Add "unnecessary local variable" inspection #KT-15958 Fixed

idea/testData/inspectionsLocal/unnecessaryVariable/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.UnnecessaryVariableInspection

idea/testData/inspectionsLocal/unnecessaryVariable/copyOfVal.kt

@@ -0,0 +1,5 @@
+fun test(): Int {
+    val x = 1
+    val <caret>y = x
+    return x + y
+}

idea/testData/inspectionsLocal/unnecessaryVariable/copyOfVal.kt.after

@@ -0,0 +1,4 @@
+fun test(): Int {
+    val x = 1
+    return x + x
+}

idea/testData/inspectionsLocal/unnecessaryVariable/copyOfValWithExplicitType.kt

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+// PROBLEM: none
+
+fun test(): Int {
+    val x = 1
+    // With explicitly given type looks dangerous
+    val <caret>y: Int = x
+    return x + y
+}

idea/testData/inspectionsLocal/unnecessaryVariable/copyOfVar.kt

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+// PROBLEM: none
+
+fun test(): Int {
+    var x = 1
+    val <caret>y = x
+    x++
+    return x + y
+}

idea/testData/inspectionsLocal/unnecessaryVariable/paramCopy.kt

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+fun sqr(arg: Int): Int {
+    val <caret>other = arg
+    return other * other
+}

idea/testData/inspectionsLocal/unnecessaryVariable/paramCopy.kt.after

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+fun sqr(arg: Int): Int {
+    return arg * arg
+}

idea/testData/inspectionsLocal/unnecessaryVariable/propertyCopy.kt

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+// PROBLEM: none
+
+class My(val x: Number)
+
+fun My.foo(): Int {
+    val <caret>y = x
+    if (y is Int) return y
+    return 0
+}

idea/testData/inspectionsLocal/unnecessaryVariable/simpleReturn.kt

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+fun sum(a: Int, b: Int): Int {
+    val <caret>c = a + b
+    return c
+}

idea/testData/inspectionsLocal/unnecessaryVariable/simpleReturn.kt.after

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+fun sum(a: Int, b: Int): Int {
+    return a + b
+}

idea/testData/inspectionsLocal/unnecessaryVariable/topLevelCopy.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+val x: Number? = null
+
+fun foo(): Int {
+    val <caret>y = x
+    if (y is Int) return y
+    return 0
+}

idea/testData/inspectionsLocal/unnecessaryVariable/varCopy.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+
+fun sqrPlusOne(arg: Int): Int {
+    var <caret>other = arg
+    other++
+    return other * other
+}