[HIR] Add new unreachable terminal

ghstack-source-id: cffdbbbd78a0aa7e1587a234ce27366ea626ee5e
Pull Request resolved: https://github.com/facebook/react-forget/pull/2864

compiler/packages/babel-plugin-react-forget/src/HIR/HIR.ts

@@ -341,6 +341,7 @@ export type BasicBlock = {
  */
 export type Terminal =
   | UnsupportedTerminal
+  | UnreachableTerminal
   | ThrowTerminal
   | ReturnTerminal
   | GotoTerminal
@@ -384,6 +385,18 @@ export type UnsupportedTerminal = {
   id: InstructionId;
   loc: SourceLocation;
 };
+
+/**
+ * Terminal for an unreachable block.
+ * Unreachable blocks are emitted when all control flow paths of a if/switch/try block diverge
+ * before reaching the fallthrough.
+ */
+export type UnreachableTerminal = {
+  kind: "unreachable";
+  id: InstructionId;
+  loc: SourceLocation;
+};
+
 export type ThrowTerminal = {
   kind: "throw";
   value: Place;

compiler/packages/babel-plugin-react-forget/src/HIR/HIRBuilder.ts

@@ -799,6 +799,7 @@ function getReversePostorderedBlocks(func: HIR): HIR["blocks"] {
         visit(terminal.block);
         break;
       }
+      case "unreachable":
       case "unsupported": {
         break;
       }

compiler/packages/babel-plugin-react-forget/src/HIR/PrintHIR.ts

@@ -121,6 +121,7 @@ export function printMixedHIR(
     case "throw":
     case "while":
     case "for":
+    case "unreachable":
     case "unsupported":
     case "goto":
     case "do-while":
@@ -261,6 +262,10 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       value = `[${terminal.id}] Sequence block=bb${terminal.block} fallthrough=bb${terminal.fallthrough}`;
       break;
     }
+    case "unreachable": {
+      value = `[${terminal.id}] Unreachable`;
+      break;
+    }
     case "unsupported": {
       value = `Unsupported`;
       break;

compiler/packages/babel-plugin-react-forget/src/HIR/visitors.ts

@@ -853,6 +853,7 @@ export function mapTerminalSuccessors(
         loc: terminal.loc,
       };
     }
+    case "unreachable":
     case "unsupported": {
       return terminal;
     }
@@ -877,6 +878,7 @@ export function terminalFallthrough(terminal: Terminal): BlockId | null {
     case "goto":
     case "return":
     case "throw":
+    case "unreachable":
     case "unsupported": {
       return null;
     }
@@ -915,6 +917,7 @@ export function mapOptionalFallthroughs(
     case "goto":
     case "return":
     case "throw":
+    case "unreachable":
     case "unsupported": {
       return;
     }
@@ -1085,6 +1088,7 @@ export function* eachTerminalSuccessor(terminal: Terminal): Iterable<BlockId> {
       yield terminal.block;
       break;
     }
+    case "unreachable":
     case "unsupported":
       break;
     default: {
@@ -1144,6 +1148,7 @@ export function mapTerminalOperands(
     case "for-of":
     case "for-in":
     case "goto":
+    case "unreachable":
     case "unsupported":
     case "scope": {
       // no-op
@@ -1201,6 +1206,7 @@ export function* eachTerminalOperand(terminal: Terminal): Iterable<Place> {
     case "for-of":
     case "for-in":
     case "goto":
+    case "unreachable":
     case "unsupported":
     case "scope": {
       // no-op

compiler/packages/babel-plugin-react-forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -817,6 +817,10 @@ class Driver {
 
         break;
       }
+      case "unreachable": {
+        // noop
+        break;
+      }
       case "unsupported": {
         CompilerError.invariant(false, {
           reason: "Unexpected unsupported terminal",