FIR checker: SENSELESS_(COMPARISON|NULL_IN_WHEN)

Currently DFA does not set "definitely equal to null" for access to variables that got assigned `null`. For example, FIR should mark the following line as SENSELESS_COMPARISON due to `s = null` above.
 https://github.com/JetBrains/kotlin/blob/d1531f9cdd5852352c0133198706125dc63b6007/compiler/testData/diagnostics/tests/smartCasts/alwaysNull.fir.kt#L6

The problem is at https://github.com/JetBrains/kotlin/blob/7e9f27436a77de1c76e3705da7aa1fbe8938336b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirDataFlowAnalyzer.kt#L1104

For null assignment, ideally the type should be `Nothing?`. This is
addressed in a followup commit instead.

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -2608,6 +2608,20 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.SENSELESS_COMPARISON) { firDiagnostic ->
+        SenselessComparisonImpl(
+            firDiagnostic.a.source!!.psi as KtExpression,
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.SENSELESS_NULL_IN_WHEN) { firDiagnostic ->
+        SenselessNullInWhenImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.UNSAFE_CALL) { firDiagnostic ->
         UnsafeCallImpl(
             firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -1825,6 +1825,16 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val unreachable: List<PsiElement>
     }
 
+    abstract class SenselessComparison : KtFirDiagnostic<KtBinaryExpression>() {
+        override val diagnosticClass get() = SenselessComparison::class
+        abstract val expression: KtExpression
+        abstract val compareResult: Boolean
+    }
+
+    abstract class SenselessNullInWhen : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = SenselessNullInWhen::class
+    }
+
     abstract class UnsafeCall : KtFirDiagnostic<PsiElement>() {
         override val diagnosticClass get() = UnsafeCall::class
         abstract val receiverType: KtType

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -2939,6 +2939,22 @@ internal class UnreachableCodeImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class SenselessComparisonImpl(
+    override val expression: KtExpression,
+    override val compareResult: Boolean,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.SenselessComparison(), KtAbstractFirDiagnostic<KtBinaryExpression> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
+internal class SenselessNullInWhenImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.SenselessNullInWhen(), KtAbstractFirDiagnostic<KtElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class UnsafeCallImpl(
     override val receiverType: KtType,
     override val receiverExpression: KtExpression?,