Enable lazy construction of label strings

core/jvm/src/test/scala/org/scalacheck/GenSpecification.scala

@@ -583,6 +583,8 @@ object GenSpecification extends Properties("Gen") with GenSpecificationVersionSp
       }
       val avg = sum / n
       s"average = $avg" |: avg >= 0.49 && avg <= 0.51
+      s"average = $avg".ensuring(false, "eager evaluation") =|= avg >= 0.49 && avg <= 0.51
+      avg >= 0.49 && avg <= 0.51 =|= s"average = $avg"
     }
 
   property("uniform long #209") = {

core/shared/src/main/scala-2/org/scalacheck/package.scala

@@ -0,0 +1,29 @@
+/*
+ * ScalaCheck
+ * Copyright (c) 2007-2021 Rickard Nilsson. All rights reserved.
+ * http://www.scalacheck.org
+ *
+ * This software is released under the terms of the Revised BSD License.
+ * There is NO WARRANTY. See the file LICENSE for the full text.
+ */
+
+package org.scalacheck
+
+object `package` {
+  implicit class `by-name label :|`(private val prop: Prop) extends AnyVal {
+    /** Label this property.
+     *
+     *  The label is evaluated lazily.
+     *  The operator name is chosen for its precedence btween
+     *  boolean operators and others.
+     */
+    def =|= (label: => String): Prop = prop.map(_.label(label))
+  }
+  // chained implicit for true =|= label
+  implicit class `by-name label bool :| label`(private val bool: Boolean) extends AnyVal {
+    def =|= (label: => String): Prop = (bool: Prop).=|=(label)
+  }
+  implicit class `by-name label |: prop`(label: => String) {
+    def =|= (prop: Prop): Prop = prop.=|=(label)
+  }
+}

core/shared/src/main/scala-3/org/scalacheck/package.scala

@@ -0,0 +1,21 @@
+/*
+ * ScalaCheck
+ * Copyright (c) 2007-2021 Rickard Nilsson. All rights reserved.
+ * http://www.scalacheck.org
+ *
+ * This software is released under the terms of the Revised BSD License.
+ * There is NO WARRANTY. See the file LICENSE for the full text.
+ */
+
+package org.scalacheck
+
+object `package` {
+
+  /** Label this property. The label is evaluated lazily.
+    *
+    * The operator name is chosen for its precedence between boolean operators and others.
+    */
+  extension (prop: Prop) def =|=(label: => String): Prop = prop.map(_.label(label))
+
+  extension (label: => String) def =|=(prop: Prop): Prop = prop.map(_.label(label))
+}

core/shared/src/main/scala/org/scalacheck/Prop.scala

@@ -9,22 +9,23 @@
 
 package org.scalacheck
 
-import scala.annotation.tailrec
-
 import rng.Seed
 import util.{Pretty, ConsoleReporter}
 
 /** Helper class to satisfy ScalaJS compilation. Do not use this directly, use `Prop.apply` instead.
   */
 sealed class PropFromFun(f: Gen.Parameters => Prop.Result) extends Prop {
+
+  /** Evaluate this property by applying the function. */
   def apply(prms: Gen.Parameters) = f(prms)
 }
 
 @Platform.EnableReflectiveInstantiation
-sealed abstract class Prop extends Serializable { self =>
+sealed abstract class Prop extends Serializable {
 
   import Prop.{Result, True, False, Undecided, provedToTrue, mergeRes}
 
+  /** Evaluate this property. */
   def apply(prms: Gen.Parameters): Result
 
   def viewSeed(name: String): Prop =
@@ -36,7 +37,7 @@ sealed abstract class Prop extends Serializable { self =>
           val sd = Seed.random()
           (prms0.withInitialSeed(sd), sd)
       }
-      val res = self(prms)
+      val res = apply(prms)
       if (res.failure) println(s"failing seed for $name is ${seed.toBase64}")
       res
     }
@@ -46,7 +47,7 @@ sealed abstract class Prop extends Serializable { self =>
     useSeed(seed)
 
   def useSeed(seed: Seed): Prop =
-    Prop(prms0 => self(prms0.withInitialSeed(seed)))
+    Prop(prms0 => apply(prms0.withInitialSeed(seed)))
 
   def contramap(f: Gen.Parameters => Gen.Parameters): Prop =
     new PropFromFun(params => apply(f(params)))
@@ -197,14 +198,12 @@ object Prop {
       labels: Set[String] = Set.empty
   ) {
     def success = status match {
-      case True => true
-      case Proof => true
+      case True | Proof => true
       case _ => false
     }
 
     def failure = status match {
-      case False => true
-      case Exception(_) => true
+      case False | Exception(_) => true
       case _ => false
     }
 
@@ -276,6 +275,10 @@ object Prop {
       case (Proof, _) => mergeRes(this, r, r.status)
       case (True, _) => mergeRes(this, r, r.status)
     }
+
+    def flatMap(f: Result => Result): Result = if (success) f(this) else this
+
+    def recover(f: Result => Result): Result = if (failure) f(this) else this
   }
 
   sealed trait Status
@@ -1274,14 +1277,12 @@ object Prop {
   /** Ensures that the property expression passed in completes within the given space of time.
     */
   def within(maximumMs: Long)(wrappedProp: => Prop): Prop = {
-    @tailrec def attempt(prms: Gen.Parameters, endTime: Long): Result = {
+    def attempt(prms: Gen.Parameters, endTime: Long): Result = {
       val result = wrappedProp.apply(prms)
-      if (System.currentTimeMillis > endTime) {
-        (if (result.failure) result else Result(status = False)).label("Timeout")
-      } else {
-        if (result.success) result
-        else attempt(prms, endTime)
-      }
+      if (System.currentTimeMillis > endTime)
+        result.flatMap(_ => Result(status = False)).label("Timeout")
+      else
+        result.recover(_ => attempt(prms, endTime))
     }
     Prop.apply(prms => attempt(prms, System.currentTimeMillis + maximumMs))
   }

core/shared/src/test/scala/org/scalacheck/StatsSpecification.scala

@@ -87,7 +87,7 @@ object StatsSpecification extends Properties("Stats") {
 
   case class Bounds(min: Double, max: Double) {
     def contains(x: Double): Prop =
-      Prop(min <= x && x <= max) :| s"($min <= $x <= $max) was false"
+      Prop(min <= x && x <= max) =|= s"($min <= $x <= $max) was false"
   }
 
   implicit class MakeBounds(val n: Double) extends AnyVal {