Add underlining to safe-init stack traces

compiler/src/dotty/tools/dotc/transform/init/Errors.scala

@@ -5,6 +5,7 @@ package init
 
 import ast.tpd._
 import core._
+import util.SourcePosition
 import Decorators._, printing.SyntaxHighlighting
 import Types._, Symbols._, Contexts._
 
@@ -26,28 +27,44 @@ object Errors {
     def toErrors: Errors = this :: Nil
 
     def stacktrace(using Context): String = if (trace.isEmpty) "" else " Calling trace:\n" + {
-      var indentCount = 0
       var last: String = ""
       val sb = new StringBuilder
       trace.foreach { tree =>
-        indentCount += 1
         val pos = tree.sourcePos
-        val prefix = s"${ " " * indentCount }-> "
+        val prefix = "-> "
         val line =
           if pos.source.exists then
             val loc = "[ " + pos.source.file.name + ":" + (pos.line + 1) + " ]"
             val code = SyntaxHighlighting.highlight(pos.lineContent.trim.nn)
             i"$code\t$loc"
           else
             tree.show
+        val positionMarkerLine =
+          if pos.exists && pos.source.exists then
+            positionMarker(pos)
+          else ""
 
-        if (last != line)  sb.append(prefix + line + "\n")
+        if (last != line)  sb.append(prefix + line + "\n" + positionMarkerLine )
 
         last = line
       }
       sb.toString
     }
 
+    /** Used to underline source positions in the stack trace
+     *  pos.source must exist
+     */
+    private def positionMarker(pos: SourcePosition): String = {
+      val trimmed = pos.lineContent.takeWhile(c => c.isWhitespace).length
+      val padding = pos.startColumnPadding.substring(trimmed).nn + "   "
+      val carets =
+        if (pos.startLine == pos.endLine)
+          "^" * math.max(1, pos.endColumn - pos.startColumn)
+        else "^"
+
+      s"$padding$carets\n"
+    }
+
     /** Flatten UnsafePromotion errors
      */
     def flatten: Errors = this match {

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

@@ -212,7 +212,7 @@ class SourceFile(val file: AbstractFile, computeContent: => Array[Char]) extends
     var idx = startOfLine(offset)
     val pad = new StringBuilder
     while (idx != offset) {
-      pad.append(if (idx < length && content()(idx) == '\t') '\t' else ' ')
+      pad.append(if (idx < content().length && content()(idx) == '\t') '\t' else ' ')
       idx += 1
     }
     pad.result()

tests/init/neg/cycle-structure.check

@@ -2,13 +2,19 @@
 2 |    val x1 = b.x      // error
   |             ^^^
   |             Access field A.this.b.x on a value with an unknown initialization status. Calling trace:
-  |              -> val x = A(this)	[ cycle-structure.scala:9 ]
-  |               -> case class A(b: B) {	[ cycle-structure.scala:1 ]
+  |             -> val x = A(this)	[ cycle-structure.scala:9 ]
+  |                        ^^^^^^^
+  |             -> case class A(b: B) {	[ cycle-structure.scala:1 ]
+  |                ^
 -- Error: tests/init/neg/cycle-structure.scala:8:15 --------------------------------------------------------------------
 8 |    val x1 = a.x      // error
   |             ^^^
   |             Access field B.this.a.x on a value with an unknown initialization status. Calling trace:
-  |              -> val x = A(this)	[ cycle-structure.scala:9 ]
-  |               -> case class A(b: B) {	[ cycle-structure.scala:1 ]
-  |                -> val x = B(this)	[ cycle-structure.scala:3 ]
-  |                 -> case class B(a: A) {	[ cycle-structure.scala:7 ]
+  |             -> val x = A(this)	[ cycle-structure.scala:9 ]
+  |                        ^^^^^^^
+  |             -> case class A(b: B) {	[ cycle-structure.scala:1 ]
+  |                ^
+  |             -> val x = B(this)	[ cycle-structure.scala:3 ]
+  |                        ^^^^^^^
+  |             -> case class B(a: A) {	[ cycle-structure.scala:7 ]
+  |                ^

tests/init/neg/default-this.check

@@ -2,4 +2,5 @@
 9 |        compare() // error
   |        ^^^^^^^
   |Cannot prove that the value is fully initialized. Only initialized values may be used as arguments. Calling trace:
-  | -> val result = updateThenCompare(5)	[ default-this.scala:11 ]
+  |-> val result = updateThenCompare(5)	[ default-this.scala:11 ]
+  |                ^^^^^^^^^^^^^^^^^^^^

tests/init/neg/enum-desugared.check

@@ -5,14 +5,18 @@
    |
    |          The unsafe promotion may cause the following problem:
    |          Calling the external method method name may cause initialization errors. Calling trace:
-   |           -> Array(this.LazyErrorId, this.NoExplanationID) // error // error	[ enum-desugared.scala:17 ]
-   |            -> override def productPrefix: String = this.name()	[ enum-desugared.scala:29 ]
+   |          -> Array(this.LazyErrorId, this.NoExplanationID) // error // error	[ enum-desugared.scala:17 ]
+   |                   ^^^^^^^^^^^^^^^^
+   |          -> override def productPrefix: String = this.name()	[ enum-desugared.scala:29 ]
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 -- Error: tests/init/neg/enum-desugared.scala:17:33 --------------------------------------------------------------------
 17 |    Array(this.LazyErrorId, this.NoExplanationID) // error // error
    |                            ^^^^^^^^^^^^^^^^^^^^
    |           Cannot prove that the value is fully initialized. May only use initialized value as method arguments.
    |
    |           The unsafe promotion may cause the following problem:
    |           Calling the external method method ordinal may cause initialization errors. Calling trace:
-   |            -> Array(this.LazyErrorId, this.NoExplanationID) // error // error	[ enum-desugared.scala:17 ]
-   |             -> def errorNumber: Int = this.ordinal() - 2	[ enum-desugared.scala:8 ]
+   |           -> Array(this.LazyErrorId, this.NoExplanationID) // error // error	[ enum-desugared.scala:17 ]
+   |                                      ^^^^^^^^^^^^^^^^^^^^
+   |           -> def errorNumber: Int = this.ordinal() - 2	[ enum-desugared.scala:8 ]
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/init/neg/enum.check

@@ -5,4 +5,5 @@
   |
   |        The unsafe promotion may cause the following problem:
   |        Calling the external method method name may cause initialization errors. Calling trace:
-  |         -> NoExplanationID // error	[ enum.scala:4 ]
+  |        -> NoExplanationID // error	[ enum.scala:4 ]
+  |           ^

tests/init/neg/inherit-non-hot.check

@@ -3,14 +3,22 @@
   |                                  ^^^^^^^^^^^
   |                        Cannot prove that the value is fully initialized. May only assign fully initialized value.
   |                         Calling trace:
-  |                         -> val c = new C	[ inherit-non-hot.scala:19 ]
-  |                            -> class C extends A {	[ inherit-non-hot.scala:15 ]
-  |                             -> val bAgain = toB.getBAgain	[ inherit-non-hot.scala:16 ]
+  |                        -> val c = new C	[ inherit-non-hot.scala:19 ]
+  |                                   ^^^^^
+  |                        -> class C extends A {	[ inherit-non-hot.scala:15 ]
+  |                           ^
+  |                        -> val bAgain = toB.getBAgain	[ inherit-non-hot.scala:16 ]
+  |                                        ^^^
   |
   |                        The unsafe promotion may cause the following problem:
   |                        Call method Foo.B.this.aCopy.toB on a value with an unknown initialization. Calling trace:
-  |                         -> val c = new C	[ inherit-non-hot.scala:19 ]
-  |                            -> class C extends A {	[ inherit-non-hot.scala:15 ]
-  |                             -> val bAgain = toB.getBAgain	[ inherit-non-hot.scala:16 ]
-  |                              -> if b == null then b = new B(this) // error	[ inherit-non-hot.scala:6 ]
-  |                               -> def getBAgain: B = aCopy.toB	[ inherit-non-hot.scala:12 ]
+  |                        -> val c = new C	[ inherit-non-hot.scala:19 ]
+  |                                   ^^^^^
+  |                        -> class C extends A {	[ inherit-non-hot.scala:15 ]
+  |                           ^
+  |                        -> val bAgain = toB.getBAgain	[ inherit-non-hot.scala:16 ]
+  |                                        ^^^
+  |                        -> if b == null then b = new B(this) // error	[ inherit-non-hot.scala:6 ]
+  |                                                 ^^^^^^^^^^^
+  |                        -> def getBAgain: B = aCopy.toB	[ inherit-non-hot.scala:12 ]
+  |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/init/neg/inlined-method.check

@@ -2,4 +2,5 @@
 8 |    scala.runtime.Scala3RunTime.assertFailed(message) // error
   |                                             ^^^^^^^
   |Cannot prove that the value is fully initialized. Only initialized values may be used as arguments. Calling trace:
-  | -> Assertion.failAssert(this)	[ inlined-method.scala:2 ]
+  |-> Assertion.failAssert(this)	[ inlined-method.scala:2 ]
+  |   ^^^^^^^^^^^^^^^^^^^^^^^^^^

tests/init/neg/local-warm4.check

@@ -2,10 +2,17 @@
 18 |                a = newA // error
    |                    ^^^^
    |       Cannot prove that the value is fully initialized. May only assign fully initialized value. Calling trace:
-   |        -> val a = new A(5)	[ local-warm4.scala:26 ]
-   |           -> class A(x: Int) extends Foo(x) {	[ local-warm4.scala:6 ]
-   |            -> val b = new B(y)	[ local-warm4.scala:10 ]
-   |               -> class B(x: Int) extends A(x) {	[ local-warm4.scala:13 ]
-   |                 -> class A(x: Int) extends Foo(x) {	[ local-warm4.scala:6 ]
-   |                  -> increment()	[ local-warm4.scala:9 ]
-   |                   -> updateA()	[ local-warm4.scala:21 ]
+   |       -> val a = new A(5)	[ local-warm4.scala:26 ]
+   |                  ^^^^^^^^
+   |       -> class A(x: Int) extends Foo(x) {	[ local-warm4.scala:6 ]
+   |          ^
+   |       -> val b = new B(y)	[ local-warm4.scala:10 ]
+   |                  ^^^^^^^^
+   |       -> class B(x: Int) extends A(x) {	[ local-warm4.scala:13 ]
+   |          ^
+   |       -> class A(x: Int) extends Foo(x) {	[ local-warm4.scala:6 ]
+   |          ^
+   |       -> increment()	[ local-warm4.scala:9 ]
+   |          ^^^^^^^^^^^
+   |       -> updateA()	[ local-warm4.scala:21 ]
+   |          ^^^^^^^^^

tests/init/neg/t3273.check

@@ -1,16 +1,18 @@
 -- Error: tests/init/neg/t3273.scala:4:42 ------------------------------------------------------------------------------
 4 |  val num1: LazyList[Int] = 1 #:: num1.map(_ + 1)                        // error
   |                                  ^^^^^^^^^^^^^^^
-  |             Cannot prove that the value is fully initialized. Only initialized values may be used as arguments.
+  |              Cannot prove that the value is fully initialized. Only initialized values may be used as arguments.
   |
-  |             The unsafe promotion may cause the following problem:
-  |             Access non-initialized value num1. Calling trace:
+  |              The unsafe promotion may cause the following problem:
+  |              Access non-initialized value num1. Calling trace:
   |              -> val num1: LazyList[Int] = 1 #:: num1.map(_ + 1)                        // error	[ t3273.scala:4 ]
+  |                                                 ^^^^
 -- Error: tests/init/neg/t3273.scala:5:61 ------------------------------------------------------------------------------
 5 |  val num2: LazyList[Int] = 1 #:: num2.iterator.map(_ + 1).to(LazyList)  // error
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |             Cannot prove that the value is fully initialized. Only initialized values may be used as arguments.
+  |              Cannot prove that the value is fully initialized. Only initialized values may be used as arguments.
   |
-  |             The unsafe promotion may cause the following problem:
-  |             Access non-initialized value num2. Calling trace:
+  |              The unsafe promotion may cause the following problem:
+  |              Access non-initialized value num2. Calling trace:
   |              -> val num2: LazyList[Int] = 1 #:: num2.iterator.map(_ + 1).to(LazyList)  // error	[ t3273.scala:5 ]
+  |                                                 ^^^^

tests/init/neg/unsound2.check

@@ -1,6 +1,8 @@
 -- Error: tests/init/neg/unsound2.scala:5:26 ---------------------------------------------------------------------------
 5 |        def getN: Int = a.n // error
   |                        ^^^
-  | Access field B.this.a.n on a value with an unknown initialization status. Calling trace:
-  |  -> println(foo(x).getB)	[ unsound2.scala:8 ]
+  |   Access field B.this.a.n on a value with an unknown initialization status. Calling trace:
+  |   -> println(foo(x).getB)	[ unsound2.scala:8 ]
+  |              ^^^^^^
   |   -> def foo(y: Int): B = if (y > 10) then B(bar(y - 1), foo(y - 1).getN) else B(bar(y), 10)	[ unsound2.scala:2 ]
+  |                                                          ^^^^^^^^^^^^^^^

tests/init/neg/unsound3.check

@@ -2,4 +2,5 @@
 10 |        if (x < 12) then foo().getC().b else newB // error
    |                         ^^^^^^^^^^^^^^
    |             Access field C.this.foo().getC().b on a value with an unknown initialization status. Calling trace:
-   |              -> val b = foo()	[ unsound3.scala:12 ]
+   |             -> val b = foo()	[ unsound3.scala:12 ]
+   |                        ^^^^^

tests/init/neg/unsound4.check

@@ -2,5 +2,7 @@
 3 |    val aAgain = foo(5) // error
   |        ^
   |        Access non-initialized value aAgain. Calling trace:
-  |         -> val aAgain = foo(5) // error	[ unsound4.scala:3 ]
-  |          -> def foo(x: Int): A = if (x < 5) then this else foo(x - 1).aAgain	[ unsound4.scala:2 ]
+  |        -> val aAgain = foo(5) // error	[ unsound4.scala:3 ]
+  |                        ^^^^^^
+  |        -> def foo(x: Int): A = if (x < 5) then this else foo(x - 1).aAgain	[ unsound4.scala:2 ]
+  |                                                          ^^^^^^^^^^^^^^^^^