sourcemap: count an ill-formed UTF-8 lead byte as one byte so the line break after it is not skipped

src/sourcemap/Chunk.rs

@@ -534,17 +534,16 @@ impl NewBuilder<'_, VLQSourceMap> {
         let mut i: usize = 0;
         let n: usize = slice.len();
         let mut c: i32;
+        // An ill-formed sequence is one U+FFFD one byte wide, so a '\n' after it is not skipped.
+        let iter = strings::CodepointIterator::init(slice);
         while i < n {
-            let len = strings::wtf8_byte_sequence_length_with_invalid(slice[i]);
-            let mut cp_bytes = [0u8; 4];
-            let take = (len as usize).min(n - i);
-            cp_bytes[..take].copy_from_slice(&slice[i..i + take]);
-            c = strings::decode_wtf8_rune_t::<i32>(
-                cp_bytes,
-                len,
-                strings::UNICODE_REPLACEMENT as i32,
-            );
-            i += len as usize;
+            let mut cursor = strings::Cursor {
+                i: i as u32,
+                ..Default::default()
+            };
+            let _ = iter.next(&mut cursor);
+            c = cursor.c;
+            i += cursor.width as usize;
 
             match c {
                 14..=127 => {

src/sourcemap/LineOffsetTable.rs

@@ -186,27 +186,18 @@ impl LineOffsetTable {
         // Hoist the base pointer so per-iteration offset math is a single sub + truncate.
         let base = contents.as_ptr() as usize;
 
+        // Same decoder as `Chunk.rs`: an ill-formed sequence is one U+FFFD one byte wide.
+        let iter = strings::CodepointIterator::init(contents);
         let mut remaining = contents;
         while !remaining.is_empty() {
-            let b0 = remaining[0];
-            let len_ = strings::wtf8_byte_sequence_length_with_invalid(b0);
-            // After the SIMD skip below lands, the loop head
-            // is overwhelmingly an ASCII '\r'/'\n' or a non-ASCII lead byte, so keep the
-            // 1-byte path branch-only and confine the zero+copy pad to the cold
-            // multibyte arm.
-            // `len_` is the lead byte's *declared* width; a source whose final bytes are
-            // a truncated multibyte sequence declares more bytes than remain, so every
-            // slice below (decode, SIMD-skip offset, advance) must use the clamped width.
-            let cp_len = (len_ as usize).min(remaining.len());
-            let c: i32 = if len_ == 1 {
-                b0 as i32
-            } else {
-                let mut cp_bytes = [0u8; 4];
-                cp_bytes[..cp_len].copy_from_slice(&remaining[..cp_len]);
-                strings::decode_wtf8_rune_t::<i32>(cp_bytes, len_, 0)
-            };
-
             let offset = (remaining.as_ptr() as usize - base) as u32;
+            let mut cursor = strings::Cursor {
+                i: offset,
+                ..Default::default()
+            };
+            let _ = iter.next(&mut cursor);
+            let c: i32 = cursor.c;
+            let cp_len = cursor.width as usize;
 
             if column == 0 {
                 line_byte_offset = offset;

test/js/bun/sourcemap/internal-sourcemap-roundtrip.test.ts

@@ -1,6 +1,7 @@
 import { internalSourceMap } from "bun:internal-for-testing";
 import { describe, expect, test } from "bun:test";
 import { bunEnv, bunExe, tempDir } from "harness";
+import { readFileSync } from "node:fs";
 import path from "node:path";
 
 // --- Reference VLQ codec (mirrors the spec; used as the oracle) ---
@@ -491,6 +492,90 @@ describe.concurrent("sourcemap of a source with a truncated trailing UTF-8 seque
   });
 });
 
+// Both sides of a mapping come from walking raw bytes: LineOffsetTable::generate
+// walks the source and the builder in Chunk.rs walks the printed output. A lead
+// byte that is not followed by the continuation bytes it declares (a Latin-1
+// file, a sequence cut short) is one character, one byte wide. Stepping over the
+// declared width instead swallows the line break after it, and every mapping
+// from there on is a line off.
+describe.concurrent("sourcemap of a source with an ill-formed UTF-8 sequence right before a line break", () => {
+  // File contents are given as latin1 text, so "\xe9" is the one byte 0xE9 on
+  // disk; the output is read back the same way. Lines are 0-based like the map's.
+  async function bundle(latin1Files: Record<string, string>, entry: string) {
+    using dir = tempDir(
+      "sourcemap-ill-formed-utf8",
+      Object.fromEntries(Object.entries(latin1Files).map(([name, text]) => [name, Buffer.from(text, "latin1")])),
+    );
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "build", "--sourcemap=external", "--outdir=out", entry],
+      env: bunEnv,
+      cwd: String(dir),
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    expect({ exitCode, stderr }).toEqual({ exitCode: 0, stderr: "" });
+    const outfile = path.join(String(dir), "out", entry);
+    const outputLines = readFileSync(outfile, "latin1").split("\n");
+    const map = JSON.parse(readFileSync(outfile + ".map", "utf8"));
+    const segs = decodeMappings(map.mappings);
+    // For each statement: the mapping at the start of the output line it was
+    // printed on, i.e. what a stack trace through that line would be remapped to.
+    return (statements: string[]) =>
+      statements.map(statement => {
+        const generatedLine = outputLines.findIndex(line => line.includes(statement));
+        expect(generatedLine).toBeGreaterThanOrEqual(0);
+        const seg = referenceFind(segs, generatedLine, 0);
+        return { statement, mapped: seg && { source: map.sources[seg.srcIdx], line: seg.origLine } };
+      });
+  }
+
+  const illFormed = [
+    ["Latin-1 0xE9, a 3-byte lead with no continuation bytes", "\xe9"],
+    ["0xC3, a 2-byte lead with no continuation byte", "\xc3"],
+    ["0xF0 0x9F 0x92, three bytes of a 4-byte sequence", "\xf0\x9f\x92"],
+  ];
+
+  // Source lines 0-1 are the comment, `first` is on line 2 and `second` on line
+  // 5. The output drops the blank lines between them, so the two walks cannot
+  // each be a line off and still agree with each other by accident.
+  const rest = '\nb */\nconsole.log("first");\n\n\nconsole.log("second");\n';
+  const statements = ['console.log("first")', 'console.log("second")'];
+  const expected = [
+    { statement: 'console.log("first")', mapped: { source: "../in.js", line: 2 } },
+    { statement: 'console.log("second")', mapped: { source: "../in.js", line: 5 } },
+  ];
+
+  // A legal comment is copied into the output, so both walks see the bytes.
+  test.each(illFormed)("in a legal comment: %s", async (_name, bytes) => {
+    const mappingsOf = await bundle({ "in.js": "/*! caf" + bytes + rest }, "in.js");
+    expect(mappingsOf(statements)).toEqual(expected);
+  });
+
+  // A plain comment is dropped from the output, so only the source walk sees them.
+  test.each(illFormed)("in a plain comment: %s", async (_name, bytes) => {
+    const mappingsOf = await bundle({ "in.js": "/* caf" + bytes + rest }, "in.js");
+    expect(mappingsOf(statements)).toEqual(expected);
+  });
+
+  // In a bundle the files' mappings are chained: each file's start where the
+  // line count of the file printed before it left off, so a file that counts
+  // one of its own line breaks short also pulls every file after it up a line.
+  test("in a legal comment of a file printed ahead of the entry point", async () => {
+    const mappingsOf = await bundle(
+      {
+        "in.js": 'import "./dep.js";\nconsole.log("entry");\n',
+        "dep.js": '/*! caf\xe9\nb */\nconsole.log("dep");\n',
+      },
+      "in.js",
+    );
+    expect(mappingsOf(['console.log("dep")', 'console.log("entry")'])).toEqual([
+      { statement: 'console.log("dep")', mapped: { source: "../dep.js", line: 2 } },
+      { statement: 'console.log("entry")', mapped: { source: "../in.js", line: 1 } },
+    ]);
+  });
+});
+
 // `sources` entries in the emitted map are URLs: the path from the chunk
 // directory to each source must use forward slashes on every platform, never
 // the host path separator.

test/js/bun/sourcemap/internal-sourcemap.test.ts

@@ -55,7 +55,7 @@ function extractPositions(stack: string): string[] {
     .filter((s): s is string => s !== null);
 }
 
-async function run(files: Record<string, string>, env: Record<string, string> = {}) {
+async function run(files: Record<string, string | Buffer>, env: Record<string, string> = {}) {
   using dir = tempDir("internal-sourcemap", files);
   await using proc = Bun.spawn({
     cmd: [bunExe(), "index.ts"],
@@ -227,4 +227,49 @@ describe("InternalSourceMap", () => {
     }
     expect(exited).toBe(0);
   });
+
+  // Both line tables behind a remapped frame are built by walking raw bytes: the
+  // source's by LineOffsetTable::generate, the transpiled output's by the builder.
+  // A lead byte without the continuation bytes it declares (the file is Latin-1,
+  // not UTF-8) is one character, one byte wide; stepping over the declared width
+  // instead swallows the line break after it and every frame below comes out a
+  // line early. The fixtures are written to disk as latin1, so "\xe9" is the one
+  // byte 0xE9.
+  describe.concurrent("an ill-formed UTF-8 byte right before a line break", () => {
+    async function framePositions(latin1Source: string) {
+      const { stdout, stderr, exited } = await run({ "index.ts": Buffer.from(latin1Source, "latin1") });
+      expect(stderr).toBe("");
+      expect(exited).toBe(0);
+      return [...stdout.matchAll(/index\.ts:(\d+:\d+)/g)].map(m => m[1]);
+    }
+
+    test("in a comment that is dropped from the output", async () => {
+      // Lines: 1 "/* caf<E9>", 2 "b */", 3 the Error, 4 console.log.
+      const positions = await framePositions(
+        '/* caf\xe9\nb */\nconst err = new Error("x");\nconsole.log(err.stack);\n',
+      );
+      expect(positions).toEqual(["3:17"]);
+    });
+
+    test("in a legal comment that is kept in the output", async () => {
+      // Lines: 1 "/*! caf<E9>", 2 "b */", 3 the Error, 6 console.log. The output
+      // has no blank lines, so the walks over the source and over the output
+      // cannot each be a line off and still produce the right answer together.
+      const positions = await framePositions(
+        '/*! caf\xe9\nb */\nconst err = new Error("x");\n\n\nconsole.log(err.stack);\n',
+      );
+      expect(positions).toEqual(["3:17"]);
+    });
+
+    test("in a template literal, next to a U+FFFD the printer wrote out for another one", async () => {
+      // Line 1 breaks inside the template literal right after the ill-formed
+      // byte. The string on line 3 is printed with a real U+FFFD (three bytes,
+      // one column) ahead of the Error on the same line, which the walk over
+      // the output must not mistake for an ill-formed byte either.
+      const positions = await framePositions(
+        'const s = `caf\xe9\n`;\nconst err = ["caf\xe9", new Error("x")][1];\nconsole.log(err.stack);\n',
+      );
+      expect(positions).toEqual(["3:26"]);
+    });
+  });
 });