fix(register): always inline source maps in auto mode so debuggers work (#1062)

The auto source-map mode introduced in 1.12.0 only inlined the map into
the emitted code when `process.sourceMapsEnabled` was true (i.e. Node was
started with `--enable-source-maps`). Otherwise it fell back to store-only
mode, which populates SourcemapMap for `@swc-node/sourcemap-support` to
rewrite `Error.stack`, but emits no inline `//# sourceMappingURL=`.

The V8 inspector reads that inline map off the running code to bind
breakpoints, and it does so regardless of `--enable-source-maps`. With no
inline map, debuggers (VS Code, `node --inspect`) stopped binding
breakpoints or landed on the wrong line. This is the 1.12.0 regression in
#1059.

Auto mode now always inlines the map, and additionally stores it only when
native source maps are off (so stack traces stay correct without retaining
a duplicate map when Node already handles them). This restores the 1.11.1
behavior of always emitting an inline map. The `inline`/`store`/`none`
env modes are unchanged.

Closes #1059


Claude-Session: https://claude.ai/code/session_01LXSDomd3mGu4SL4TJ9q6T2

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Brooooooklyn

packages/register/__test__/register-runtime-tuning.spec.ts

@@ -124,6 +124,37 @@ test.serial('supports sourcemap store-only mode to avoid inline map payload', (t
   t.true(SourcemapMap.has(filename))
 })
 
+test.serial('auto source map mode inlines the map so debuggers can bind breakpoints', (t) => {
+  // Regression guard for https://github.com/swc-project/swc-node/issues/1059.
+  // In auto mode (no SWC_NODE_SOURCE_MAP_MODE) the emitted code must carry an
+  // inline sourceMappingURL even when process.sourceMapsEnabled is false. The V8
+  // inspector reads that inline map to bind breakpoints, and it does so
+  // regardless of --enable-source-maps. Dropping the inline map here is what
+  // broke the debugger in 1.12.0.
+  delete process.env.SWC_NODE_SOURCE_MAP_MODE
+
+  const previousSourceMaps = process.sourceMapsEnabled
+  process.setSourceMapsEnabled(false)
+  t.teardown(() => process.setSourceMapsEnabled(previousSourceMaps))
+
+  sinon.stub(swcCore, 'transformSync').returns({
+    code: 'console.log("auto-inline")',
+    map: emptyMap,
+  })
+
+  const filename = uniquePath('sourcemap-auto', 'ts')
+  const output = compile('const x = 1', filename, {
+    module: ts.ModuleKind.CommonJS,
+    sourceMap: true,
+  })
+
+  // Inline map present -> the inspector can map breakpoints back to source.
+  t.true(output.includes('sourceMappingURL'))
+  // Stored map present -> sourcemap-support keeps Error.stack correct when native
+  // source maps are off.
+  t.true(SourcemapMap.has(filename))
+})
+
 test.serial('skips transform for plain js in commonjs mode', (t) => {
   const transformSyncStub = sinon.stub(swcCore, 'transformSync')
 

packages/register/read-default-tsconfig.ts

@@ -28,9 +28,17 @@ export function getSourceMapMode(): { inline: boolean; store: boolean } {
       return { inline: false, store: false }
   }
 
-  // In auto mode, follow runtime capability: native source maps favor inline,
-  // non-native stacks favor store mode.
-  return process.sourceMapsEnabled ? { inline: true, store: false } : { inline: false, store: true }
+  // In auto mode, always inline the map: the V8 inspector reads the inline
+  // sourceMappingURL off the running code to bind breakpoints, and it does so
+  // regardless of `process.sourceMapsEnabled` (which only governs how Node
+  // rewrites Error.stack). Gating inline emission on that flag broke debuggers
+  // in 1.12.0 (https://github.com/swc-project/swc-node/issues/1059).
+  //
+  // Additionally store the map when native source maps are off so
+  // @swc-node/sourcemap-support keeps Error.stack line numbers correct; when
+  // native maps are on, Node handles stack traces and storing would just retain
+  // a duplicate map.
+  return process.sourceMapsEnabled ? { inline: true, store: false } : { inline: true, store: true }
 }
 
 export function readDefaultTsConfig(