Merge pull request #134 from scala/backport-lts-3.3-21927

Backport "Allow discarding "Discarded non-Unit" warnings with `: Unit`" to 3.3 LTS

Author: tgodzik

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2429,7 +2429,7 @@ class PureExpressionInStatementPosition(stat: untpd.Tree, val exprOwner: Symbol)
 class PureUnitExpression(stat: untpd.Tree, tpe: Type)(using Context)
   extends Message(PureUnitExpressionID) {
   def kind = MessageKind.PotentialIssue
-  def msg(using Context) = i"Discarded non-Unit value of type ${tpe.widen}. You may want to use `()`."
+  def msg(using Context) = i"Discarded non-Unit value of type ${tpe.widen}. Add `: Unit` to discard silently."
   def explain(using Context) =
     i"""As this expression is not of type Unit, it is desugared into `{ $stat; () }`.
        |Here the `$stat` expression is a pure statement that can be discarded.
@@ -3133,7 +3133,7 @@ class InlinedAnonClassWarning()(using Context)
 class ValueDiscarding(tp: Type)(using Context)
   extends Message(ValueDiscardingID):
     def kind = MessageKind.PotentialIssue
-    def msg(using Context) = i"discarded non-Unit value of type $tp"
+    def msg(using Context) = i"discarded non-Unit value of type ${tp.widen}. Add `: Unit` to discard silently."
     def explain(using Context) = ""
 
 class UnusedNonUnitValue(tp: Type)(using Context)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -74,6 +74,9 @@ object Typer {
   /** An attachment for GADT constraints that were inferred for a pattern. */
   val InferredGadtConstraints = new Property.StickyKey[core.GadtConstraint]
 
+  /** Indicates that an expression is explicitly ascribed to [[Unit]] type. */
+  val AscribedToUnit = new Property.StickyKey[Unit]
+
   /** An attachment on a Select node with an `apply` field indicating that the `apply`
    *  was inserted by the Typer.
    */
@@ -992,7 +995,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         else tpt
       val expr1 =
         if isWildcard then tree.expr.withType(underlyingTreeTpe.tpe)
-        else typed(tree.expr, underlyingTreeTpe.tpe.widenSkolem)
+        else
+          if underlyingTreeTpe.tpe.isRef(defn.UnitClass) then
+            untpd.unsplice(tree.expr).putAttachment(AscribedToUnit, ())
+          typed(tree.expr, underlyingTreeTpe.tpe.widenSkolem)
       assignType(cpy.Typed(tree)(expr1, tpt), underlyingTreeTpe)
         .withNotNullInfo(expr1.notNullInfo)
     }
@@ -3009,7 +3015,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       else if (ctx.mode.is(Mode.Pattern))
         typedUnApply(cpy.Apply(tree)(op, l :: r :: Nil), pt)
       else {
-        val app = typedApply(desugar.binop(l, op, r), pt)
+        val app = typedApply(desugar.binop(l, op, r).withAttachmentsFrom(tree), pt)
         if op.name.isRightAssocOperatorName && !ctx.mode.is(Mode.QuotedPattern) then
           val defs = new mutable.ListBuffer[Tree]
           def lift(app: Tree): Tree = (app: @unchecked) match
@@ -4221,9 +4227,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         // so will take the code path that decides on inlining
         val tree1 = adapt(tree, WildcardType, locked)
         checkStatementPurity(tree1)(tree, ctx.owner, isUnitExpr = true)
-        if (!ctx.isAfterTyper && !tree.isInstanceOf[Inlined] && ctx.settings.Whas.valueDiscard && !isThisTypeResult(tree)) {
+
+        if ctx.settings.Whas.valueDiscard
+           && !ctx.isAfterTyper
+           && !tree.isInstanceOf[Inlined]
+           && !isThisTypeResult(tree)
+           && !tree.hasAttachment(AscribedToUnit) then
           report.warning(ValueDiscarding(tree.tpe), tree.srcPos)
-        }
+
         return tpd.Block(tree1 :: Nil, unitLiteral)
       }
 
@@ -4578,6 +4589,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         // sometimes we do not have the original anymore and use the transformed tree instead.
         // But taken together, the two criteria are quite accurate.
         missingArgs(tree, tree.tpe.widen)
+      case _ if tree.hasAttachment(AscribedToUnit) =>
+        // The tree was ascribed to `Unit` explicitly to silence the warning.
+        ()
       case _ if isUnitExpr =>
         report.warning(PureUnitExpression(original, tree.tpe), original.srcPos)
       case _ =>

tests/neg-custom-args/captures/real-try.check

@@ -1,7 +1,7 @@
 -- [E190] Potential Issue Warning: tests/neg-custom-args/captures/real-try.scala:30:4 ----------------------------------
 30 |  b.x
    |  ^^^
-   |  Discarded non-Unit value of type () -> Unit. You may want to use `()`.
+   |  Discarded non-Unit value of type () -> Unit. Add `: Unit` to discard silently.
    |
    | longer explanation available when compiling with `-explain`
 -- Error: tests/neg-custom-args/captures/real-try.scala:12:2 -----------------------------------------------------------

tests/neg/i13091.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i13091.scala:7:16 ----------------------------------------------------------------------------------
+7 |def test = (new Foo): Unit // error: class Foo is marked @experimental ...
+  |                ^^^
+  |                class Foo is marked @experimental and therefore may only be used in an experimental scope.

tests/neg/i13091.scala

@@ -4,4 +4,4 @@ import annotation.experimental
 
 @experimental class Foo
 
-def test: Unit = new Foo // error: class Foo is marked @experimental ...
+def test = (new Foo): Unit // error: class Foo is marked @experimental ...

tests/neg/i18408a.check

@@ -7,7 +7,7 @@
 -- [E190] Potential Issue Warning: tests/neg/i18408a.scala:3:15 --------------------------------------------------------
 3 |def test1 = fa(42)
   |               ^^
-  |               Discarded non-Unit value of type Int. You may want to use `()`.
+  |               Discarded non-Unit value of type Int. Add `: Unit` to discard silently.
   |
   | longer explanation available when compiling with `-explain`
 -- [E129] Potential Issue Warning: tests/neg/i18408a.scala:4:16 --------------------------------------------------------

tests/neg/i18408b.check

@@ -7,7 +7,7 @@
 -- [E190] Potential Issue Warning: tests/neg/i18408b.scala:3:15 --------------------------------------------------------
 3 |def test1 = fa(42)
   |               ^^
-  |               Discarded non-Unit value of type Int. You may want to use `()`.
+  |               Discarded non-Unit value of type Int. Add `: Unit` to discard silently.
   |
   | longer explanation available when compiling with `-explain`
 -- [E129] Potential Issue Warning: tests/neg/i18408b.scala:4:16 --------------------------------------------------------

tests/neg/i18408c.check

@@ -7,7 +7,7 @@
 -- [E190] Potential Issue Warning: tests/neg/i18408c.scala:3:15 --------------------------------------------------------
 3 |def test1 = fa(42)
   |               ^^
-  |               Discarded non-Unit value of type Int. You may want to use `()`.
+  |               Discarded non-Unit value of type Int. Add `: Unit` to discard silently.
   |
   | longer explanation available when compiling with `-explain`
 -- [E129] Potential Issue Warning: tests/neg/i18408c.scala:4:16 --------------------------------------------------------

tests/neg/spaces-vs-tabs.check

@@ -28,9 +28,3 @@
    |	  The start of this line does not match any of the previous indentation widths.
    |	  Indentation width of current line : 1 tab, 2 spaces
    |	  This falls between previous widths: 1 tab and 1 tab, 4 spaces
--- [E190] Potential Issue Warning: tests/neg/spaces-vs-tabs.scala:13:7 -------------------------------------------------
-13 |	      1
-   |	      ^
-   |	      Discarded non-Unit value of type Int. You may want to use `()`.
-   |
-   | longer explanation available when compiling with `-explain`

tests/neg/spaces-vs-tabs.scala

@@ -10,6 +10,6 @@ object Test:
 	object Test2:
 
 	    if true then
-	      1
+	      ()
 	  else 2 // error