improvement: Add fallback for inlay hints

For Scala 3.nightly, it maps old synthetic decorations to inlay hints.
Its needed because inlay hints are not available for every Scala version

metals/src/main/scala/scala/meta/internal/metals/Compilers.scala

@@ -35,6 +35,7 @@ import scala.meta.pc.HoverSignature
 import scala.meta.pc.OffsetParams
 import scala.meta.pc.PresentationCompiler
 import scala.meta.pc.SymbolSearch
+import scala.meta.pc.SyntheticDecorationsParams
 
 import ch.epfl.scala.bsp4j.BuildTargetIdentifier
 import ch.epfl.scala.bsp4j.CompileReport
@@ -46,6 +47,7 @@ import org.eclipse.lsp4j.Diagnostic
 import org.eclipse.lsp4j.DocumentHighlight
 import org.eclipse.lsp4j.InitializeParams
 import org.eclipse.lsp4j.InlayHint
+import org.eclipse.lsp4j.InlayHintKind
 import org.eclipse.lsp4j.InlayHintParams
 import org.eclipse.lsp4j.RenameParams
 import org.eclipse.lsp4j.SelectionRange
@@ -569,8 +571,24 @@ class Compilers(
       token: CancelToken,
   ): Future[ju.List[InlayHint]] = {
     withPCAndAdjustLsp(params) { (pc, pos, adjust) =>
-      val rangeParams =
-        CompilerRangeParamsUtils.fromPos(pos, token)
+      def inlayHintsFallback(
+          params: SyntheticDecorationsParams
+      ): Future[ju.List[InlayHint]] = {
+        pc.syntheticDecorations(params)
+          .asScala
+          .map(
+            _.map { d =>
+              val hint = new InlayHint()
+              hint.setPosition(d.range().getStart())
+              hint.setLabel(d.label())
+              val kind =
+                if (d.kind() <= 2) InlayHintKind.Type
+                else InlayHintKind.Parameter
+              hint.setKind(kind)
+              hint
+            }
+          )
+      }
 
       def adjustInlayHints(
           inlayHints: ju.List[InlayHint]
@@ -588,6 +606,8 @@ class Compilers(
           .asJava
       }
 
+      val rangeParams =
+        CompilerRangeParamsUtils.fromPos(pos, token)
       val pcParams = CompilerInlayHintsParams(
         rangeParams,
         typeParameters = userConfig().showInferredType.contains("true"),
@@ -596,10 +616,18 @@ class Compilers(
         implicitParameters = userConfig().showImplicitArguments,
         implicitConversions = userConfig().showImplicitConversionsAndClasses,
       )
+
       pc
         .inlayHints(pcParams)
         .asScala
-        .map(adjustInlayHints)
+        .flatMap { hints =>
+          if (hints.isEmpty) {
+            inlayHintsFallback(pcParams.toSyntheticDecorationsParams)
+              .map(adjustInlayHints)
+          } else {
+            Future.successful(adjustInlayHints(hints))
+          }
+        }
     }
       .getOrElse(Future.successful(Nil.asJava))
   }

mtags-shared/src/main/scala/scala/meta/internal/metals/CompilerInlayHintsParams.scala

@@ -17,4 +17,14 @@ case class CompilerInlayHintsParams(
   override def token(): CancelToken = rangeParams.token
   override def offset(): Int = rangeParams.offset
   override def endOffset(): Int = rangeParams.endOffset
+
+  def toSyntheticDecorationsParams: CompilerSyntheticDecorationsParams = {
+    CompilerSyntheticDecorationsParams(
+      rangeParams,
+      inferredTypes = inferredTypes,
+      typeParameters = typeParameters,
+      implicitConversions = implicitConversions,
+      implicitParameters = implicitParameters
+    )
+  }
 }

mtags-shared/src/main/scala/scala/meta/internal/metals/CompilerSyntheticDecorationsParams.scala

@@ -0,0 +1,19 @@
+package scala.meta.internal.metals
+
+import java.net.URI
+
+import scala.meta.pc.CancelToken
+import scala.meta.pc.SyntheticDecorationsParams
+import scala.meta.pc.VirtualFileParams
+
+case class CompilerSyntheticDecorationsParams(
+    virtualFileParams: VirtualFileParams,
+    inferredTypes: Boolean,
+    typeParameters: Boolean,
+    implicitParameters: Boolean,
+    implicitConversions: Boolean
+) extends SyntheticDecorationsParams {
+  override def uri(): URI = virtualFileParams.uri
+  override def text(): String = virtualFileParams.text
+  override def token(): CancelToken = virtualFileParams.token
+}