diff --git a/build.xml b/build.xml
index 0ee4aea0164..585da896cb8 100644
--- a/build.xml
+++ b/build.xml
@@ -1022,6 +1022,9 @@
         <copy file="${basedir}/libraries/stdlib/src/kotlin/jvm/JvmVersion.kt" todir="${output}/mock-runtime-src"/>
         <copy file="${basedir}/libraries/stdlib/src/kotlin/util/Standard.kt" todir="${output}/mock-runtime-src"/>
         <copy file="${basedir}/libraries/stdlib/src/kotlin/internal/Annotations.kt" todir="${output}/mock-runtime-src"/>
+        <!-- Contracts -->
+        <copy file="${basedir}/libraries/stdlib/src/kotlin/internal/contracts/ContractBuilder.kt" todir="${output}/mock-runtime-src"/>
+        <copy file="${basedir}/libraries/stdlib/src/kotlin/internal/contracts/Effect.kt" todir="${output}/mock-runtime-src"/>
 
         <new-kotlinc output="${output}/classes/mock-runtime" moduleName="kotlin-stdlib">
             <src>
diff --git a/libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt b/libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt
index 3c104fb9126..b942ff92627 100644
--- a/libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt
+++ b/libraries/kotlin.test/common/src/main/kotlin/kotlin/test/Assertions.kt
@@ -24,6 +24,7 @@
 package kotlin.test
 
 import kotlin.internal.*
+import kotlin.internal.contracts.*
 import kotlin.reflect.KClass
 
 /**
@@ -40,6 +41,7 @@ fun assertTrue(message: String? = null, block: () -> Boolean): Unit = assertTrue
 
 /** Asserts that the expression is `true` with an optional [message]. */
 fun assertTrue(actual: Boolean, message: String? = null) {
+    contract { returns() implies actual }
     return asserter.assertTrue(message ?: "Expected value to be true.", actual)
 }
 
@@ -48,6 +50,7 @@ fun assertFalse(message: String? = null, block: () -> Boolean): Unit = assertFal
 
 /** Asserts that the expression is `false` with an optional [message]. */
 fun assertFalse(actual: Boolean, message: String? = null) {
+    contract { returns() implies (!actual) }
     return asserter.assertTrue(message ?: "Expected value to be false.", !actual)
 }
 
@@ -63,12 +66,14 @@ fun <@OnlyInputTypes T> assertNotEquals(illegal: T, actual: T, message: String?
 
 /** Asserts that the [actual] value is not `null`, with an optional [message]. */
 fun <T : Any> assertNotNull(actual: T?, message: String? = null): T {
+    contract { returns() implies (actual != null) }
     asserter.assertNotNull(message, actual)
     return actual!!
 }
 
 /** Asserts that the [actual] value is not `null`, with an optional [message] and a function [block] to process the not-null value. */
 fun <T : Any, R> assertNotNull(actual: T?, message: String? = null, block: (T) -> R) {
+    contract { returns() implies (actual != null) }
     asserter.assertNotNull(message, actual)
     if (actual != null) {
         block(actual)
diff --git a/libraries/stdlib/src/kotlin/text/Strings.kt b/libraries/stdlib/src/kotlin/text/Strings.kt
index 1b8d1f57d34..2a94223f400 100644
--- a/libraries/stdlib/src/kotlin/text/Strings.kt
+++ b/libraries/stdlib/src/kotlin/text/Strings.kt
@@ -21,6 +21,7 @@
 package kotlin.text
 
 import kotlin.comparisons.*
+import kotlin.internal.contracts.*
 
 
 /**
@@ -225,7 +226,13 @@ public fun String.padEnd(length: Int, padChar: Char = ' '): String
  * Returns `true` if this nullable char sequence is either `null` or empty.
  */
 @kotlin.internal.InlineOnly
-public inline fun CharSequence?.isNullOrEmpty(): Boolean = this == null || this.length == 0
+public inline fun CharSequence?.isNullOrEmpty(): Boolean {
+    contract {
+        returns(false) implies (this@isNullOrEmpty != null)
+    }
+
+    return this == null || this.length == 0
+}
 
 /**
  * Returns `true` if this char sequence is empty (contains no characters).
@@ -253,7 +260,13 @@ public inline fun CharSequence.isNotBlank(): Boolean = !isBlank()
  * Returns `true` if this nullable char sequence is either `null` or empty or consists solely of whitespace characters.
  */
 @kotlin.internal.InlineOnly
-public inline fun CharSequence?.isNullOrBlank(): Boolean = this == null || this.isBlank()
+public inline fun CharSequence?.isNullOrBlank(): Boolean {
+    contract {
+        returns(false) implies (this@isNullOrBlank != null)
+    }
+
+    return this == null || this.isBlank()
+}
 
 /**
  * Iterator for characters of the given char sequence.
diff --git a/libraries/stdlib/src/kotlin/util/Preconditions.kt b/libraries/stdlib/src/kotlin/util/Preconditions.kt
index 7ae8d724907..7aabd40aa87 100644
--- a/libraries/stdlib/src/kotlin/util/Preconditions.kt
+++ b/libraries/stdlib/src/kotlin/util/Preconditions.kt
@@ -2,13 +2,20 @@
 @file:kotlin.jvm.JvmName("PreconditionsKt")
 package kotlin
 
+import kotlin.internal.contracts.*
+
 /**
  * Throws an [IllegalArgumentException] if the [value] is false.
  *
  * @sample samples.misc.Preconditions.failRequireWithLazyMessage
  */
 @kotlin.internal.InlineOnly
-public inline fun require(value: Boolean): Unit = require(value) { "Failed requirement." }
+public inline fun require(value: Boolean): Unit {
+    contract {
+        returns() implies value
+    }
+    require(value) { "Failed requirement." }
+}
 
 /**
  * Throws an [IllegalArgumentException] with the result of calling [lazyMessage] if the [value] is false.
@@ -17,6 +24,9 @@ public inline fun require(value: Boolean): Unit = require(value) { "Failed requi
  */
 @kotlin.internal.InlineOnly
 public inline fun require(value: Boolean, lazyMessage: () -> Any): Unit {
+    contract {
+        returns() implies value
+    }
     if (!value) {
         val message = lazyMessage()
         throw IllegalArgumentException(message.toString())
@@ -27,7 +37,12 @@ public inline fun require(value: Boolean, lazyMessage: () -> Any): Unit {
  * Throws an [IllegalArgumentException] if the [value] is null. Otherwise returns the not null value.
  */
 @kotlin.internal.InlineOnly
-public inline fun <T:Any> requireNotNull(value: T?): T = requireNotNull(value) { "Required value was null." }
+public inline fun <T:Any> requireNotNull(value: T?): T {
+    contract {
+        returns() implies (value != null)
+    }
+    return requireNotNull(value) { "Required value was null." }
+}
 
 /**
  * Throws an [IllegalArgumentException] with the result of calling [lazyMessage] if the [value] is null. Otherwise
@@ -37,6 +52,10 @@ public inline fun <T:Any> requireNotNull(value: T?): T = requireNotNull(value) {
  */
 @kotlin.internal.InlineOnly
 public inline fun <T:Any> requireNotNull(value: T?, lazyMessage: () -> Any): T {
+    contract {
+        returns() implies (value != null)
+    }
+
     if (value == null) {
         val message = lazyMessage()
         throw IllegalArgumentException(message.toString())
@@ -51,7 +70,12 @@ public inline fun <T:Any> requireNotNull(value: T?, lazyMessage: () -> Any): T {
  * @sample samples.misc.Preconditions.failCheckWithLazyMessage
  */
 @kotlin.internal.InlineOnly
-public inline fun check(value: Boolean): Unit = check(value) { "Check failed." }
+public inline fun check(value: Boolean): Unit {
+    contract {
+        returns() implies value
+    }
+    check(value) { "Check failed." }
+}
 
 /**
  * Throws an [IllegalStateException] with the result of calling [lazyMessage] if the [value] is false.
@@ -60,6 +84,9 @@ public inline fun check(value: Boolean): Unit = check(value) { "Check failed." }
  */
 @kotlin.internal.InlineOnly
 public inline fun check(value: Boolean, lazyMessage: () -> Any): Unit {
+    contract {
+        returns() implies value
+    }
     if (!value) {
         val message = lazyMessage()
         throw IllegalStateException(message.toString())
@@ -83,6 +110,10 @@ public inline fun <T:Any> checkNotNull(value: T?): T = checkNotNull(value) { "Re
  */
 @kotlin.internal.InlineOnly
 public inline fun <T:Any> checkNotNull(value: T?, lazyMessage: () -> Any): T {
+    contract {
+        returns() implies (value != null)
+    }
+
     if (value == null) {
         val message = lazyMessage()
         throw IllegalStateException(message.toString())
diff --git a/libraries/stdlib/src/kotlin/util/Standard.kt b/libraries/stdlib/src/kotlin/util/Standard.kt
index e524197ebab..98421de1651 100644
--- a/libraries/stdlib/src/kotlin/util/Standard.kt
+++ b/libraries/stdlib/src/kotlin/util/Standard.kt
@@ -2,6 +2,8 @@
 @file:kotlin.jvm.JvmName("StandardKt")
 package kotlin
 
+import kotlin.internal.contracts.*
+
 /**
  * An exception is thrown to indicate that a method body remains to be implemented.
  */
@@ -28,52 +30,91 @@ public inline fun TODO(reason: String): Nothing = throw NotImplementedError("An
  * Calls the specified function [block] and returns its result.
  */
 @kotlin.internal.InlineOnly
-public inline fun <R> run(block: () -> R): R = block()
+public inline fun <R> run(block: () -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    return block()
+}
 
 /**
  * Calls the specified function [block] with `this` value as its receiver and returns its result.
  */
 @kotlin.internal.InlineOnly
-public inline fun <T, R> T.run(block: T.() -> R): R = block()
+public inline fun <T, R> T.run(block: T.() -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    return block()
+}
 
 /**
  * Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
  */
 @kotlin.internal.InlineOnly
-public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
+public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    return receiver.block()
+}
 
 /**
  * Calls the specified function [block] with `this` value as its receiver and returns `this` value.
  */
 @kotlin.internal.InlineOnly
-public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
+public inline fun <T> T.apply(block: T.() -> Unit): T {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    block()
+    return this
+}
 
 /**
  * Calls the specified function [block] with `this` value as its argument and returns `this` value.
  */
 @kotlin.internal.InlineOnly
 @SinceKotlin("1.1")
-public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
+public inline fun <T> T.also(block: (T) -> Unit): T {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    block(this)
+    return this
+}
 
 /**
  * Calls the specified function [block] with `this` value as its argument and returns its result.
  */
 @kotlin.internal.InlineOnly
-public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
+public inline fun <T, R> T.let(block: (T) -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    return block(this)
+}
 
 /**
  * Returns `this` value if it satisfies the given [predicate] or `null`, if it doesn't.
  */
 @kotlin.internal.InlineOnly
 @SinceKotlin("1.1")
-public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
+public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? {
+    contract {
+        callsInPlace(predicate, InvocationKind.EXACTLY_ONCE)
+    }
+    return if (predicate(this)) this else null
+}
 
 /**
  * Returns `this` value if it _does not_ satisfy the given [predicate] or `null`, if it does.
  */
 @kotlin.internal.InlineOnly
 @SinceKotlin("1.1")
-public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
+public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? {
+    return if (!predicate(this)) this else null
+}
 
 /**
  * Executes the given function [action] specified number of [times].
@@ -82,7 +123,9 @@ public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predica
  */
 @kotlin.internal.InlineOnly
 public inline fun repeat(times: Int, action: (Int) -> Unit) {
-    for (index in 0 until times) {
+    contract { callsInPlace(action) }
+
+    for (index in 0..times - 1) {
         action(index)
     }
 }
diff --git a/prepare/mock-runtime-for-test/build.gradle.kts b/prepare/mock-runtime-for-test/build.gradle.kts
index 6e4ee448696..d04024e2c40 100644
--- a/prepare/mock-runtime-for-test/build.gradle.kts
+++ b/prepare/mock-runtime-for-test/build.gradle.kts
@@ -36,7 +36,9 @@ val copySources by task<Copy> {
             .include("kotlin/collections/TypeAliases.kt",
                     "kotlin/jvm/JvmVersion.kt",
                     "kotlin/util/Standard.kt",
-                    "kotlin/internal/Annotations.kt")
+                    "kotlin/internal/Annotations.kt",
+                    "kotlin/internal/contracts/ContractBuilder.kt",
+                    "kotlin/internal/contracts/Effect.kt")
     into(File(buildDir, "src"))
 }