[FIR] Add REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE diagnostic

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -1192,6 +1192,8 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
         val ILLEGAL_INLINE_PARAMETER_MODIFIER by error<KtElement>(PositioningStrategy.INLINE_PARAMETER_MODIFIER)
 
         val INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED by error<KtParameter>()
+
+        val REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE by warning<KtElement>(PositioningStrategy.SUSPEND_MODIFIER)
     }
 
     val IMPORTS by object : DiagnosticGroup("Imports") {

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -616,6 +616,7 @@ object FirErrors {
     val INLINE_PROPERTY_WITH_BACKING_FIELD by error0<KtDeclaration>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE)
     val ILLEGAL_INLINE_PARAMETER_MODIFIER by error0<KtElement>(SourceElementPositioningStrategies.INLINE_PARAMETER_MODIFIER)
     val INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED by error0<KtParameter>()
+    val REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE by warning0<KtElement>(SourceElementPositioningStrategies.SUSPEND_MODIFIER)
 
     // Imports
     val CANNOT_ALL_UNDER_IMPORT_FROM_SINGLETON by error1<KtImportDirective, Name>(SourceElementPositioningStrategies.IMPORT_LAST_NAME)

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirInlineDeclarationChecker.kt

@@ -364,8 +364,12 @@ object FirInlineDeclarationChecker : FirFunctionChecker() {
                 )
             }
 
-            if (!function.isSuspend && isSuspendFunctionalType && !param.isCrossinline) {
+            if (isSuspendFunctionalType && !param.isCrossinline) {
-                reporter.reportOn(param.source, FirErrors.INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED, context)
+                if (function.isSuspend) {
+                    reporter.reportOn(param.returnTypeRef.source, FirErrors.REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE, context)
+                } else {
+                    reporter.reportOn(param.source, FirErrors.INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED, context)
+                }
             }
 
             if (coneType.isNullable && isFunctionalType) {

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDefaultErrorMessages.kt

@@ -333,6 +333,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDECLARATION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_ANNOTATION_TARGET
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_CALL_OF_CONVERSION_METHOD
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_EXPLICIT_TYPE
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_MODALITY_MODIFIER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_MODIFIER
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.REDUNDANT_OPEN_IN_INTERFACE
@@ -1524,6 +1525,11 @@ class FirDefaultErrorMessages {
                 "Suspend inline lambda parameters of non-suspend function type are not supported. Add 'noinline' or 'crossinline' modifier."
             )
 
+            map.put(
+                REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE,
+                "Redundant 'suspend' modifier: lambda parameters of suspend function type uses existing continuation."
+            )
+
             //imports
             map.put(
                 CANNOT_ALL_UNDER_IMPORT_FROM_SINGLETON,

compiler/testData/diagnostics/testsWithStdLib/coroutines/inlineCrossinline/inlineSuspendOfSuspend.fir.kt

@@ -16,7 +16,7 @@ interface SuspendRunnable {
 // Block is NOT allowed to be called from nested classes/lambdas (as common crossinlines)
 // It is NOT possible to call startCoroutine on the parameter
 // suspend calls possible inside lambda matching to the parameter
-suspend inline fun test(c: suspend () -> Unit) {
+suspend inline fun test(c: <!REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE!>suspend<!> () -> Unit) {
     c()
     val o = object: SuspendRunnable {
         override suspend fun run() {

compiler/testData/diagnostics/testsWithStdLib/coroutines/suspendFunctionType/inline.fir.kt

@@ -1,45 +0,0 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER
-// SKIP_TXT
-inline fun foo1(<!INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED!>x: suspend () -> Unit<!>) {}
-inline fun foo2(crossinline x: suspend () -> Unit) {}
-<!NOTHING_TO_INLINE!>inline<!> fun foo3(noinline x: suspend () -> Unit) {}
-<!NOTHING_TO_INLINE!>inline<!> fun foo4(<!INCOMPATIBLE_MODIFIERS!>noinline<!> <!INCOMPATIBLE_MODIFIERS!>crossinline<!> x: suspend () -> Unit) {}
-
-suspend inline fun bar1(x: suspend () -> Unit) {}
-suspend inline fun bar2(crossinline x: suspend () -> Unit) {}
-suspend inline fun bar3(noinline x: suspend () -> Unit) {}
-suspend inline fun bar4(<!INCOMPATIBLE_MODIFIERS!>noinline<!> <!INCOMPATIBLE_MODIFIERS!>crossinline<!> x: suspend () -> Unit) {}
-
-suspend fun baz() {
-    foo1 {
-        return@baz
-    }
-
-    foo2 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-
-    foo3 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-
-    foo4 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-
-    bar1 {
-        return@baz
-    }
-
-    bar2 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-
-    bar3 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-
-    bar4 {
-        <!RETURN_NOT_ALLOWED!>return@baz<!>
-    }
-}

compiler/testData/diagnostics/testsWithStdLib/coroutines/suspendFunctionType/inline.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -UNUSED_PARAMETER
 // SKIP_TXT
 inline fun foo1(<!INLINE_SUSPEND_FUNCTION_TYPE_UNSUPPORTED!>x: suspend () -> Unit<!>) {}

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

@@ -3217,6 +3217,12 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.REDUNDANT_INLINE_SUSPEND_FUNCTION_TYPE) { firDiagnostic ->
+        RedundantInlineSuspendFunctionTypeImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.CANNOT_ALL_UNDER_IMPORT_FROM_SINGLETON) { firDiagnostic ->
         CannotAllUnderImportFromSingletonImpl(
             firDiagnostic.a,

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

@@ -2244,6 +2244,10 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = InlineSuspendFunctionTypeUnsupported::class
     }
 
+    abstract class RedundantInlineSuspendFunctionType : KtFirDiagnostic<KtElement>() {
+        override val diagnosticClass get() = RedundantInlineSuspendFunctionType::class
+    }
+
     abstract class CannotAllUnderImportFromSingleton : KtFirDiagnostic<KtImportDirective>() {
         override val diagnosticClass get() = CannotAllUnderImportFromSingleton::class
         abstract val objectName: Name

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

@@ -3625,6 +3625,13 @@ internal class InlineSuspendFunctionTypeUnsupportedImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class RedundantInlineSuspendFunctionTypeImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.RedundantInlineSuspendFunctionType(), KtAbstractFirDiagnostic<KtElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class CannotAllUnderImportFromSingletonImpl(
     override val objectName: Name,
     firDiagnostic: FirPsiDiagnostic,