Avoid unnecessary file IO when computing positions

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -52,7 +52,14 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
 
   def maybeIncomplete: Boolean = _maybeInComplete
 
-  def this(file: AbstractFile, codec: Codec) = this(file, new String(file.toByteArray, codec.charSet).toCharArray)
+  def this(file: AbstractFile, codec: Codec) =
+    // It would be cleaner to check if the file exists instead of catching
+    // an exception, but it turns out that Files.exists is remarkably slow,
+    // at least on Java 8 (https://rules.sonarsource.com/java/tag/performance/RSPEC-3725),
+    // this is significant enough to show up in our benchmarks.
+    this(file,
+      try new String(file.toByteArray, codec.charSet).toCharArray
+      catch case _: java.nio.file.NoSuchFileException => Array[Char]())
 
   /** Tab increment; can be overridden */
   def tabInc: Int = 8

compiler/src/dotty/tools/dotc/util/SourcePosition.scala

@@ -42,16 +42,16 @@ extends interfaces.SourcePosition with Showable {
   def beforeAndAfterPoint: (List[Int], List[Int]) =
     lineOffsets.partition(_ <= point)
 
-  def column: Int = if (source.file.exists) source.column(point) else -1
+  def column: Int = if (source.content().length != 0) source.column(point) else -1
 
   def start: Int = span.start
-  def startLine: Int = if (source.file.exists) source.offsetToLine(start) else -1
-  def startColumn: Int = if (source.file.exists) source.column(start) else -1
+  def startLine: Int = if (source.content().length != 0) source.offsetToLine(start) else -1
+  def startColumn: Int = if (source.content().length != 0) source.column(start) else -1
   def startColumnPadding: String = source.startColumnPadding(start)
 
   def end: Int = span.end
-  def endLine: Int = if (source.file.exists) source.offsetToLine(end) else -1
-  def endColumn: Int = if (source.file.exists) source.column(end) else -1
+  def endLine: Int = if (source.content().length != 0) source.offsetToLine(end) else -1
+  def endColumn: Int = if (source.content().length != 0) source.column(end) else -1
 
   def withOuter(outer: SourcePosition): SourcePosition = SourcePosition(source, span, outer)
   def withSpan(range: Span) = SourcePosition(source, range, outer)

sbt-bridge/src/xsbt/DelegatingReporter.java

@@ -91,7 +91,7 @@ public Optional<java.io.File> sourceFile() {
           return Optional.ofNullable(src.file().file());
         }
         public Optional<Integer> line() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           int line = pos.line() + 1;
@@ -101,7 +101,7 @@ public Optional<Integer> line() {
           return Optional.of(line);
         }
         public String lineContent() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return "";
 
           String line = pos.lineContent();
@@ -116,13 +116,13 @@ public Optional<Integer> offset() {
           return Optional.of(pos.point());
         }
         public Optional<Integer> pointer() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           return Optional.of(pos.point() - src.startOfLine(pos.point()));
         }
         public Optional<String> pointerSpace() {
-          if (!src.file().exists())
+          if (src.content().length == 0)
             return Optional.empty();
 
           String lineContent = this.lineContent();