HIRVisitor - change labels to block ids

HIRTreeVisitor previously passed a string label (for certain blocks). This 
changes to pass the raw BlockId, and have codegen convert that to a string. I'm 
not sure if we'll need this but it would be helpful for eg visiting the IR and 
emitting a new IR, while mapping block ids forward. Even if we don't need that 
it makes sense for Codegen to decide how to convert a block id into a label 
(which has to obey the rules of an identifier, not the visitor's concern).

compiler/forget/src/HIR/Codegen.ts

@@ -9,6 +9,7 @@ import * as t from "@babel/types";
 import { assertExhaustive } from "../Common/utils";
 import { invariant } from "../CompilerError";
 import {
+  BlockId,
   GeneratedSource,
   HIRFunction,
   Identifier,
@@ -168,14 +169,16 @@ class CodegenVisitor
     switch (terminal.kind) {
       case "break": {
         if (terminal.label) {
-          return t.breakStatement(t.identifier(terminal.label));
+          return t.breakStatement(t.identifier(codegenLabel(terminal.label)));
         } else {
           return t.breakStatement();
         }
       }
       case "continue": {
         if (terminal.label) {
-          return t.continueStatement(t.identifier(terminal.label));
+          return t.continueStatement(
+            t.identifier(codegenLabel(terminal.label))
+          );
         } else {
           return t.continueStatement();
         }
@@ -223,13 +226,19 @@ class CodegenVisitor
   appendBlock(
     block: t.Statement[],
     item: t.Statement,
-    label?: string | undefined
+    blockId?: BlockId | undefined
   ): void {
     if (item.type === "EmptyStatement") {
       return;
     }
-    if (label !== undefined) {
-      block.push(createLabelledStatement(item.loc, t.identifier(label), item));
+    if (blockId !== undefined) {
+      block.push(
+        createLabelledStatement(
+          item.loc,
+          t.identifier(codegenLabel(blockId)),
+          item
+        )
+      );
     } else {
       block.push(item);
     }
@@ -240,6 +249,10 @@ class CodegenVisitor
   }
 }
 
+function codegenLabel(id: BlockId): string {
+  return `bb${id}`;
+}
+
 function codegenInstructionValue(
   temp: Temporaries,
   instrValue: InstructionValue

compiler/forget/src/HIR/HIRTreeVisitor.ts

@@ -138,7 +138,7 @@ class Driver<TBlock, TValue, TItem, TCase> {
               consequent: consequent ?? this.emptyBlock(),
               alternate: alternate,
             }),
-            `bb${fallthroughId}` //
+            fallthroughId
           );
           this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
         } else {
@@ -217,7 +217,7 @@ class Driver<TBlock, TValue, TItem, TCase> {
               test,
               cases,
             }),
-            `bb${fallthroughId}`
+            fallthroughId
           );
           this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
         } else {
@@ -290,7 +290,7 @@ class Driver<TBlock, TValue, TItem, TCase> {
               test: testValue,
               loop: loopBody,
             }),
-            `bb${fallthroughId}`
+            fallthroughId
           );
           this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
         } else {
@@ -359,7 +359,7 @@ class Driver<TBlock, TValue, TItem, TCase> {
       case "labeled": {
         return this.visitor.visitTerminal({
           kind: "break",
-          label: `bb${target.block}`,
+          label: target.block,
         });
       }
     }
@@ -375,7 +375,7 @@ class Driver<TBlock, TValue, TItem, TCase> {
       case "labeled": {
         return this.visitor.visitTerminal({
           kind: "continue",
-          label: `bb${target.block}`,
+          label: target.block,
         });
       }
       case "unlabeled": {
@@ -687,7 +687,7 @@ export interface Visitor<TBlock, TValue, TItem, TCase> {
    * Appends an item onto the given block, with an optional label. The label
    * indicates that a break/continue will proceed to code *after* the given item.
    */
-  appendBlock(block: TBlock, item: TItem, label?: string): void;
+  appendBlock(block: TBlock, item: TItem, label?: BlockId): void;
 
   /**
    * Converts the visitor's block representation into the representation of a
@@ -713,5 +713,5 @@ export type BlockTerminal<TBlock, TValue, TItem, TCase> =
       test: TValue;
       loop: TItem;
     }
-  | { kind: "break"; label: string | null }
-  | { kind: "continue"; label: string | null };
+  | { kind: "break"; label: BlockId | null }
+  | { kind: "continue"; label: BlockId | null };

compiler/forget/src/HIR/InferReactiveScopeDependencies.ts

@@ -7,6 +7,7 @@
 
 import { assertExhaustive, retainWhere } from "../Common/utils";
 import {
+  BlockId,
   HIRFunction,
   Identifier,
   Instruction,
@@ -190,6 +191,6 @@ class ScopeDependenciesVisitor
   enterBlock(): void {}
   visitImplicitTerminal(): void | null {}
   visitCase(test: InstructionValue, block: void): void {}
-  appendBlock(block: void, item: void, label?: string | undefined): void {}
+  appendBlock(block: void, item: void, label?: BlockId | undefined): void {}
   leaveBlock(block: void): void {}
 }

compiler/forget/src/HIR/InferReactiveScopes.ts

@@ -9,6 +9,7 @@ import invariant from "invariant";
 import { retainWhere } from "../Common/utils";
 import DisjointSet from "./DisjointSet";
 import {
+  BlockId,
   HIRFunction,
   Instruction,
   InstructionId,
@@ -282,7 +283,7 @@ class MergeOverlappingReactiveScopesVisitor
   visitImplicitTerminal(): void | null {}
   visitTerminal(terminal: BlockTerminal<void, void, void, void>): void {}
   visitCase(test: void | null, block: void): void {}
-  appendBlock(block: void, item: void, label?: string | undefined): void {}
+  appendBlock(block: void, item: void, label?: BlockId | undefined): void {}
   leaveBlock(block: void): void {
     this.scopes.pop();
     if (this.scopes.length === 0) {
@@ -381,7 +382,7 @@ class AlignReactiveScopesToBlockScopeRangeVisitor
   // no-ops
   visitValue(value: InstructionValue): void {}
   visitCase(test: void | null, block: void): void {}
-  appendBlock(block: void, item: void, label?: string | undefined): void {}
+  appendBlock(block: void, item: void, label?: BlockId | undefined): void {}
 }
 
 function getInstructionScope(instr: Instruction): ReactiveScope | null {

compiler/forget/src/HIR/PrintHIRTree.ts

@@ -6,6 +6,7 @@
  */
 
 import { assertExhaustive } from "../Common/utils";
+import { BlockId } from "../ControlFlowGraph";
 import {
   HIRFunction,
   Instruction,
@@ -116,13 +117,17 @@ class PrintVisitor implements Visitor<Array<string>, string, string, string> {
       return `${prefix}case ${test}: ${block.trimStart()}`;
     }
   }
-  appendBlock(block: string[], item: string, label?: string | undefined): void {
+  appendBlock(
+    block: string[],
+    item: string,
+    label?: BlockId | undefined
+  ): void {
     const prefix = "  ".repeat(this.depth);
     if (item !== "") {
       block.push(`${prefix}${item.trimStart()}`);
     }
     if (label !== undefined) {
-      block.push(`${prefix}${label}:`);
+      block.push(`${prefix}bb${label}:`);
     }
   }
   leaveBlock(block: string[]): string {