new Rule: No line break before assignment

Line break is allowed only after assignment (eg. equals sign)
Auto formatting is supported via simplistic approach by putting
same line break with same indent from left side to right side of assignment

Closes pinterest/ktlint#105

ktlint-ruleset-standard/src/main/kotlin/com/github/shyiko/ktlint/ruleset/standard/NoLineBreakBeforeAssignmentRule.kt

@@ -0,0 +1,23 @@
+package com.github.shyiko.ktlint.ruleset.standard
+
+import com.github.shyiko.ktlint.core.Rule
+import org.jetbrains.kotlin.com.intellij.lang.ASTNode
+import org.jetbrains.kotlin.com.intellij.psi.PsiWhiteSpace
+import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.LeafPsiElement
+import org.jetbrains.kotlin.lexer.KtTokens
+
+class NoLineBreakBeforeAssignmentRule : Rule("no-line-break-before-assignment") {
+
+    override fun visit(node: ASTNode, autoCorrect: Boolean, emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit) {
+        if (node.elementType == KtTokens.EQ) {
+            val prevElement = node.treePrev?.psi
+            if (prevElement is PsiWhiteSpace && prevElement.text.contains("\n")) {
+                emit(node.startOffset, "Line break before assignment is not allowed", true)
+                if (autoCorrect) {
+                    (node.treeNext?.psi as LeafPsiElement).replaceWithText(prevElement.text)
+                    (prevElement as LeafPsiElement).replaceWithText(" ")
+                }
+            }
+        }
+    }
+}

ktlint-ruleset-standard/src/test/kotlin/com/github/shyiko/ktlint/ruleset/standard/NoLineBreakBeforeAssignmentRuleTest.kt

@@ -0,0 +1,69 @@
+package com.github.shyiko.ktlint.ruleset.standard
+
+import com.github.shyiko.ktlint.core.LintError
+import com.github.shyiko.ktlint.test.format
+import com.github.shyiko.ktlint.test.lint
+import org.assertj.core.api.Assertions.assertThat
+import org.testng.annotations.Test
+
+const val ruleId = "no-line-break-before-assignment"
+
+class NoLineBreakBeforeAssignmentRuleTest {
+    @Test
+    fun testAllPartsOnSameLineIsValid() {
+        assertThat(NoLineBreakBeforeAssignmentRule().lint(
+            """
+              val valA = ""
+              """.trimIndent()
+        )).isEmpty()
+    }
+
+    @Test
+    fun testLineBreakAfterAssignmentIsValid() {
+        assertThat(NoLineBreakBeforeAssignmentRule().lint(
+            """
+                val valA =
+                      ""
+              """.trimIndent()
+        )).isEmpty()
+    }
+
+    @Test
+    fun testLineBreakBeforeAssignmentIsViolation() {
+        assertThat(NoLineBreakBeforeAssignmentRule().lint(
+            """
+              val valA
+                    = ""
+              """.trimIndent()
+        )).isEqualTo(listOf(
+            LintError(2, 7, ruleId, "Line break before assignment is not allowed")
+        ))
+    }
+
+    @Test
+    fun testViolationInFunction() {
+        assertThat(NoLineBreakBeforeAssignmentRule().lint(
+            """
+              fun funA()
+                    = ""
+              """.trimIndent()
+        )).isEqualTo(listOf(
+            LintError(2, 7, ruleId, "Line break before assignment is not allowed")
+        ))
+    }
+
+    @Test
+    fun testFixViolationByRemovingLineBreakFromLeftAndPutItOnRightSide() {
+        assertThat(NoLineBreakBeforeAssignmentRule().format(
+            """
+              fun funA()
+                    = ""
+              """.trimIndent()
+        )).isEqualTo(
+            """
+              fun funA() =
+                    ""
+              """.trimIndent()
+        )
+    }
+}