SI-8205 Avoid long, slow march to AIIOBE in SourceFile#lineContent

Fixing a regression from SI-8015.

The failure mode is kind of amusing: a while loop in `lineToString`
would count all the way to `Int.MaxValue`, and integer overflow
would foil a bounds check when looking for the 'LF' in 'CR'-'LF'.

Given that we're not a style checker to enforce that source files
end in a new-line, this commit accounts for EOF, and fixed the
overflow problem too.

A JUnit test exercises the bug and a few other variations of
`lineContent`.

While i was in the neighbourhood, I opted for a more efficient
means to slice out that line.

1 files changed, 33 insertions, 0 deletions

diff --git a/test/junit/scala/reflect/internal/util/SourceFileTest.scala b/test/junit/scala/reflect/internal/util/SourceFileTest.scala
new file mode 100644
index 0000000000..bbd5685ef7
--- /dev/null
+++ b/test/junit/scala/reflect/internal/util/SourceFileTest.scala
@@ -0,0 +1,33 @@
+package scala.reflect.internal.util
+
+import org.junit.Assert._
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(classOf[JUnit4])
+class SourceFileTest {
+  def lineContentOf(code: String, offset: Int) =
+    Position.offset(new BatchSourceFile("", code), offset).lineContent
+
+  @Test
+  def si8205_overflow(): Unit = {
+    val file = new BatchSourceFile("", "code no newline")
+    // the bug in lineToString counted until MaxValue, and the AIOOBE came from here
+    assertFalse(file.isEndOfLine(Int.MaxValue))
+  }
+
+  @Test
+  def si8205_lineToString(): Unit = {
+    assertEquals("", lineContentOf("", 0))
+    assertEquals("abc", lineContentOf("abc", 0))
+    assertEquals("abc", lineContentOf("abc", 3))
+    assertEquals("code no newline", lineContentOf("code no newline", 1))
+    assertEquals("", lineContentOf("\n", 0))
+    assertEquals("abc", lineContentOf("abc\ndef", 0))
+    assertEquals("abc", lineContentOf("abc\ndef", 3))
+    assertEquals("def", lineContentOf("abc\ndef", 4))
+    assertEquals("def", lineContentOf("abc\ndef", 6))
+    assertEquals("def", lineContentOf("abc\ndef\n", 7))
+  }
+}