FIR checker: add convention check for ++ and +=, etc

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

@@ -3027,6 +3027,20 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INC_DEC_SHOULD_NOT_RETURN_UNIT) { firDiagnostic ->
+        IncDecShouldNotReturnUnitImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT) { firDiagnostic ->
+        AssignmentOperatorShouldReturnUnitImpl(
+            firSymbolBuilder.functionLikeBuilder.buildFunctionSymbol(firDiagnostic.a.fir),
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.TOPLEVEL_TYPEALIASES_ONLY) { firDiagnostic ->
         ToplevelTypealiasesOnlyImpl(
             firDiagnostic as FirPsiDiagnostic,

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

@@ -2113,6 +2113,16 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val rightType: KtType
     }
 
+    abstract class IncDecShouldNotReturnUnit : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = IncDecShouldNotReturnUnit::class
+    }
+
+    abstract class AssignmentOperatorShouldReturnUnit : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = AssignmentOperatorShouldReturnUnit::class
+        abstract val functionSymbol: KtFunctionLikeSymbol
+        abstract val operator: String
+    }
+
     abstract class ToplevelTypealiasesOnly : KtFirDiagnostic<KtTypeAlias>() {
         override val diagnosticClass get() = ToplevelTypealiasesOnly::class
     }

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

@@ -3401,6 +3401,22 @@ internal class IncompatibleEnumComparisonErrorImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class IncDecShouldNotReturnUnitImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.IncDecShouldNotReturnUnit(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
+internal class AssignmentOperatorShouldReturnUnitImpl(
+    override val functionSymbol: KtFunctionLikeSymbol,
+    override val operator: String,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.AssignmentOperatorShouldReturnUnit(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class ToplevelTypealiasesOnlyImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,