feat: show inferred type on holes in hover [skip ci]

Author: kasiaMarek

presentation-compiler/src/main/dotty/tools/pc/HoverProvider.scala

@@ -25,6 +25,8 @@ import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.pc.printer.ShortenedTypePrinter
 import dotty.tools.pc.printer.ShortenedTypePrinter.IncludeDefaultParam
 import dotty.tools.pc.utils.InteractiveEnrichments.*
+import dotty.tools.dotc.ast.untpd.InferredTypeTree
+import dotty.tools.dotc.core.StdNames
 
 object HoverProvider:
 
@@ -131,7 +133,12 @@ object HoverProvider:
             .flatMap(symTpe => search.symbolDocumentation(symTpe._1, contentType))
             .map(_.docstring())
             .mkString("\n")
-          printer.expressionType(exprTpw) match
+
+          val expresionTypeOpt = 
+            if symbol.name == StdNames.nme.??? then
+              InferExpectedType(search, driver, params).infer()
+            else printer.expressionType(exprTpw)
+          expresionTypeOpt match
             case Some(expressionType) =>
               val forceExpressionType =
                 !pos.span.isZeroExtent || (

presentation-compiler/test/dotty/tools/pc/tests/hover/HoverHoleSuite.scala

@@ -0,0 +1,68 @@
+package dotty.tools.pc.tests.hover
+
+import dotty.tools.pc.base.BaseHoverSuite
+
+import org.junit.Test
+
+class HoverHoleSuite extends BaseHoverSuite:
+  @Test def basic =
+    check(
+      """object a {
+        |  val x: Int = ?@@??
+        |}
+        |""".stripMargin,
+      """|**Expression type**:
+         |```scala
+         |Int
+         |```
+         |**Symbol signature**:
+         |```scala
+         |def ???: Nothing
+         |```
+         |""".stripMargin
+    )
+
+  @Test def function =
+    check(
+      """object a {
+        |  def m(i: Int) = ???
+        |  val x = m(??@@?)
+        |}
+        |""".stripMargin,
+      """|**Expression type**:
+         |```scala
+         |Int
+         |```
+         |**Symbol signature**:
+         |```scala
+         |def ???: Nothing
+         |```
+         |""".stripMargin
+    )
+
+  @Test def constructor =
+    check(
+      """object a {
+        |  val x: List[Int] = List(?@@??)
+        |}
+        |""".stripMargin,
+      """|def ???: Nothing
+         |""".stripMargin.hover
+    )
+
+  @Test def bounds =
+    check(
+      """|trait Foo
+         |def foo[T <: Foo](a: T): Boolean = ???
+         |val _ = foo(?@@??)
+         |""".stripMargin,
+      """|**Expression type**:
+         |```scala
+         |Foo
+         |```
+         |**Symbol signature**:
+         |```scala
+         |def ???: Nothing
+         |```
+         |""".stripMargin
+    )