feat: implement Convert to collect code action

docs/features/code-actions.md

@@ -55,6 +55,12 @@ It converts a chain of `map`, `flatMap`, `filter` and `filterNot` methods into a
 
 ![To For Comprehension](./gifs/FlatMapToForComprehension.gif)
 
+## filter then map to collect
+
+It converts a chain of `filter` and `map` methods into a `collect` method.
+
+![To Collect](./gifs/FilterMapToCollect.gif)
+
 ## Implement Abstract Members of the Parent Type
 
 Upon inheriting from a type, you also have to implement its abstract members. But manually looking them all up and copying their signature is time consuming, isn't it? You can just use this code action instead.

metals/src/main/scala/scala/meta/internal/metals/codeactions/CodeActionProvider.scala

@@ -44,6 +44,7 @@ final class CodeActionProvider(
     new InlineValueCodeAction(trees, compilers, languageClient),
     new ConvertToNamedArguments(trees, compilers, languageClient),
     new FlatMapToForComprehensionCodeAction(trees, buffers),
+    new FilterMapToCollectCodeAction(trees),
     new MillifyDependencyCodeAction(buffers),
     new MillifyScalaCliDependencyCodeAction(buffers),
     new ConvertCommentCodeAction(buffers),

metals/src/main/scala/scala/meta/internal/metals/codeactions/FilterMapToCollectCodeAction.scala

@@ -0,0 +1,155 @@
+package scala.meta.internal.metals.codeactions
+
+import scala.concurrent.ExecutionContext
+import scala.concurrent.Future
+
+import scala.meta._
+import scala.meta.internal.metals.MetalsEnrichments._
+import scala.meta.internal.metals.codeactions.CodeAction
+import scala.meta.internal.metals.codeactions.CodeActionBuilder
+import scala.meta.internal.parsing.Trees
+import scala.meta.pc.CancelToken
+
+import org.eclipse.lsp4j.CodeActionParams
+import org.eclipse.{lsp4j => l}
+
+class FilterMapToCollectCodeAction(trees: Trees) extends CodeAction {
+  override def kind: String = l.CodeActionKind.RefactorRewrite
+
+  override def contribute(params: CodeActionParams, token: CancelToken)(implicit
+      ec: ExecutionContext
+  ): Future[Seq[l.CodeAction]] = Future {
+    val uri = params.getTextDocument().getUri()
+
+    val path = uri.toAbsolutePath
+    val range = params.getRange()
+
+    trees
+      .findLastEnclosingAt[Term.Apply](path, range.getStart())
+      .flatMap(findFilterMapChain)
+      .map(toTextEdit(_))
+      .map(toCodeAction(uri, _))
+      .toSeq
+  }
+
+  private def toTextEdit(chain: FilterMapChain) = {
+    val param = chain.filterFn.params.head
+    val paramName = Term.Name(param.name.value)
+    val paramPatWithType = param.decltpe match {
+      case Some(tpe) => Pat.Typed(Pat.Var(paramName), tpe)
+      case None => Pat.Var(paramName)
+    }
+
+    val collectCall = Term.Apply(
+      fun = Term.Select(chain.qual, Term.Name("collect")),
+      argClause = Term.ArgClause(
+        values = List(
+          Term.PartialFunction(
+            cases = List(
+              Case(
+                pat = paramPatWithType,
+                cond = Some(chain.filterFn.renameParam(paramName)),
+                body = chain.mapFn.renameParam(paramName),
+              )
+            )
+          )
+        )
+      ),
+    )
+    val indented = collectCall.syntax.linesIterator.zipWithIndex
+      .map {
+        case (line, 0) => line
+        case (line, _) => "  " + line
+      }
+      .mkString("\n")
+
+    new l.TextEdit(chain.tree.pos.toLsp, indented)
+  }
+
+  private def toCodeAction(uri: String, textEdit: l.TextEdit): l.CodeAction =
+    CodeActionBuilder.build(
+      title = FilterMapToCollectCodeAction.title,
+      kind = this.kind,
+      changes = List(uri.toAbsolutePath -> List(textEdit)),
+    )
+
+  private implicit class FunctionOps(fn: Term.Function) {
+    def renameParam(to: Term.Name): Term = {
+      val fnParamName = fn.params.head.name.value
+      fn.body
+        .transform { case Term.Name(name) if name == fnParamName => to }
+        .asInstanceOf[Term]
+    }
+  }
+
+  private def findFilterMapChain(tree: Term.Apply): Option[FilterMapChain] = {
+    val x = Term.Name("x")
+    def extractFunction(arg: Tree): Option[Term.Function] = arg match {
+      case fn: Term.Function => Some(fn)
+      case Term.Block(List(fn: Term.Function)) => extractFunction(fn)
+      case ref: Term.Name => {
+        Some(
+          Term.Function(
+            UnaryParameterList(x),
+            Term.Apply(ref, Term.ArgClause(List(x))),
+          )
+        )
+      }
+      case _ => None
+    }
+
+    def findChain(tree: Term.Apply): Option[FilterMapChain] =
+      tree match {
+        case MapFunctionApply(FilterFunctionApply(base, filterArg), mapArg) =>
+          for {
+            filterFn <- extractFunction(filterArg)
+            mapFn <- extractFunction(mapArg)
+          } yield FilterMapChain(tree, base, filterFn, mapFn)
+        case _ => None
+      }
+
+    findChain(tree).orElse {
+      // If we're inside the chain, look at our parent
+      tree.parent.flatMap {
+        // We're in a method call or function, look at parent apply
+        case Term.Select(_, Term.Name("map" | "filter")) | Term.Function(_) =>
+          tree.parent
+            .flatMap(_.parent)
+            .collectFirst { case parent: Term.Apply => parent }
+            .flatMap(findChain)
+        case _ => None
+      }
+    }
+  }
+
+  private object UnaryParameterList {
+    def unapply(tree: Tree): Option[Name] = tree match {
+      case Term.Param(_, name, _, _) => Some(name)
+      case _ => None
+    }
+    def apply(name: Name): List[Term.Param] = List(
+      Term.Param(Nil, name, None, None)
+    )
+  }
+
+  private case class FunctionApply(val name: String) {
+    def unapply(tree: Tree): Option[(Term, Term)] = tree match {
+      case Term.Apply(Term.Select(base, Term.Name(`name`)), List(args)) =>
+        Some((base, args))
+      case _ => None
+    }
+  }
+  private val FilterFunctionApply = new FunctionApply("filter")
+  private val MapFunctionApply = new FunctionApply("map")
+
+  private case class FilterMapChain(
+      tree: Term.Apply,
+      qual: Term,
+      filterFn: Term.Function,
+      mapFn: Term.Function,
+  )
+}
+
+object FilterMapToCollectCodeAction {
+  val title = "Convert to collect"
+}