Remove obsolete Rust parser experiment, superseded by the much more mature new Rust parser integration (#3815)

We've had an old attempt at integrating a Rust parser with our Scala/Java projects. It seems to have been abandoned and is not used anywhere - it is also superseded by the new integration of the Rust parser. I think it was used as an experiment to see how to approach such an integration.

Since it is not used anymore - it make sense to remove it, because it only adds some (slight, but non-zero) maintenance effort. We can always bring it back from git history if necessary.

build.sbt

@@ -1193,32 +1193,6 @@ lazy val `language-server` = (project in file("engine/language-server"))
   .dependsOn(`library-manager-test` % Test)
   .dependsOn(`runtime-version-manager-test` % Test)
 
-lazy val ast = (project in file("lib/scala/ast"))
-  .settings(
-    version := ensoVersion,
-    Compile / sourceGenerators += GenerateAST.task
-  )
-
-lazy val parser = (project in file("lib/scala/parser"))
-  .settings(
-    fork := true,
-    Compile / compile / compileInputs := (Compile / compile / compileInputs)
-      .dependsOn(Cargo("build --project parser"))
-      .value,
-    javaOptions += {
-      val root = baseDirectory.value.getParentFile.getParentFile.getParentFile
-      s"-Djava.library.path=$root/target/rust/debug"
-    },
-    libraryDependencies ++= Seq(
-      "org.scalatest" %%% "scalatest" % scalatestVersion % Test
-    ),
-    testFrameworks := List(
-      new TestFramework("org.scalatest.tools.Framework"),
-      new TestFramework("org.scalameter.ScalaMeterFramework")
-    )
-  )
-  .dependsOn(ast)
-
 lazy val cleanInstruments = taskKey[Unit](
   "Cleans fragile class files to force a full recompilation and preserve" +
   "consistency of instrumentation configuration."

lib/scala/parser/src/main/scala/org/enso/parser/Parser.scala

@@ -1,41 +0,0 @@
-package org.enso.parser
-
-import org.enso.ast.Ast
-
-import scala.annotation.unused
-
-/** This is the Enso language parser.
-  *
-  * It is a wrapper of parser written in Rust that uses JNI to efficiently
-  * construct scala AST directly without any serialization overhead.
-  *
-  * The methods are loaded from a native shared library `parser` that is located
-  * in a directory specified by `-Djava.library.path` and has one of the extensions
-  * `.dll`, `.so` or `dylib` depending on the platform (windows, linux or mac).
-  *
-  * The shared library itself is generated into `target/rust/debug` by executing
-  * `cargo build -p parser`. Each method marked by `@native` must have a
-  * corresponding counterpart in rust, otherwise the loading of the shared library
-  * is going to fail at runtime with `UnsatisfiedLinkingError`.
-  */
-class Parser private () {
-
-  /** Parses a content of a single source file. */
-  @native def parseStr(@unused input: String): Ast.AnyAst
-
-  /** Parses a single source file. */
-  @native def parseFile(@unused filename: String): Ast.AnyAst
-
-  /** Parses a content of a single source file into a stream of tokens. */
-  @native def lexStr(@unused input: String): Ast.AnyAst
-
-  /** Parses a single source file into a stream of tokens. */
-  @native def lexFile(@unused filename: String): Ast.AnyAst
-}
-
-object Parser {
-  System.loadLibrary("parser")
-
-  /** Constructs a new parser */
-  def apply(): Parser = new Parser()
-}

lib/scala/parser/src/test/scala/org/enso/parser/ParserTest.scala

@@ -1,19 +0,0 @@
-package org.enso.parser
-
-import org.enso.ast.Ast
-
-import org.scalatest.flatspec.AnyFlatSpec
-import org.scalatest.matchers.should.Matchers
-
-class ParserTest extends AnyFlatSpec with Matchers {
-  val parser: Parser = Parser()
-
-  it should "parse file" in {
-    val expected =
-      Ast.Ast(uid = None, len = 0, off = 0, ast = Ast.Txt.Text("Hello!"))
-    assert(expected == parser.parseStr("Hello!"))
-    assert(expected == parser.parseFile("Hello!"))
-    assert(expected == parser.lexStr("Hello!"))
-    assert(expected == parser.lexFile("Hello!"))
-  }
-}

project/GenerateAST.scala

@@ -1,42 +0,0 @@
-import sbt.Keys._
-import sbt._
-import sbt.internal.util.ManagedLogger
-
-object GenerateAST {
-
-  lazy val task = Def.task {
-    val log    = state.value.log
-    val lib    = baseDirectory.value.getParentFile.getParentFile
-    val source = lib / "rust/ast/src/ast.rs"
-    val output = sourceManaged.value / "main/org/enso/ast/Ast.scala"
-    val cache  = streams.value.cacheStoreFactory.make("ast_source")
-
-    Tracked.diffInputs(cache, FileInfo.lastModified)(Set(source)) {
-      source: ChangeReport[File] =>
-        if (source.modified.nonEmpty) {
-          output.getParentFile.mkdirs
-          generateAST(output, log)
-        }
-    }
-
-    Seq(output)
-  }
-
-  /** Generates the Scala AST in the specified file. All errors are reported in
-    * stderr and raise a runtime exception.
-    *
-    * @param out the file where the generated AST is going to be placed
-    */
-  def generateAST(out: File, log: ManagedLogger): Unit = {
-    val args = s"run -p ast -- --generate-scala-ast $out"
-
-    log.info(s"Generating Scala AST from Rust definitions.")
-
-    try Cargo.run(args, log)
-    catch {
-      case ex: RuntimeException =>
-        log.error(s"Generation of the Scala AST failed.")
-        throw ex
-    }
-  }
-}