SwiftFixIt: Always rely on the UTF-8 offset of an input source location

As it turns out, contrary to an old comment in the diagnostic
deserializer, the Swift compiler *does* serialize the offset of a
source location, so stop assuming that it may have been left blank. This
slightly simplifies the diagnostic conversion logic and also enables us
to stop hashing the line and column in the diagnostic filter.

Author: AnthonyLatsis

Sources/SwiftFixIt/SwiftFixIt.swift

@@ -43,7 +43,7 @@ private enum Error: Swift.Error {
 }
 
 // FIXME: An abstraction for tests to work around missing memberwise initializers in `TSCUtility.SerializedDiagnostics`.
-protocol AnySourceLocation: Hashable {
+protocol AnySourceLocation {
     var filename: String { get }
     var line: UInt64 { get }
     var column: UInt64 { get }
@@ -76,11 +76,11 @@ protocol AnyDiagnostic {
 
 extension AnyDiagnostic {
     var isPrimary: Bool {
-        self.level != .note
+        !self.isNote
     }
 
     var isNote: Bool {
-        !self.isPrimary
+        self.level == .note
     }
 
     var isIgnored: Bool {
@@ -97,13 +97,7 @@ extension AnyDiagnostic {
 }
 
 extension SerializedDiagnostics.Diagnostic: AnyDiagnostic {}
-extension SerializedDiagnostics.SourceLocation: AnySourceLocation, @retroactive Hashable {
-    public func hash(into hasher: inout Hasher) {
-        hasher.combine(self.filename)
-        hasher.combine(self.line)
-        hasher.combine(self.column)
-    }
-}
+extension SerializedDiagnostics.SourceLocation: AnySourceLocation {}
 
 extension SerializedDiagnostics.FixIt: AnyFixIt {}
 
@@ -112,16 +106,18 @@ private struct PrimaryDiagnosticFilter<Diagnostic: AnyDiagnostic>: ~Copyable {
     /// A hashable type storing the minimum data necessary to uniquely identify
     /// a diagnostic for our purposes.
     private struct DiagnosticID: Hashable {
-        private let location: Diagnostic.SourceLocation
         private let message: String
+        private let filename: String
+        private let utf8Offset: UInt64
         private let level: SerializedDiagnostics.Diagnostic.Level
 
         init(diagnostic: Diagnostic) {
             self.level = diagnostic.level
             self.message = diagnostic.text
             // Force the location. We should be filtering out diagnostics
             // without a location.
-            self.location = diagnostic.location!
+            self.filename = diagnostic.location!.filename
+            self.utf8Offset = diagnostic.location!.offset
         }
     }
 
@@ -379,17 +375,10 @@ private struct SourceFile {
             throw Error.failedToResolveSourceLocation
         }
 
-        guard location.offset == 0 else {
-            return AbsolutePosition(utf8Offset: Int(location.offset))
-        }
-
-        return self.sourceLocationConverter.position(
-            ofLine: Int(location.line),
-            column: Int(location.column)
-        )
+        return AbsolutePosition(utf8Offset: Int(location.offset))
     }
 
-    func node(at location: some AnySourceLocation) throws -> Syntax {
+    func node(at location: borrowing some AnySourceLocation) throws -> Syntax {
         let position = try position(of: location)
 
         if let token = syntax.token(at: position) {