Provides Ask and Local instances backed by FiberRef. (#665)

* Provides Ask and Local instances backed by FiberRef.

* formatting

zio-interop-cats-tests/shared/src/test/scala/zio/interop/CatsMtlSpec.scala

@@ -21,6 +21,20 @@ class CatsMtlSpec extends ZioSpecBase {
     implicit tc => LocalTests[ZIO[Ctx, Error, _], ZEnvironment[Ctx]].local[ZEnvironment[Ctx], Int]
   )
 
+  Unsafe.unsafe { implicit unsafe =>
+    implicit val f: FiberRef[Ctx] = FiberRef.unsafe.make("")
+
+    checkAllAsync(
+      "FiberRef Ask[ZIO[Ctx, Error, _]]",
+      implicit tc => AskTests[ZIO[Ctx, Error, _], Ctx].ask[Ctx]
+    )
+
+    checkAllAsync(
+      "FiberRef Local[ZIO[Ctx, Error, _]]",
+      implicit tc => LocalTests[ZIO[Ctx, Error, _], Ctx].local[Ctx, Int]
+    )
+  }
+
   checkAllAsync(
     "Raise[ZIO[Ctx, Error, _]]",
     implicit tc => RaiseTests[ZIO[Ctx, Error, _], Error].raise[Int]

zio-interop-cats/shared/src/main/scala/zio/interop/catsmtl.scala

@@ -18,7 +18,7 @@ package zio.interop
 
 import cats.Applicative
 import cats.mtl.*
-import zio.{ CanFail, ZEnvironment, ZIO }
+import zio.{ CanFail, FiberRef, ZEnvironment, ZIO }
 import zio.internal.stacktracer.InteropTracer
 
 abstract class CatsMtlPlatform extends CatsMtlInstances
@@ -47,4 +47,21 @@ abstract class CatsMtlInstances {
         fa.catchAll(f)(implicitly[CanFail[E]], InteropTracer.newTrace(f))
     }
 
+  implicit def fiberRefLocal[R, E](implicit
+    fiberRef: FiberRef[R],
+    ev: Applicative[ZIO[R, E, _]]
+  ): Local[ZIO[R, E, _], R] = new Local[ZIO[R, E, _], R] {
+    override def local[A](fa: ZIO[R, E, A])(f: R => R): ZIO[R, E, A] = fiberRef.locallyWith(f)(fa)
+
+    override def applicative: Applicative[ZIO[R, E, *]] = ev
+
+    override def ask[E2 >: R]: ZIO[R, E, E2] = fiberRef.get
+  }
+
+  implicit def fiberRefAsk[R, E](implicit fiberRef: FiberRef[R], ev: Applicative[ZIO[R, E, _]]): Ask[ZIO[R, E, _], R] =
+    new Ask[ZIO[R, E, _], R] {
+      override def applicative: Applicative[ZIO[R, E, *]] = ev
+      override def ask[E2 >: R]: ZIO[R, E, E2]            = fiberRef.get
+    }
+
 }