From 120c65bfbbd3c3298dc0232aacc5a3b2e85c8e08 Mon Sep 17 00:00:00 2001 From: Chris Tate Date: Sat, 24 Oct 2026 20:24:11 -0500 Subject: [PATCH] Read an iterator's next once, and await return()'s value in for await JavaScript reads an iterator's `next` when the iterator opens and keeps calling that function, so a `next` the iterator assigns while it runs is not called. The for-of loop and the spread drain over an object-literal iterator read the field on every step; they now read it once after the open (classIteratorNextRead) and call the function they read. The checked-dynamic spread drain (%iterator.pack.checked) does the same, using the stepping the for-of and Array.from drains already use. `for await` over a synchronous iterator closes it on an early exit the way the async-from-sync iterator does: return()'s result is read for done and value, and the value is awaited, so a pending promise settles before the loop continues and a rejected one rejects the loop (after a throw from the body, the body's error still wins). --- .../lowering/expressions/array-literals.ts | 123 ++++++++++++++---- .../src/frontend/lowering/lower-classes.ts | 45 ++++++- .../src/frontend/lowering/lower-stmts.ts | 41 +++++- .../src/frontend/lowering/lower-stream.ts | 2 +- .../compiler/src/frontend/lowering/lowerer.ts | 14 +- tests/corpus/6928-class-iterator-protocol.ts | 19 +++ tests/corpus/6929-class-iterator-protocol.mjs | 19 +++ .../6935-class-async-iterator-protocol.ts | 40 ++++++ .../6936-class-async-iterator-protocol.mjs | 39 ++++++ 9 files changed, 306 insertions(+), 36 deletions(-) diff --git a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts index 6153b87f2..09f3e1a2c 100644 --- a/packages/compiler/src/frontend/lowering/expressions/array-literals.ts +++ b/packages/compiler/src/frontend/lowering/expressions/array-literals.ts @@ -29,9 +29,10 @@ import { lowerCollectionSpread } from "../containers/collection-methods.js"; import { strCharsCall } from "../containers/array-construction.js"; import { generatorDrain } from "../iterator-adapters.js"; import { isSearchParamsIterable } from "../builtins/url.js"; -import { varRef } from "../../../ir/build.js"; +import { dynUndefinedExpr, varRef } from "../../../ir/build.js"; import { arrayValueStore } from "../array-values.js"; import { classGeneratorIterable } from "../class-iterables.js"; +import { iteratorCanStep } from "../iterator-consumption.js"; /** JavaScript: whether `expr` is or holds an existing checked-dynamic * object — a binding stored as a dyn value whose checker type is an @@ -717,13 +718,17 @@ function lowerTupleSpreadLiteral( return { kind: "seqExpr", stmts, result: { kind: "recordLit", fields, type, loc }, type, loc }; } -/** Drain using checked dispatch so native class and generator iterators keep their methods. - * `spell` is the TypeError message for a source that is not iterable - * (notIterableSpelling). */ +/** Drain a checked-dynamic iterable: `next` is read once when the iterator + * opens, as JS keeps it in the iterator record, so a `next` the iterator + * assigns while it runs is not called. Runtime iterators step natively, and + * other iterators call the `next` read (class methods keep their emitted + * dispatch). `spell` is the TypeError message for a source that is not + * iterable (notIterableSpelling). */ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, loc: SrcLoc): IrExpr { const name = "%iterator.pack.checked"; if (!lowerer.liftedFns.some((fn) => fn.name === name)) { const ref = (id: string): IrExpr => ({ kind: "varRef", localId: id, type: DYN, loc }); + const flag = (id: string): IrExpr => ({ kind: "varRef", localId: id, type: BOOL, loc }); const property = (key: string): IrExpr => ({ kind: "dynKeyGet", value: ref("step"), @@ -731,6 +736,7 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, lo type: DYN, loc, }); + const finished: IrStmt = { kind: "return", value: ref("result"), loc }; lowerer.liftedFns.push({ name, params: [ @@ -739,12 +745,13 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, lo ], returnType: DYN, locals: [ - ...["source", "iterator", "step", "result"].map((id) => ({ + ...["source", "iterator", "next", "step", "value", "result"].map((id) => ({ id, name: id, type: DYN, mutable: id !== "source", })), + { id: "fast", name: "fast", type: BOOL, mutable: false }, { id: "spell", name: "spell", type: STRING, mutable: false }, ], body: [ @@ -760,6 +767,24 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, lo }, loc, }, + { + kind: "varDecl", + localId: "next", + init: { + kind: "dynKeyGet", + value: ref("iterator"), + key: { kind: "strLit", value: "next", type: STRING, loc }, + type: DYN, + loc, + }, + loc, + }, + { + kind: "varDecl", + localId: "fast", + init: iteratorCanStep(ref("iterator"), ref("next"), loc), + loc, + }, { kind: "varDecl", localId: "result", @@ -770,25 +795,77 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, lo kind: "while", cond: { kind: "boolLit", value: true, type: BOOL, loc }, body: [ - { - kind: "varDecl", - localId: "step", - init: { - kind: "dynInvoke", - recv: ref("iterator"), - method: "next", - calleeName: "iterator.next", - args: [], - type: DYN, - loc, - }, - loc, - }, + // Declared before the branches, which each assign it: a local + // declared in a branch is released when the branch ends. + { kind: "varDecl", localId: "value", init: dynUndefinedExpr(loc), loc }, { kind: "if", - cond: { kind: "dynTest", test: "truthy", value: property("done"), type: BOOL, loc }, - then: [{ kind: "return", value: ref("result"), loc }], - else_: null, + cond: flag("fast"), + then: [ + { + kind: "assign", + localId: "value", + value: { + kind: "libCall", + fn: "dyn.iteratorStep", + args: [ref("iterator")], + type: DYN, + loc, + }, + loc, + }, + { + kind: "if", + cond: { + kind: "libCall", + fn: "dyn.iteratorStepDone", + args: [ref("iterator")], + type: BOOL, + loc, + }, + then: [finished], + else_: null, + loc, + }, + ], + else_: [ + { + kind: "varDecl", + localId: "step", + init: { + kind: "libCall", + fn: "dyn.iteratorResult", + args: [ + { + kind: "dynCall", + callee: ref("next"), + receiver: ref("iterator"), + calleeName: "iterator.next", + args: [], + type: DYN, + loc, + }, + ], + type: DYN, + loc, + }, + loc, + }, + { + kind: "if", + cond: { + kind: "dynTest", + test: "truthy", + value: property("done"), + type: BOOL, + loc, + }, + then: [finished], + else_: null, + loc, + }, + { kind: "assign", localId: "value", value: property("value"), loc }, + ], loc, }, { @@ -796,7 +873,7 @@ function checkedIteratorPack(lowerer: Lowerer, source: IrExpr, spell: string, lo expr: { kind: "libCall", fn: "dyn.packPush", - args: [ref("result"), property("value")], + args: [ref("result"), ref("value")], type: VOID, loc, }, diff --git a/packages/compiler/src/frontend/lowering/lower-classes.ts b/packages/compiler/src/frontend/lowering/lower-classes.ts index 9d994b76b..2e077bfcf 100644 --- a/packages/compiler/src/frontend/lowering/lower-classes.ts +++ b/packages/compiler/src/frontend/lowering/lower-classes.ts @@ -8016,15 +8016,42 @@ export function classIteratorOf( }; } -/** The `it.next()` step of a class iterator as an ordinary (possibly - * virtual) method call. */ -export function classIteratorNextCall( +/** The `next` an object-literal iterator's protocol calls, read once when + * the iterator opens: JS keeps the `next` it reads then in the iterator + * record, so a `next` the iterator assigns while it runs is not called. + * Null for an iterator class, whose `next` is a method. */ +export function classIteratorNextRead( lowerer: Lowerer, cit: ClassIteratorInfo, itRef: IrExpr, loc: SrcLoc, +): (IrExpr & { type: IrType & { kind: "func" } }) | null { + const iterT = cit.iterT; + if (iterT.kind === "object") return null; + const field = lowerer.shapes.get(iterT.shapeId)!.fields.find((f) => f.name === "next")!; + return { + kind: "recordGet", + obj: itRef, + shapeId: iterT.shapeId, + field: "next", + type: field.type as IrType & { kind: "func" }, + loc, + }; +} + +/** The `it.next()` step of a class iterator: an ordinary (possibly + * virtual) method call, or the call of `next`, the object-literal + * iterator's `next` that classIteratorNextRead read when it opened. */ +export function classIteratorNextCall( + lowerer: Lowerer, + cit: ClassIteratorInfo, + itRef: IrExpr, + next: IrExpr | null, + loc: SrcLoc, ): IrExpr { - return classIteratorMethodCall(lowerer, cit, "next", itRef, loc); + if (next === null || next.type.kind !== "func") + return classIteratorMethodCall(lowerer, cit, "next", itRef, loc); + return { kind: "callValue", callee: next, args: [], type: next.type.ret, loc }; } /** `it.next()` or `it.return()` of a class iterator: a (possibly virtual) @@ -8108,6 +8135,8 @@ export function classIteratorDrainCall( const recvT: IrType = { kind: "object", className: cit.className }; const recvRef: IrExpr = { kind: "varRef", localId: "r.0", type: recvT, loc }; const itRef: IrExpr = { kind: "varRef", localId: "it.0", type: cit.iterT, loc }; + const nextRead = classIteratorNextRead(lowerer, cit, itRef, loc); + const nextRef: IrExpr | null = nextRead ? varRef("next.0", nextRead.type, loc) : null; const outRef: IrExpr = { kind: "varRef", localId: "out.0", type: outT, loc }; const resRef: IrExpr = { kind: "varRef", localId: "res.0", type: cit.resultT, loc }; const valueRead: IrExpr = { @@ -8122,7 +8151,7 @@ export function classIteratorDrainCall( { kind: "varDecl", localId: "res.0", - init: classIteratorNextCall(lowerer, cit, itRef, loc), + init: classIteratorNextCall(lowerer, cit, itRef, nextRef, loc), loc, }, ...(cit.hasDone @@ -8163,6 +8192,7 @@ export function classIteratorDrainCall( locals: [ { id: "r.0", name: "r", type: recvT, mutable: false }, { id: "it.0", name: "it", type: cit.iterT, mutable: false }, + ...(nextRead ? [{ id: "next.0", name: "next", type: nextRead.type, mutable: false }] : []), { id: "out.0", name: "out", type: outT, mutable: false }, { id: "res.0", name: "res", type: cit.resultT, mutable: true }, ], @@ -8179,6 +8209,9 @@ export function classIteratorDrainCall( init: classIteratorOpenCall(lowerer, cit, recvRef, loc), loc, }, + ...(nextRead + ? [{ kind: "varDecl", localId: "next.0", init: nextRead, loc } satisfies IrStmt] + : []), { kind: "while", cond: { kind: "boolLit", value: true, type: BOOL, loc }, @@ -8226,7 +8259,7 @@ export function classIteratorRestDrainCall( { kind: "varDecl", localId: "res.0", - init: classIteratorNextCall(lowerer, cit, itRef, loc), + init: classIteratorNextCall(lowerer, cit, itRef, null, loc), loc, }, ...(cit.hasDone diff --git a/packages/compiler/src/frontend/lowering/lower-stmts.ts b/packages/compiler/src/frontend/lowering/lower-stmts.ts index bee887e0f..b942a538b 100644 --- a/packages/compiler/src/frontend/lowering/lower-stmts.ts +++ b/packages/compiler/src/frontend/lowering/lower-stmts.ts @@ -2578,7 +2578,7 @@ export function lowerBindingPattern( const itRef = (): IrExpr => ({ kind: "varRef", localId: it.id, type: citPat.iterT, loc }); for (const el of pattern.elements.nodes) { const stepLoc = locOf(el); - const step = lowerer.classIteratorNextCall(citPat, itRef(), stepLoc); + const step = lowerer.classIteratorNextCall(citPat, itRef(), null, stepLoc); if (ts.isOmittedExpression(el) || el.name === undefined) { out.push({ kind: "exprStmt", expr: step, loc: stepLoc }); continue; @@ -10726,6 +10726,8 @@ function lowerForOfClassIterator( const r = lowerer.declareHiddenLocal("%cir", cit.resultT); const recvRef: IrExpr = { kind: "varRef", localId: recv.id, type: iterable.type, loc }; const itRef = (): IrExpr => ({ kind: "varRef", localId: it.id, type: cit.iterT, loc }); + const nextRead = lowerer.classIteratorNextRead(cit, itRef(), loc); + const next = nextRead ? lowerer.declareHiddenLocal("%cin", nextRead.type) : null; const rRef = (): IrExpr => ({ kind: "varRef", localId: r.id, type: cit.resultT, loc }); const done = cit.hasReturn ? lowerer.declareHiddenLocal("%cid", BOOL) : null; if (done) done.mutable = true; @@ -10756,7 +10758,12 @@ function lowerForOfClassIterator( { kind: "varDecl", localId: r.id, - init: lowerer.classIteratorNextCall(cit, itRef(), loc), + init: lowerer.classIteratorNextCall( + cit, + itRef(), + next ? varRef(next.id, next.type, loc) : null, + loc, + ), loc, }, ...(cit.hasDone @@ -10820,6 +10827,9 @@ function lowerForOfClassIterator( init: lowerer.classIteratorOpenCall(cit, recvRef, loc), loc, }, + ...(next && nextRead + ? [{ kind: "varDecl", localId: next.id, init: nextRead, loc } satisfies IrStmt] + : []), ...(done ? [ { @@ -10887,6 +10897,7 @@ function lowerForOfDyn( done.mutable = true; const value = lowerer.declareHiddenLocal("%dofValue", DYN); const returned = lowerer.declareHiddenLocal("%dofReturn", DYN); + const closed = lowerer.declareHiddenLocal("%dofClosed", DYN); const needsClose = lowerer.declareHiddenLocal("%dofNeedsClose", BOOL); needsClose.mutable = true; const error = lowerer.declareHiddenLocal("%dofError", CAUGHT); @@ -10952,9 +10963,10 @@ function lowerForOfDyn( }, then: [ { - kind: "exprStmt", + kind: "varDecl", + localId: closed.id, // return()'s (awaited) result must be an object, as for next(). - expr: asyncLoop + init: asyncLoop ? check( awaitDyn({ kind: "dynCall", @@ -10969,6 +10981,27 @@ function lowerForOfDyn( : call(varRef(returned.id, DYN, loc)), loc, }, + // A synchronous iterator's return() result is read as the + // async-from-sync iterator reads it: done, then the value, which is + // awaited, so a rejected promise there rejects the loop. + ...(syncIterator + ? [ + { + kind: "if", + cond: varRef(syncIterator.id, BOOL, loc), + then: [ + { kind: "exprStmt", expr: get(varRef(closed.id, DYN, loc), "done"), loc }, + { + kind: "exprStmt", + expr: awaitDyn(get(varRef(closed.id, DYN, loc), "value")), + loc, + }, + ], + else_: null, + loc, + } satisfies IrStmt, + ] + : []), ], else_: null, loc, diff --git a/packages/compiler/src/frontend/lowering/lower-stream.ts b/packages/compiler/src/frontend/lowering/lower-stream.ts index 90bdf917e..e17dee274 100644 --- a/packages/compiler/src/frontend/lowering/lower-stream.ts +++ b/packages/compiler/src/frontend/lowering/lower-stream.ts @@ -1982,7 +1982,7 @@ function classIteratorObject( { kind: "varDecl", localId: r.id, - init: classIteratorNextCall(lowerer, cit, varRef(it.id, cit.iterT, loc), loc), + init: classIteratorNextCall(lowerer, cit, varRef(it.id, cit.iterT, loc), null, loc), loc, }, { diff --git a/packages/compiler/src/frontend/lowering/lowerer.ts b/packages/compiler/src/frontend/lowering/lowerer.ts index ea503e1f1..76c4e5c0d 100644 --- a/packages/compiler/src/frontend/lowering/lowerer.ts +++ b/packages/compiler/src/frontend/lowering/lowerer.ts @@ -362,6 +362,7 @@ import { classIteratorDrainCall, classIteratorMethodCall, classIteratorNextCall, + classIteratorNextRead, classIteratorOf, classIteratorOpenCall, classIteratorRestDrainCall, @@ -13063,8 +13064,17 @@ export class Lowerer { return classIteratorOpenCall(this, cit, recv, loc); } - classIteratorNextCall(cit: ClassIteratorInfo, itRef: IrExpr, loc: SrcLoc): IrExpr { - return classIteratorNextCall(this, cit, itRef, loc); + classIteratorNextRead(cit: ClassIteratorInfo, itRef: IrExpr, loc: SrcLoc): IrExpr | null { + return classIteratorNextRead(this, cit, itRef, loc); + } + + classIteratorNextCall( + cit: ClassIteratorInfo, + itRef: IrExpr, + next: IrExpr | null, + loc: SrcLoc, + ): IrExpr { + return classIteratorNextCall(this, cit, itRef, next, loc); } classIteratorDrainCall(src: IrExpr, loc: SrcLoc, elemT?: IrType): IrExpr | null { diff --git a/tests/corpus/6928-class-iterator-protocol.ts b/tests/corpus/6928-class-iterator-protocol.ts index f097368de..2c2eecccc 100644 --- a/tests/corpus/6928-class-iterator-protocol.ts +++ b/tests/corpus/6928-class-iterator-protocol.ts @@ -91,3 +91,22 @@ class Range { let total = 0; for (const v of new Range(4)) total += v; console.log(total, [...new Range(3)].join()); + +// An object-literal iterator that replaces its next() while it runs: the +// loop and the spread keep calling the next() they read when it opened. +class Swapping { + [Symbol.iterator]() { + let i = 0; + const it = { + next(): { value: number; done: boolean } { + i++; + if (i === 2) it.next = () => ({ value: -1, done: true }); + return { value: i, done: i > 4 }; + }, + }; + return it; + } +} +const swapped: number[] = []; +for (const v of new Swapping()) swapped.push(v); +console.log(swapped.join(), [...new Swapping()].join()); diff --git a/tests/corpus/6929-class-iterator-protocol.mjs b/tests/corpus/6929-class-iterator-protocol.mjs index b5255ba35..6a7827f15 100644 --- a/tests/corpus/6929-class-iterator-protocol.mjs +++ b/tests/corpus/6929-class-iterator-protocol.mjs @@ -76,3 +76,22 @@ try { } catch (e) { console.log("throw:", e.message); } + +// An object-literal iterator that replaces its next() while it runs: the +// loop and the spread keep calling the next() they read when it opened. +class Swapping { + [Symbol.iterator]() { + let i = 0; + const it = { + next() { + i++; + if (i === 2) it.next = () => ({ value: -1, done: true }); + return { value: i, done: i > 4 }; + }, + }; + return it; + } +} +const swapped = []; +for (const v of new Swapping()) swapped.push(v); +console.log(swapped.join(), [...new Swapping()].join()); diff --git a/tests/corpus/6935-class-async-iterator-protocol.ts b/tests/corpus/6935-class-async-iterator-protocol.ts index 9ec5a61a5..25626a773 100644 --- a/tests/corpus/6935-class-async-iterator-protocol.ts +++ b/tests/corpus/6935-class-async-iterator-protocol.ts @@ -56,5 +56,45 @@ async function main(): Promise { console.log("found", await find()); for await (const p of new Pages()) console.log(p); for await (const l of new Letters()) console.log("letter", l); + // return()'s value is awaited when the loop leaves early: a pending + // promise settles first, and a rejection rejects the loop after a break + // (after a throw from the body, the body's error wins). + for await (const v of new Closing((resolve) => setTimeout(() => resolve(1), 5))) + if (v === 1) break; + console.log("after break"); + try { + for await (const v of new Closing((_, reject) => reject(new Error("closing failed")))) + if (v === 1) break; + } catch (e) { + console.log("break:", (e as Error).message); + } + try { + for await (const v of new Closing((_, reject) => reject(new Error("closing failed")))) + if (v === 1) throw new Error("body failed"); + } catch (e) { + console.log("throw:", (e as Error).message); + } +} +class Closing { + readonly settle: (resolve: (v: number) => void, reject: (e: Error) => void) => void; + constructor(settle: (resolve: (v: number) => void, reject: (e: Error) => void) => void) { + this.settle = settle; + } + [Symbol.iterator]() { + let i = 0; + const settle = this.settle; + return { + next(): IteratorResult { + return { value: i++, done: false }; + }, + return(): IteratorResult { + console.log("return called"); + const value = new Promise((resolve, reject) => + settle((v) => (console.log("value settled"), resolve(v)), reject), + ); + return { value: value as unknown as number, done: true }; + }, + }; + } } main(); diff --git a/tests/corpus/6936-class-async-iterator-protocol.mjs b/tests/corpus/6936-class-async-iterator-protocol.mjs index cb7b1e68a..4cc67b0d1 100644 --- a/tests/corpus/6936-class-async-iterator-protocol.mjs +++ b/tests/corpus/6936-class-async-iterator-protocol.mjs @@ -56,5 +56,44 @@ async function main() { console.log("found", await find()); for await (const p of new Pages()) console.log(p); for await (const l of new Letters()) console.log("letter", l); + // return()'s value is awaited when the loop leaves early: a pending + // promise settles first, and a rejection rejects the loop after a break + // (after a throw from the body, the body's error wins). + for await (const v of new Closing((resolve) => setTimeout(() => resolve(1), 5))) + if (v === 1) break; + console.log("after break"); + try { + for await (const v of new Closing((_, reject) => reject(new Error("closing failed")))) + if (v === 1) break; + } catch (e) { + console.log("break:", e.message); + } + try { + for await (const v of new Closing((_, reject) => reject(new Error("closing failed")))) + if (v === 1) throw new Error("body failed"); + } catch (e) { + console.log("throw:", e.message); + } +} +class Closing { + constructor(settle) { + this.settle = settle; + } + [Symbol.iterator]() { + let i = 0; + const settle = this.settle; + return { + next() { + return { value: i++, done: false }; + }, + return() { + console.log("return called"); + const value = new Promise((resolve, reject) => + settle((v) => (console.log("value settled"), resolve(v)), reject), + ); + return { value, done: true }; + }, + }; + } } main();