Add pattern completion based on the Unapply argument type

The idea was to collect patterns given the Unapply tree as the parent.
We need to deconstruct the UnApply type tree and get the type of the placeholder ident.

Author: natsukagami

presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -366,6 +366,22 @@ class Completions(
           true,
         )
 
+      // unapply pattern
+      case Ident(name) :: (unapp : UnApply) :: _ =>
+        (
+          CaseKeywordCompletion.contribute(
+            EmptyTree, // no selector
+            completionPos,
+            indexedContext,
+            config,
+            search,
+            parent = unapp,
+            autoImports,
+            patternOnly = Some(name.decoded)
+          ),
+          false,
+        )
+
       // class FooImpl extends Foo:
       //   def x|
       case OverrideExtractor(td, completing, start, exhaustive, fallbackName) =>

presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala

@@ -28,6 +28,8 @@ import dotty.tools.dotc.core.Types.NoType
 import dotty.tools.dotc.core.Types.OrType
 import dotty.tools.dotc.core.Types.Type
 import dotty.tools.dotc.core.Types.TypeRef
+import dotty.tools.dotc.core.Types.AppliedType
+import dotty.tools.dotc.typer.Applications.unapplyArgs
 import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.pc.AutoImports.AutoImportsGenerator
 import dotty.tools.pc.AutoImports.SymbolImport
@@ -76,10 +78,23 @@ object CaseKeywordCompletion:
       patternOnly,
       hasBind
     )
+
     val printer = ShortenedTypePrinter(search, IncludeDefaultParam.Never)(using indexedContext)
     val selTpe = selector match
       case EmptyTree =>
         parent match
+          /* Parent is an unapply pattern */
+          case UnApply(fn, implicits, patterns) if !fn.tpe.isErroneous =>
+            patternOnly match
+              case None => None
+              case Some(value) =>
+                val argPts = unapplyArgs(fn.tpe.widen.finalResultType, fn, patterns, parent.srcPos)
+                patterns.zipWithIndex
+                  .find:
+                    case (id@Ident(v), tpe) => v.decoded == value
+                    case _ => false
+                  .map((_, id) => argPts(id).widen.metalsDealias)
+          /* Parent is a function expecting a case match expression */
           case TreeApply(fun, _) if !fun.tpe.isErroneous =>
             fun.tpe.paramInfoss match
               case (head :: Nil) :: _
@@ -106,7 +121,8 @@ object CaseKeywordCompletion:
           if patternOnly.isEmpty then
             val selectorTpe = selTpe.show
             val tpeLabel =
-              if !selectorTpe.contains("x$1") then selectorTpe
+              if !selectorTpe.contains("x$1") /* selector of a function type? */ then
+                selectorTpe
               else selector.symbol.info.show
             val label = s"case ${tpeLabel} =>"
             List(