[FIR] Implement UNNECESSARY_NOT_NULL_ASSERTION checker for java warning level types

compiler/fir/checkers/checkers.jvm/src/org/jetbrains/kotlin/fir/analysis/jvm/checkers/JvmExpressionCheckers.kt

@@ -77,4 +77,9 @@ object JvmExpressionCheckers : ExpressionCheckers() {
         get() = setOf(
             FirJavaUnnecessarySafeCallChecker,
         )
+
+    override val checkNotNullCallCheckers: Set<FirCheckNotNullCallChecker>
+        get() = setOf(
+            FirJavaUnnecessaryNotNullChecker,
+        )
 }

compiler/fir/checkers/checkers.jvm/src/org/jetbrains/kotlin/fir/analysis/jvm/checkers/expression/FirJavaUnnecessaryNotNullChecker.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2010-2024 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.analysis.jvm.checkers.expression
+
+import org.jetbrains.kotlin.config.LanguageFeature
+import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.expression.FirCheckNotNullCallChecker
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.expressions.FirCheckNotNullCall
+import org.jetbrains.kotlin.fir.expressions.arguments
+import org.jetbrains.kotlin.fir.java.enhancement.EnhancedForWarningConeSubstitutor
+import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
+import org.jetbrains.kotlin.fir.types.canBeNull
+import org.jetbrains.kotlin.fir.types.resolvedType
+import org.jetbrains.kotlin.fir.types.typeContext
+
+object FirJavaUnnecessaryNotNullChecker: FirCheckNotNullCallChecker() {
+    override fun check(expression: FirCheckNotNullCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val argument = expression.arguments.singleOrNull() ?: return
+        val argumentType = EnhancedForWarningConeSubstitutor(context.session.typeContext)
+            .substituteOrNull(argument.resolvedType)
+            ?.fullyExpandedType(context.session) ?: return
+
+        if (!argumentType.canBeNull(context.session) && context.languageVersionSettings.supportsFeature(LanguageFeature.EnableDfaWarningsInK2)) {
+            reporter.reportOn(expression.source, FirErrors.UNNECESSARY_NOT_NULL_ASSERTION, argumentType, context)
+        }
+    }
+}