error: report default class constructor frames at the class definition - #38507
error: report default class constructor frames at the class definition#38507robobun wants to merge 5 commits into
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A class without an explicit constructor gets one that JSC compiles from BuiltinExecutables::defaultConstructorSourceCode(), a one-line source with no URL, so its stack frames rendered as "new X (unknown:1:17)" in error.stack, CallSites and the uncaught error printer, and err.sourceURL and err.line were empty and 1 when such a frame was on top. Attribute these frames to the class's own SourceCode instead, positioned at the `class` keyword like V8 does, in all three renderers. The column is counted from the line start in the source text because the SourceCode's own start column is relative to the function start when the class is on the first line of a lazily parsed function body.
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Reproduced on the released binary (1.4.0) and on main: a class without an explicit constructor renders as Fix in this PR: frames of the constructor JSC synthesizes for such a class are attributed to the class's own source ( Tests: |
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WalkthroughThe change maps stack frames from default class constructors to their class source. It updates line, column, source ID, source URL, formatting, exception reporting, and regression tests. ChangesDefault Constructor Stack Mapping
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In `@test/js/bun/test/stack.test.ts`:
- Around line 258-264: Update the Bun.spawn test to drain proc.stdout
concurrently with proc.stderr and proc.exited in the existing Promise.all call,
then assert stdout is empty before asserting the exit code.
In `@test/js/node/v8/capture-stack-trace.test.js`:
- Around line 587-593: Update the assertions for derivedSite and innerSite to
verify their exact expected class-keyword columns, using the appropriate
expected-column values for each source line rather than only comparing them with
derivedClassSite and innerClassSite. Keep the existing file, line, and
description assertions unchanged.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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📒 Files selected for processing (7)
src/jsc/bindings/ErrorStackFrame.cppsrc/jsc/bindings/ErrorStackFrame.hsrc/jsc/bindings/ErrorStackTrace.cppsrc/jsc/bindings/FormatStackTraceForJS.cppsrc/jsc/bindings/ZigException.cpptest/js/bun/test/stack.test.tstest/js/node/v8/capture-stack-trace.test.js
… stdout in the uncaught fixture
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Review feedback so far is addressed:
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I reviewed this PR and didn't find any bugs. Because it touches native stack-frame rendering across four C++ bindings files (ErrorStackFrame, ErrorStackTrace, FormatStackTraceForJS, ZigException) with hand-rolled column arithmetic and a stated interaction with #37396, a human look would still be worthwhile.
What was reviewed:
defaultClassConstructorClassSource/classSourceStartPosition— theisBuiltinDefaultClassConstructor()gate, the line-terminator scan back to line start, and the first-linestartPosition().m_columnadd-back for node:vm columnOffset.- All four call sites (sourceURL, sourceID, getAdjustedPositionForBytecode, computeLineAndColumn, populateStackFramePosition's provider swap) agree on which SourceProvider the position/byte_offset/excerpt come from.
- Tests cover the variant matrix (derived, base-with-fields, in-function, same-line, class expression, extends null, source-mapped .ts, uncaught printer, CallSite getFileName/LineNumber/ColumnNumber/ScriptId) and the earlier stdout-drain / exact-column feedback is addressed.
Extended reasoning...
Overview
The PR fixes stack frames for classes without an explicit constructor, which JSC compiles from a template string and previously rendered as at new E (unknown:1:28). It adds Bun::defaultClassConstructorClassSource() (returns FunctionExecutable::classSource() when isBuiltinDefaultClassConstructor() is set) and Bun::classSourceStartPosition() (computes the class keyword's line/column by scanning the provider text back to the previous line terminator), then wires those into all three stack renderers: error.stack (FormatStackTraceForJS via a new Bun::computeLineAndColumn wrapper), CallSites (JSCStackFrame::sourceID, getAdjustedPositionForBytecode), and the uncaught-error printer (populateStackFramePosition's provider selection). Zig::sourceURL(CodeBlock&) is also taught to return the class source's URL. Four new tests exercise raw positions (// @bun), source-mapped .ts, the uncaught printer's caret, and Error.prepareStackTrace CallSites.
Security risks
None. This is diagnostic output only — file/line/column reported in stack traces. No user input reaches allocation sizing or path resolution; the only text scan is bounded by source.length() on a StringView held by an already-live SourceProvider.
Level of scrutiny
Moderate-to-high. Error stack rendering is a core diagnostic path shared by every thrown error, Bun.inspect, and the uncaught-error printer, and the change spans four C++ files that each read frame positions slightly differently. The column computation deliberately does not trust classSource.startColumn() (documented as wrong on the first line of a lazily-parsed function body) and instead walks the source; that reasoning is sound and tested by the sameLine case, but it is exactly the kind of position arithmetic a maintainer familiar with JSC's SourceCode/SourceProvider offset conventions should sanity-check. The PR description also flags a merge-order interaction with #37396 that whichever lands second must reconcile.
Other factors
All prior review feedback is addressed: the coderabbit / claude stdout-drain nit and exact-column assertion are fixed in e9f4c3a, and the comment-cop long-comment warnings on the new helpers were shortened in 68894c4 / e67c4d7. The bug-hunting pass found nothing. Test coverage is thorough (exact positions asserted for six class shapes, plus source-map remapping, plus the uncaught printer's caret line, plus CallSite getFileName/getLineNumber/getColumnNumber/getScriptId). I did not approve because this is non-trivial native code across multiple renderers with subtle offset semantics — the change looks correct, but it is not the simple/mechanical class the auto-approval bar targets.
The column already took the provider's start column as-is; JSC clamps a negative lineOffset away in SourceCode, so add it back to the line the same way. Covers both offsets, both signs, in stack.test.ts with the positions node prints for the same scripts.
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Self-review outcome. The only concerns that held up were about how this PR sits next to the other open PRs on the same lines (#37396, #38248, #38344), not about the fix itself. Changes made:
No change to the URL / scriptId / excerpt-provider sites since f143308. |
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I reviewed this PR and didn't find any bugs. The author has addressed all prior feedback (stdout draining, exact column assertions, comment length). Because it touches C++ position arithmetic across three stack renderers with subtle node:vm offset and lazily-parsed-function-body handling, a human look would still be worthwhile.
What was reviewed:
classSourceStartPosition()line-terminator scan bounds and thestd::min(providerStart.m_line, 0)negative-offset compensation — traced against the four node:vm test cases and they check out.ZigException.cppprovider swap for the source excerpt — the existing ref/deref path still applies to whichever provider is chosen.sourceURL(CodeBlock&)change funnels through to all callers (StackVisitor, StackFrame overloads,retrieveSourceURL) as the description claims.- Test coverage: derived/base/nested/same-line/expression classes, source-mapped .ts, uncaught printer caret, CallSite fields — all assert exact positions.
Extended reasoning...
Overview
This PR fixes stack frames for JavaScript classes without an explicit constructor. JSC synthesizes such constructors from a fixed template string ((function () { }) / (function (...args) { super(...args); })), so their frames previously rendered as at new E (unknown:1:28). The fix redirects them to the class keyword in the user's source via FunctionExecutable::classSource(), matching V8/Node. Five C++ bindings files are touched (ErrorStackFrame.{h,cpp}, ErrorStackTrace.cpp, FormatStackTraceForJS.cpp, ZigException.cpp) plus two test files with five new tests.
Security risks
None. This is read-only position reporting — no user input parsing, no allocation on untrusted sizes, no new JS entry points. The backward line-scan in classSourceStartPosition() is bounds-guarded by static_cast<unsigned>(start) <= source.length() before indexing.
Level of scrutiny
Moderate-to-high. The change is well-scoped and the mechanism is sound (keying on JSC's own isBuiltinDefaultClassConstructor() flag), but it threads through three independent stack renderers (error.stack formatter, CallSite/prepareStackTrace, uncaught-error printer) and the ZigException source-excerpt path. The column computation deliberately does NOT use classSource.startColumn() because it's function-relative on the first line of a lazily parsed body — this is subtle and covered by the sameLine test case, but the negative-offset clamping compensation (std::min(providerStart.m_line.zeroBasedInt(), 0)) and the first-line column-offset addition are the kind of arithmetic a maintainer familiar with JSC's SourceProvider/TextPosition semantics should sanity-check.
Other factors
- Test coverage is thorough: raw JSC positions (
// @bun), source-mapped .ts positions, the uncaught printer's caret line, CallSitegetFileName/getLineNumber/getColumnNumber/getScriptId, and node:vmlineOffset/columnOffsetincluding negatives. All assert exact values. - All prior review feedback (CodeRabbit, comment-cop, my own stdout-drain nit) is addressed and marked resolved; the code logic is unchanged since f143308 per the author's summary.
- The PR description explicitly calls out an interaction with #37396 that whichever lands second must handle — this coordination is a human decision.
#pragma oncewas added to ErrorStackFrame.h (was missing before) — good incidental fix.
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Problem
at new E (unknown:1:28)for a derived class,at new B (unknown:1:17)for a base class (e.g. a class with field initializers). Node printsat new E (/tmp/e.js:2:1), the position of theclasskeyword.error.stack,Error.prepareStackTraceCallSites (getFileName()is[source:3],getLineNumber()is 1,getScriptId()is a different script per class), and the uncaught error printer. When such a frame is on top (class E extends null {}; new E(), where the synthesizedsuper(...args)throws) the printer showsat new E (1:23)with the caret under the wrong line, anderr.sourceURL/err.lineare unset / 1.BuiltinExecutables::defaultConstructorSourceCode()("(function () { })"/"(function (...args) { super(...args); })"), andUnlinkedFunctionExecutable::linkedSourceCode()makes that string the executable's source. Bun reads the URL and position straight off that code block:Zig::sourceURL(CodeBlock&)(src/jsc/bindings/ErrorStackTrace.cpp),getAdjustedPositionForBytecode()(src/jsc/bindings/ErrorStackFrame.cpp),frame.computeLineAndColumn()informatStackTrace(src/jsc/bindings/FormatStackTraceForJS.cpp) and the provider used for the source excerpt inpopulateStackFramePosition(src/jsc/bindings/ZigException.cpp).Fix
Bun::defaultClassConstructorClassSource(executable)(ErrorStackFrame.cpp) returnsFunctionExecutable::classSource()when the executable'sUnlinkedFunctionExecutable::isBuiltinDefaultClassConstructor()is set, and a null SourceCode otherwise.isBuiltinDefaultClassConstructoris the flaglinkedSourceCode()itself keys the source swap on, andclassSource()is the class's real SourceCode: the user's provider, starting at theclasskeyword.Bun::classSourceStartPosition()turns that into a position: the class source'sfirstLine(), the column counted back to the line start in the provider's text,byte_position= the class start offset.classSource.startColumn()is not used because JSC's lexer counts columns from wherever it started, so for a class on the first line of a lazily parsed function body it is relative to the function (thesameLinetest case reports9:6with it,9:23with this). node:vmlineOffset/columnOffsetare taken from the provider'sstartPosition()the way V8 applies them: the column offset on the first line only, and both signs (JSC itself clamps negative offsets away inSourceCode, sofirstLine()only carries a positive line offset and the negative part is added back). The vm test case asserts the positions node prints for the same scripts, both signs of both offsets.Zig::sourceURL(CodeBlock&)returns the class source's URL (this also coversJSCStackFrame::retrieveSourceURL,Zig::sourceURL(StackVisitor&)and the twoStackFrameoverloads, which all funnel through it),getAdjustedPositionForBytecode()returns the class position (CallSites viacalculateSourcePositions, and the error printer),formatStackTracepass 1 uses aBun::computeLineAndColumn(frame)wrapper that does the same on top ofStackFrame::computeLineAndColumn()(error.stack's columns for other frames are unchanged), andpopulateStackFramePositiontakes the excerpt lines from the class source's provider so they match the position it just computed.JSCStackFrame::sourceID()(CallSitegetScriptId()and the[source:N]placeholder) reports the class's provider for the same reason.at new E (file:2:1),export classreports theclasskeyword, a class expression reports itsclasskeyword), and giving the frame a real URL and position is also what letsBun__remapStackFramePositionssource-map it for transpiled files, so TypeScript classes now report their original line.Bun__CallFrame__getCallerSrcLoc,Bun__CallFrame__getLineNumberandInspectorTestReporterAgent::reportTestFoundstill pairZig::sourceURL(visitor)withvisitor->computeLineAndColumn(). They locate the JS caller of a non-constructor native function, which a synthesized constructor (whose body is onlysuper(...args)) can never be, so only their URL half is affected, and it now agrees with the other renderers. The arrow header node:vm prepends to an error escaping a vm script (NodeVM.cpp, JSC's ownsourceURL()/computeLineAndColumn()on the top frame) still shows the template source when that top frame is a default constructor; several open node:vm PRs are editing that function, so it is left for a follow-up. The frames in that error's.stackare fixed by this PR.getAdjustedLineColumnForBytecode), node:vm: apply negative lineOffset/columnOffset to stack frames and the error header #38248 (negative vm offsets for the frames JSC positions itself), node:vm: don't remap vm code through the source map of the file its filename names #38344 (vm sources and source maps). This PR is deliberately not stacked on them: it is a different bug and can land in any order. Its position rule is one early return at the top ofgetAdjustedPositionForBytecodeplus theBun::computeLineAndColumnwrapper for error.stack, and whichever of error.stack: report frames at new X(...) at the new keyword #37396 / node:vm: apply negative lineOffset/columnOffset to stack frames and the error header #38248 lands second adds that same early return to its per-frame position function and drops the wrapper (a few lines; the error.stack tests here fail if it is missed). node:vm: apply negative lineOffset/columnOffset to stack frames and the error header #38248'sapplyNegativeSourceStartmust not run on these frames and does not need to: the class position already includes both offsets, which is what the negative cases in the vm test pin down.bun bd test test/js/bun/test/stack.test.ts(new: error.stack for a// @bunfile with derived / base-with-fields / class in a function / class on a function's first line / class expression,err.sourceURL+err.lineforextends nullon top; the same through a source map for a.tsfile; the uncaught printer's frame line and caret;vm.Scriptwith positive and negativelineOffset/columnOffset) andbun bd test test/js/node/v8/capture-stack-trace.test.js(new: CallSitegetFileName/getLineNumber/getColumnNumber/getScriptId/isConstructorfor a top-level and an in-function class; the expected file and line come from a frame captured in each class'sextendsclause, the expected column from theclasskeyword's offset in that line). All five new tests fail on the released binary with the output quoted above.inspect-error.test.js,reportError.test.ts,vm.test.ts,vm-sourceUrl.test.ts,internal-sourcemap.test.ts,coverage.test.ts. The only failures (inspect-error"Error inside minified file",error-gc-test,diffexample"no color") fail identically on main with a debug build and are tracked separately.Background
class E extends B {}JSC does not parse a constructor;BytecodeGenerator::emitNewDefaultConstructorcreates anUnlinkedFunctionExecutablefrom a fixed source string (BuiltinExecutables.cpp), marks itisBuiltinDefaultClassConstructor, and stores the class's own source range on it viasetClassSource(). It is a normal (not builtin-visibility) function, so it shows up in stacks like any user function.SourceCode/SourceProvider: a provider holds one file's text and URL; aSourceCodeis a range in it (startOffset,firstLine,startColumn).classSource()is the range from theclasskeyword to the closing brace; it is also whatFunction.prototype.toStringprints for a class.lineOffset/columnOffset: stored on the script's SourceProvider asstartPosition(). JSC adds a positive line offset to every line and a positive column offset to the first line when it reports positions; V8 does the same but also honors negative offsets.ZigStackFramePosition: zero-based line/column plus a byte offset into the provider's text, shared with the Rust error printer; the offset is what the printer's source excerpt is cut around, which is why the excerpt provider has to match it.formatStackTracebuilds theerror.stackstring (and setserr.line/err.sourceURL),JSCStackFrame/CallSitebackError.prepareStackTrace, andZigExceptionfeeds the uncaught error /Bun.inspectprinter. All three hand positions toBun__remapStackFramePositions, which applies the transpiler's source map when the frame has a URL.