FIR checker: warn useless elvis

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

@@ -1798,6 +1798,19 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.USELESS_ELVIS) { firDiagnostic ->
+        UselessElvisImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
+    add(FirErrors.USELESS_ELVIS_RIGHT_IS_NULL) { firDiagnostic ->
+        UselessElvisRightIsNullImpl(
+            firDiagnostic as FirPsiDiagnostic<*>,
+            token,
+        )
+    }
     add(FirErrors.NO_ELSE_IN_WHEN) { firDiagnostic ->
         NoElseInWhenImpl(
             firDiagnostic.a.map { whenMissingCase ->

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

@@ -1265,6 +1265,15 @@ sealed class KtFirDiagnostic<PSI: PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = NotNullAssertionOnCallableReference::class
     }
 
+    abstract class UselessElvis : KtFirDiagnostic<KtBinaryExpression>() {
+        override val diagnosticClass get() = UselessElvis::class
+        abstract val receiverType: KtType
+    }
+
+    abstract class UselessElvisRightIsNull : KtFirDiagnostic<KtBinaryExpression>() {
+        override val diagnosticClass get() = UselessElvisRightIsNull::class
+    }
+
     abstract class NoElseInWhen : KtFirDiagnostic<KtWhenExpression>() {
         override val diagnosticClass get() = NoElseInWhen::class
         abstract val missingWhenCases: List<WhenMissingCase>

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

@@ -2049,6 +2049,21 @@ internal class NotNullAssertionOnCallableReferenceImpl(
     override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
 }
 
+internal class UselessElvisImpl(
+    override val receiverType: KtType,
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.UselessElvis(), KtAbstractFirDiagnostic<KtBinaryExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
+internal class UselessElvisRightIsNullImpl(
+    firDiagnostic: FirPsiDiagnostic<*>,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.UselessElvisRightIsNull(), KtAbstractFirDiagnostic<KtBinaryExpression> {
+    override val firDiagnostic: FirPsiDiagnostic<*> by weakRef(firDiagnostic)
+}
+
 internal class NoElseInWhenImpl(
     override val missingWhenCases: List<WhenMissingCase>,
     firDiagnostic: FirPsiDiagnostic<*>,