WASM: Implement finally blocks for try/catch'es

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/FinallyBlocksLowering.kt

@@ -217,7 +217,7 @@ class FinallyBlocksLowering(val context: CommonBackendContext, private val throw
             val syntheticTry = IrTryImpl(
                 startOffset = startOffset,
                 endOffset = endOffset,
-                type = context.irBuiltIns.unitType
+                type = context.irBuiltIns.nothingType
             ).apply {
                 this.catches += irCatch(
                     catchParameter,
@@ -241,7 +241,7 @@ class FinallyBlocksLowering(val context: CommonBackendContext, private val throw
                     val transformedTry = IrTryImpl(
                         startOffset = startOffset,
                         endOffset = endOffset,
-                        type = context.irBuiltIns.unitType
+                        type = context.irBuiltIns.nothingType
                     )
                     transformedTry.tryResult = returnedResult
                     for (aCatch in aTry.catches) {

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/WasmLoweringPhases.kt

@@ -291,13 +291,6 @@ private val excludeDeclarationsFromCodegenPhase = makeCustomWasmModulePhase(
     description = "Move excluded declarations to separate place"
 )
 
-private val returnableBlockLoweringPhase = makeWasmModulePhase(
-    ::ReturnableBlockLowering,
-    name = "ReturnableBlockLowering",
-    description = "Replace returnable block with do-while loop",
-    prerequisite = setOf(functionInliningPhase)
-)
-
 private val tryCatchCanonicalization = makeWasmModulePhase(
     ::TryCatchCanonicalization,
     name = "TryCatchCanonicalization",
@@ -305,6 +298,13 @@ private val tryCatchCanonicalization = makeWasmModulePhase(
     prerequisite = setOf(functionInliningPhase)
 )
 
+private val returnableBlockLoweringPhase = makeWasmModulePhase(
+    ::ReturnableBlockLowering,
+    name = "ReturnableBlockLowering",
+    description = "Replace returnable block with do-while loop",
+    prerequisite = setOf(functionInliningPhase)
+)
+
 private val bridgesConstructionPhase = makeWasmModulePhase(
     ::WasmBridgesConstruction,
     name = "BridgesConstruction",
@@ -467,8 +467,8 @@ val wasmPhases = NamedCompilerPhase(
 //            suspendFunctionsLoweringPhase then
 
             stringConstructorLowering then
-            returnableBlockLoweringPhase then
             tryCatchCanonicalization then
+            returnableBlockLoweringPhase then
 
             forLoopsLoweringPhase then
             propertyAccessorInlinerLoweringPhase then

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/ir2wasm/BodyGenerator.kt

@@ -68,7 +68,6 @@ class BodyGenerator(val context: WasmFunctionCodegenContext) : IrElementVisitorV
 
     override fun visitTry(aTry: IrTry) {
         assert(aTry.isCanonical(irBuiltIns)) { "expected canonical try/catch" }
-        assert(aTry.type != irBuiltIns.nothingType) { "shouldn't happen" }
 
         val resultType = context.transformBlockResultType(aTry.type)
         body.buildTry(null, resultType)

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/TryCatchCanonicalization.kt

@@ -40,12 +40,38 @@ import org.jetbrains.kotlin.name.Name
 //            is Bar -> ...exprs
 //        }
 //    }
-// TODO: Describe finally transformation
+// With finally we transform this:
+//    try {
+//        ...exprs
+//    } catch (e: Throwable) {
+//        ...exprs
+//    } finally {
+//        ...<finally exprs>
+//    }
+// Into something like this (tmp variable is used only if we return some result):
+//    val tmp = block { // this is where we return if we return from original try/catch with the result
+//      try {
+//        try {
+//            return@block ...exprs
+//        } catch (e: Throwable) {
+//            return@block ...exprs
+//        }
+//     }
+//     catch (e: Throwable) {
+//       ...<finally exprs>
+//       throw e // rethrow exception if it happened inside of the catch statement
+//     }
+//   }
+//   ...<finally exprs>
+//   tmp // result
+
 
 internal class TryCatchCanonicalization(private val ctx: WasmBackendContext) : FileLoweringPass {
     override fun lower(irFile: IrFile) {
         irFile.transformChildrenVoid(CatchMerger(ctx))
 
+        irFile.transformChildrenVoid(FinallyBlocksLowering(ctx, ctx.irBuiltIns.throwableType))
+
         irFile.acceptVoid(object : IrElementVisitorVoid {
             override fun visitElement(element: IrElement) {
                 element.acceptChildrenVoid(this)
@@ -58,7 +84,7 @@ internal class TryCatchCanonicalization(private val ctx: WasmBackendContext) : F
     }
 }
 
-internal class CatchMerger(private val ctx: WasmBackendContext): IrElementTransformerVoidWithContext() {
+internal class CatchMerger(private val ctx: WasmBackendContext) : IrElementTransformerVoidWithContext() {
     override fun visitTry(aTry: IrTry): IrExpression {
         // First, handle all nested constructs
         aTry.transformChildrenVoid(this)