Use string based SourceLocationConverter init because it's faster.

I'm not sure why, but the string based initializer for `SourceLocationConverter` is substantially faster than the syntax tree based initializer.

The impact is a few hundred ms per file, so it is substantial when formatting many files.

Results:
- swift-protobuf 21.4 seconds before, 17.9 seconds after
- 10k LOC file - 2.5 seconds before, 2.2 seconds after

Author: dylansturg

Sources/SwiftFormat/SwiftFormatter.swift

@@ -58,7 +58,8 @@ public final class SwiftFormatter {
       throw SwiftFormatError.isDirectory
     }
     let sourceFile = try SyntaxParser.parse(url)
-    try format(syntax: sourceFile, assumingFileURL: url, to: &outputStream)
+    let source = try String(contentsOf: url, encoding: .utf8)
+    try format(syntax: sourceFile, assumingFileURL: url, source: source, to: &outputStream)
   }
 
   /// Formats the given Swift source code and writes the result to an output stream.
@@ -75,7 +76,7 @@ public final class SwiftFormatter {
     source: String, assumingFileURL url: URL?, to outputStream: inout Output
   ) throws {
     let sourceFile = try SyntaxParser.parse(source: source)
-    try format(syntax: sourceFile, assumingFileURL: url, to: &outputStream)
+    try format(syntax: sourceFile, assumingFileURL: url, source: source, to: &outputStream)
   }
 
   /// Formats the given Swift syntax tree and writes the result to an output stream.
@@ -85,11 +86,14 @@ public final class SwiftFormatter {
   ///   - url: A file URL denoting the filename/path that should be assumed for this syntax tree,
   ///     which is associated with any diagnostics emitted during formatting. If this is nil, a
   ///     dummy value will be used.
+  ///   - source: The Swift source code. This can be provided to improve the formatter's
+  ///     performance.
   ///   - outputStream: A value conforming to `TextOutputStream` to which the formatted output will
   ///     be written.
   /// - Throws: If an unrecoverable error occurs when formatting the code.
   public func format<Output: TextOutputStream>(
-    syntax: SourceFileSyntax, assumingFileURL url: URL?, to outputStream: inout Output
+    syntax: SourceFileSyntax, assumingFileURL url: URL?, source: String? = nil,
+    to outputStream: inout Output
   ) throws {
     if let position = firstInvalidSyntaxPosition(in: Syntax(syntax)) {
       throw SwiftFormatError.fileContainsInvalidSyntax(position: position)
@@ -98,7 +102,7 @@ public final class SwiftFormatter {
     let assumedURL = url ?? URL(fileURLWithPath: "source")
     let context = Context(
       configuration: configuration, diagnosticEngine: diagnosticEngine, fileURL: assumedURL,
-      sourceFileSyntax: syntax)
+      sourceFileSyntax: syntax, source: source)
     let pipeline = FormatPipeline(context: context)
     let transformedSyntax = pipeline.visit(Syntax(syntax))
 

Sources/SwiftFormat/SwiftLinter.swift

@@ -54,7 +54,8 @@ public final class SwiftLinter {
       throw SwiftFormatError.isDirectory
     }
     let sourceFile = try SyntaxParser.parse(url)
-    try lint(syntax: sourceFile, assumingFileURL: url)
+    let source = try String(contentsOf: url, encoding: .utf8)
+    try lint(syntax: sourceFile, assumingFileURL: url, source: source)
   }
 
   /// Lints the given Swift source code.
@@ -65,23 +66,26 @@ public final class SwiftLinter {
   /// - Throws: If an unrecoverable error occurs when formatting the code.
   public func lint(source: String, assumingFileURL url: URL) throws {
     let sourceFile = try SyntaxParser.parse(source: source)
-    try lint(syntax: sourceFile, assumingFileURL: url)
+    try lint(syntax: sourceFile, assumingFileURL: url, source: source)
   }
 
   /// Lints the given Swift syntax tree.
   ///
   /// - Parameters:
   ///   - syntax: The Swift syntax tree to be converted to be linted.
   ///   - url: A file URL denoting the filename/path that should be assumed for this syntax tree.
+  ///   - source: The Swift source code. This can be provided to improve the linter's performance.
   /// - Throws: If an unrecoverable error occurs when formatting the code.
-  public func lint(syntax: SourceFileSyntax, assumingFileURL url: URL) throws {
+  public func lint(
+    syntax: SourceFileSyntax, assumingFileURL url: URL, source: String? = nil
+  ) throws {
     if let position = firstInvalidSyntaxPosition(in: Syntax(syntax)) {
       throw SwiftFormatError.fileContainsInvalidSyntax(position: position)
     }
-    
+
     let context = Context(
       configuration: configuration, diagnosticEngine: diagnosticEngine, fileURL: url,
-      sourceFileSyntax: syntax)
+      sourceFileSyntax: syntax, source: source)
     let pipeline = LintPipeline(context: context)
     pipeline.walk(Syntax(syntax))
 

Sources/SwiftFormatCore/Context.swift

@@ -57,14 +57,16 @@ public class Context {
     configuration: Configuration,
     diagnosticEngine: DiagnosticEngine?,
     fileURL: URL,
-    sourceFileSyntax: SourceFileSyntax
+    sourceFileSyntax: SourceFileSyntax,
+    source: String? = nil
   ) {
     self.configuration = configuration
     self.diagnosticEngine = diagnosticEngine
     self.fileURL = fileURL
     self.importsXCTest = .notDetermined
-    self.sourceLocationConverter = SourceLocationConverter(
-      file: fileURL.path, tree: sourceFileSyntax)
+    self.sourceLocationConverter =
+      source.map { SourceLocationConverter(file: fileURL.path, source: $0) }
+      ?? SourceLocationConverter(file: fileURL.path, tree: sourceFileSyntax)
     self.ruleMask = RuleMask(
       syntaxNode: Syntax(sourceFileSyntax),
       sourceLocationConverter: sourceLocationConverter