'Kind'"),
-        FindingSpec("2️⃣", message: "rename the type alias 'x' using UpperCamelCase; for example, 'X'"),
-        FindingSpec("3️⃣", message: "rename the type alias 'data' using UpperCamelCase; for example, 'Data'"),
+        FindingSpec("1️⃣", message: "rename the associated type 'kind' using UpperCamelCase; for example, 'Kind'", severity: .convention),
+        FindingSpec("2️⃣", message: "rename the type alias 'x' using UpperCamelCase; for example, 'X'", severity: .convention),
+        FindingSpec("3️⃣", message: "rename the type alias 'data' using UpperCamelCase; for example, 'Data'", severity: .convention),
       ]
     )
   }
@@ -107,17 +107,18 @@ final class TypeNamesShouldBeCapitalizedTests: LintOrFormatRuleTestCase {
       distributed actor __InternalGreeter {}
       """,
       findings: [
-        FindingSpec("1️⃣", message: "rename the protocol '_p' using UpperCamelCase; for example, '_P'"),
-        FindingSpec("2️⃣", message: "rename the associated type '_value' using UpperCamelCase; for example, '_Value'"),
-        FindingSpec("3️⃣", message: "rename the struct '_data' using UpperCamelCase; for example, '_Data'"),
-        FindingSpec("4️⃣", message: "rename the type alias '_x' using UpperCamelCase; for example, '_X'"),
+        FindingSpec("1️⃣", message: "rename the protocol '_p' using UpperCamelCase; for example, '_P'", severity: .convention),
+        FindingSpec("2️⃣", message: "rename the associated type '_value' using UpperCamelCase; for example, '_Value'", severity: .convention),
+        FindingSpec("3️⃣", message: "rename the struct '_data' using UpperCamelCase; for example, '_Data'", severity: .convention),
+        FindingSpec("4️⃣", message: "rename the type alias '_x' using UpperCamelCase; for example, '_X'", severity: .convention),
         FindingSpec(
           "5️⃣",
-          message: "rename the actor '_internalActor' using UpperCamelCase; for example, '_InternalActor'"
+          message: "rename the actor '_internalActor' using UpperCamelCase; for example, '_InternalActor'",
+          severity: .convention
         ),
-        FindingSpec("6️⃣", message: "rename the enum '__e' using UpperCamelCase; for example, '__E'"),
-        FindingSpec("7️⃣", message: "rename the class '_myClass' using UpperCamelCase; for example, '_MyClass'"),
-        FindingSpec("8️⃣", message: "rename the actor '__greeter' using UpperCamelCase; for example, '__Greeter'"),
+        FindingSpec("6️⃣", message: "rename the enum '__e' using UpperCamelCase; for example, '__E'", severity: .convention),
+        FindingSpec("7️⃣", message: "rename the class '_myClass' using UpperCamelCase; for example, '_MyClass'", severity: .convention),
+        FindingSpec("8️⃣", message: "rename the actor '__greeter' using UpperCamelCase; for example, '__Greeter'", severity: .convention),
       ]
     )
   }