bundler: fix source map names, closing-brace mappings, null segments, and URL escaping by robobun · Pull Request #40834 · oven-sh/bun

robobun · 2026-08-28T17:35:56Z

Problem

bun build --sourcemap wrote "names": [] for every map. add_source_mapping_for_name (src/js_printer/lib.rs) ignored its name and ref arguments and the chunk builder had no names table, so --minify --sourcemap maps could not un-minify locals in a debugger.
The } that closes a function, arrow, class static block, try, catch, finally, else, or switch body had no mapping. Its line mapped to the last token inside the body (gen 5:0 -> src 3:2 for the } of a function whose body ends with an object literal). G::FnBody, Catch, Finally, S::Try, S::Switch, and ClassStaticBlock carried no close_brace_loc.
A file that contributes code but no mappings (a file or text loader shim) got no segment at all, so the previous file's last mapping covered its code on a minified line. And //# sourceMappingURL= plus sources entries were raw file names: a space was written as a space.

Fix

The chunk builder (src/sourcemap/Chunk.rs) records the original name of a renamed symbol as the fifth VLQ field and collects a chunk-local, JSON-quoted names table. The linker (generate_source_map_for_chunk) offsets each chunk's name indices by the names of the chunks before it, rewrites the first name field through Chunk::first_name_offset, and writes the joined names array. The printer passes the original name from every symbol print site that esbuild does (bindings, identifiers, import and export names, labels, private names, function and class names).
The parser records close_brace_loc for the six node kinds above and the printer passes it to print_block, which already mapped the } of plain blocks.
postProcessJSChunk and postProcessCSSChunk add a null entry for a printed file with code but no chunk. The linker emits a 1-field segment for it (append_null_source_map_segment). SourceMapPieces::finalize accepts 1-field segments.
sourceMappingURL and sources go through append_url_escaped_path, the same character set as esbuild (Go's EscapedPath). A space is %20 in the comment and stays literal in sources, as in esbuild.
The runtime transpiler emits the same } mappings, so bun test --coverage attributes the bytes after a function's } to the } line. That exposed an off-by-one in CodeCoverage.rs: a never-executed function marked its lines min..max exclusive and skipped the first byte of each line, so its last statement line (and a } at column zero) was never reported. Multi-line functions now mark through their }. Coverage numbers move a little (see Notes).
Verified: test/bundler/bundler_sourcemap.test.ts (5 new tests, all fail on the current release). Also test/bundler/ (esbuild, edgecase, npm, minify, compile, splitting, banner, html, plugin, api), test/js/bun/sourcemap/, test/js/node/module/*sourcemap*, test/bake/dev/sourcemap.test.ts, test/cli/install/bun-pm-diff.test.ts.

Background

A source map mappings segment has 1, 4, or 5 VLQ fields: generated column, then source index, original line, original column, then an optional index into names. Every field is a delta from the previous segment, which is why chunks printed in parallel are rebased when the linker joins them.
Each input file is printed into its own Chunk with its own VLQ buffer. append_source_map_chunk rewrites the first segment of each chunk so it is relative to the end state of the previous one. Name indices get the same treatment now.
A 1-field segment maps generated code to no original position. Readers stop extending the previous mapping there.
record_names is on only for the bundler's output path. The dev server joins chunk buffers under a fixed "names":[] and the runtime's internal format stores positions only, so both keep the old behavior.

Notes

Repros (before / after, debug build):

// smnames.js:  export function fn(firstArgument) { return firstArgument * 2 }
bun build smnames.js --minify --sourcemap=external --outdir=o
before: "names": []
after:  "names": ["fn","firstArgument"]; gen 0:9 -> 0:16 name=fn; gen 0:11 -> 0:19 name=firstArgument

// in.js: function whose body ends with an object literal
before: gen 5:0 -> src 3:2   (the object literal's `}`)
after:  gen 5:0 -> src 4:0   (the function's `}`)

// entry.js imports other.js and a text-loader data.txt, --minify
var o=1;var t="hello";console.log(t,o);
before: gen 0:6 -> other.js 0:17, then gen 0:22 -> entry.js (the shim inherits other.js)
after:  gen 0:8 -> (null) between them

bun build "my file.js" --sourcemap=linked --outdir=out
before: //# sourceMappingURL=my file.js.map
after:  //# sourceMappingURL=my%20file.js.map     sources: ["../my file.js"]

Other changes the new output required:

SourceMapPieces::finalize decoded four fields per segment unconditionally and panicked (assertion failed: index != U7_MAX) on a 1-field segment when an asset path placeholder shifted the mappings. It now reads 1, 4, or 5 fields.
Coverage (bun test --coverage). Before: the bytes after a never-executed function's } were attributed to its last statement line (the nearest mapping), so that line showed as executed with 1 hit, and the function loop's exclusive min_line..max_line never marked it either. After: the last statement line is reported as uncovered, and so is the function's } line, which the function range now includes (include-me.ts | 50.00 | 50.00 | 6-8 instead of 66.67 | 6). Executed functions' } lines count as covered lines (LF and LH both grow by one per function). One-line functions keep the old behavior: their line also holds the statement that created them, which ran. Three coverage expectations and two inspect-error stack frame columns (a call frame now maps to the arrow's }, one column closer) are updated.
A labeled statement starts at its label, so the extra add_source_mapping(stmt.loc) before the label's name mapping made the name a duplicate (same location, same generated length). The label mapping is the statement mapping now; sourcemap/NamesAcrossFiles covers a minified label.
bun pm diff folds punctuation-only lines (}) as layout. It did so only for lines with no mapping, which the function } happened to be. It now folds them whether or not they have a position, so its output is unchanged.
test/bundler/expectBundled.ts gains decodeSourceMappings (whole-map decoder with names and 1-field segments) and accepts null mappings in its SourceMapConsumer validation.
Updated expectations: the exact mappings in edgecase/EmitInvalidSourceMap2 (arrow {/} now mapped, react name recorded), the inline map snapshot in bun-serve-html.test.ts (arrow }), and test/regression/issue/22003.test.ts (a tab in a file name is %09 in sources).
The dedup rule in the builder follows esbuild: a mapping at the same location is skipped unless the generated position moved and it carries a different original name.
Names are not shared across chunks (esbuild does the same), so names can repeat an entry for a symbol used in two files. Indices are still correct.
Not covered: shorthand object properties ({ a } where the value was renamed to the key) do not get a name entry. The EIdentifier path covers the non-shorthand form.

Suites run locally on a debug build. Three failures in bundler_compile.test.ts, bun-build-api.test.ts (bytecode) and test/regression/issue/10139.test.ts are 5 s timeouts or a Bun.version vs -debug mismatch that do not involve source maps and reproduce with --sourcemap=none.

no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/regression/issue/22003.test.ts, test/bundler/bundler_edgecase.test.ts

… and URL escaping `bun build --sourcemap` output had four accuracy problems: - `names` was always `[]`. The printer's `add_source_mapping_for_name` ignored its arguments and the chunk builder had no names table. The builder now records the original name of a renamed symbol as the fifth VLQ field, with a chunk-local names table, and the linker rebases the indices when it joins chunks (`Chunk::first_name_offset`, like esbuild). - The `}` that closes a function, arrow, class static block, try, catch, finally, else, or switch body had no mapping, so its line mapped to the last token inside the body. The parser records `close_brace_loc` for those nodes and the printer maps it. - A file that contributes code but no mappings (a `file` or `text` loader shim) let the previous file's last mapping cover its code on a minified line. The linker now emits a 1-field segment where that code starts. - `//# sourceMappingURL=` and `sources` entries were raw file names. They are percent-encoded URL paths now; like esbuild, `sources` keeps a literal space for readability. `SourceMapPieces::finalize` and `bun pm diff` learn to handle the new segments. Tests decode the emitted maps and assert positions and names.

coderabbitai · 2026-08-28T17:36:06Z

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Walkthrough

Changes

The AST records closing-brace locations for functions, static blocks, and control-flow nodes. Printers emit mappings for closing braces and renamed symbols. Source-map assembly supports names, null segments, merged chunks, and escaped URLs. Coverage includes closing-brace offsets.

Source-map and AST changes

Layer / File(s)	Summary
AST location metadata and generated-node initialization `src/ast/`, `src/js_parser/`, `src/react_compiler/`, `src/bundler/`	AST nodes store closing-brace locations. Parsers capture them. Generated nodes initialize empty locations. Cloning preserves the metadata.
Source-map names and chunk joining `src/sourcemap/*`, `src/js_printer/lib.rs`	Source maps record renamed symbols, optional name fields, closing-brace mappings, and merged chunk name metadata.
Bundler source-map assembly `src/bundler/*`	The bundler escapes paths, merges source entries and names, and emits null segments for unmapped generated code.
Coverage and validation updates `src/sourcemap_jsc/CodeCoverage.rs`, `test/bundler/`, `test/cli/test/coverage.test.ts`, `test/js/`, `test/regression/issue/22003.test.ts`	Coverage includes closing-brace offsets. Tests validate source-map names, null mappings, closing-brace mappings, URL escaping, and updated snapshots.

Semantic diff folding

Layer / File(s)	Summary
Punctuation-line folding `src/runtime/cli/pm_diff_semantic.rs`	Punctuation-only and whitespace-only lines are treated as layout before image checks.

Suggested reviewers: jarred-sumner

Merge Risk: ⚪ Minimal · up to c8048

The PR improves source-map names, closing-brace locations, null segments, URL escaping, and related coverage behavior. No actionable merge-blocking risk remains beyond normal checks and review.

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Title check	✅ Passed	The title clearly summarizes the primary changes: source map names, closing-brace mappings, null segments, and URL escaping.
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robobun · 2026-08-28T17:36:14Z

^{Updated 12:41 PM PT - Aug 28th, 2026}

❌ @robobun, your commit c8048d7 has 2 failures in Build #107932 (All Failures):

test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64
test/js/sql/sql-prepare-false.test.ts - code 1: error: Failed to start service postgres_plain (via coordinator): Docker is not available. Please … on 🐧 3.23 x64

🧪 To try this PR locally:

bunx bun-pr 40834

That installs a local version of the PR into your bun-40834 executable, so you can run:

bun-40834 --bun

… by name The `}` that closes a function body now has a source mapping. The runtime's coverage report attributed the bytes after a never-executed function's `}` to the function's last statement line, which made that line look executed. Those bytes now land on the `}` line, and the function's own range has to mark its last line as executable. Both coverage loops in CodeCoverage.rs marked `min_line..max_line`, which left out the last line the function's bytes map to. A function on one line keeps the old behavior because that line also holds the statement that created it. A labeled statement starts at its label, so the extra statement mapping at the same location made the label's name mapping a duplicate. The label mapping is the statement mapping now. Updates the coverage and inspect-error snapshots to the new positions.

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Actionable comments posted: 2

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Inline comments:
In `@src/sourcemap_jsc/CodeCoverage.rs`:
- Around line 781-782: Update the line-start predicates in both scans to use a
strict greater-than comparison instead of greater-than-or-equal:
src/sourcemap_jsc/CodeCoverage.rs lines 781-782 and 918-919. This preserves the
inclusive end_offset scan and includes closing braces that begin at column zero.

In `@src/sourcemap/Chunk.rs`:
- Around line 709-714: Update the duplicate-mapping guard in the relevant Chunk
method so it suppresses a mapping only when the source location, generated
position, and original name all match; preserve mappings when either the
generated position or original name differs.

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src/ast/e.rs
src/ast/expr.rs
src/ast/g.rs
src/ast/nodes.rs
src/ast/s.rs
src/bundler/LinkerContext.rs
src/bundler/ServerComponentParseTask.rs
src/bundler/bundle_v2.rs
src/bundler/lib.rs
src/bundler/linker_context/doStep5.rs
src/bundler/linker_context/generateChunksInParallel.rs
src/bundler/linker_context/generateCodeForFileInChunkJS.rs
src/bundler/linker_context/postProcessCSSChunk.rs
src/bundler/linker_context/postProcessJSChunk.rs
src/bundler/linker_context/writeOutputFilesToDisk.rs
src/js_parser/lower/lower_decorators.rs
src/js_parser/lower/lower_esm_exports_hmr.rs
src/js_parser/p.rs
src/js_parser/parse/parse_fn.rs
src/js_parser/parse/parse_property.rs
src/js_parser/parse/parse_stmt.rs
src/js_parser/repl_transforms.rs
src/js_parser/visit/mod.rs
src/js_printer/lib.rs
src/react_compiler/codegen.rs
src/react_compiler/pipeline.rs
src/react_compiler/program.rs
src/runtime/bake/dev_server/source_map_store.rs
src/runtime/cli/pm_diff_semantic.rs
src/sourcemap/Chunk.rs
src/sourcemap/InternalSourceMap.rs
src/sourcemap/lib.rs
src/sourcemap_jsc/CodeCoverage.rs
test/bundler/bundler_edgecase.test.ts
test/bundler/bundler_sourcemap.test.ts
test/bundler/expectBundled.ts
test/cli/test/coverage.test.ts
test/js/bun/http/bun-serve-html.test.ts
test/js/bun/util/inspect-error.test.js
test/regression/issue/22003.test.ts

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The function-range scans skipped the first byte of every line, so a `}` at column zero never extended a never-executed function's line range and its line kept the hit from the program block that follows the function. The scans now take every byte of the range.

robobun · 2026-08-28T19:30:25Z

Ready for review. CI on c8048d7 (build 107932): 179 of 181 jobs pass. The two red lanes do not involve this diff: alpine x64, where test/js/sql/sql-prepare-false.test.ts could not start its postgres service (Docker unavailable on the runner), and darwin x64, where test/js/web/url/url.test.ts fails on main as well. Both are reported to main-break triage. The tests this PR touches (test/bundler/bundler_sourcemap.test.ts, coverage, inspect-error, the bundler suites) pass on every lane.

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Code review found no issues

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robobun assigned dylan-conway Aug 28, 2026

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