bundler: label conditional CSS @import records as import-rule

src/ast/lib.rs

@@ -79,8 +79,8 @@ pub enum ImportKind {
 // arms inside label()/error_label() below — zero runtime init (PORTING.md §Concurrency: prefer no-lock over OnceLock
 // when the data is pure const).
 //
-// If these are changed, make sure to update
-// - src/js/builtins/codegen/replacements.ts
+// If the variants or the labels below change, make sure to update
+// - src/codegen/replacements.ts (`enums.ImportKind`, indexed by discriminant)
 // - packages/bun-types/bun.d.ts
 
 impl ImportKind {
@@ -94,7 +94,7 @@ impl ImportKind {
             ImportKind::Dynamic => b"dynamic-import",
             ImportKind::RequireResolve => b"require-resolve",
             ImportKind::At => b"import-rule",
-            ImportKind::AtConditional => b"",
+            ImportKind::AtConditional => b"import-rule",
             ImportKind::Url => b"url-token",
             ImportKind::Composes => b"composes",
             ImportKind::Internal => b"internal",
@@ -112,7 +112,7 @@ impl ImportKind {
             ImportKind::Dynamic => b"import()",
             ImportKind::RequireResolve => b"require.resolve()",
             ImportKind::At => b"@import",
-            ImportKind::AtConditional => b"",
+            ImportKind::AtConditional => b"@import",
             ImportKind::Url => b"url()",
             ImportKind::Internal => b"<bun internal>",
             ImportKind::Composes => b"composes",

src/codegen/replacements.ts

@@ -117,16 +117,20 @@ export const enums = {
     "md",
     "xml",
   ],
+  // Ids are the `bun_ast::ImportKind` discriminants; labels are `ImportKind::label()`.
   ImportKind: [
-    "entry-point-run",
-    "entry-point-build",
-    "import-statement",
-    "require-call",
-    "dynamic-import",
-    "require-resolve",
-    "import-rule",
-    "url-token",
-    "internal",
+    "entry-point-run", // EntryPointRun
+    "entry-point-build", // EntryPointBuild
+    "import-statement", // Stmt
+    "require-call", // Require
+    "dynamic-import", // Dynamic
+    "require-resolve", // RequireResolve
+    "import-rule", // At
+    "import-rule", // AtConditional
+    "url-token", // Url
+    "composes", // Composes
+    "html_manifest", // HtmlManifest
+    "internal", // Internal
   ],
 };
 

src/jsc/ResolveMessage.rs

@@ -35,27 +35,6 @@ impl Default for ResolveMessage {
     }
 }
 
-/// `ImportKind.label()` — the canonical table lives in
-/// `bun_ast::ImportKind::label`, but
-/// `bun_ast::MetadataResolve.import_kind` is the type-only `bun_ast::ImportKind`.
-/// Replicate the table here verbatim.
-fn import_kind_label(kind: ImportKind) -> &'static [u8] {
-    match kind {
-        ImportKind::EntryPointRun => b"entry-point-run",
-        ImportKind::EntryPointBuild => b"entry-point-build",
-        ImportKind::Stmt => b"import-statement",
-        ImportKind::Require => b"require-call",
-        ImportKind::Dynamic => b"dynamic-import",
-        ImportKind::RequireResolve => b"require-resolve",
-        ImportKind::At => b"import-rule",
-        ImportKind::AtConditional => b"",
-        ImportKind::Url => b"url-token",
-        ImportKind::Composes => b"composes",
-        ImportKind::Internal => b"internal",
-        ImportKind::HtmlManifest => b"html_manifest",
-    }
-}
-
 /// Host-agnostic bare-specifier check for Node ESM error shaping. Must not vary by host:
 /// relative, separator-led, and ASCII-letter drive forms are path-like; everything else is a
 /// package. Unlike `bun_paths::is_absolute`, the drive byte must be alphabetic.
@@ -479,7 +458,7 @@ impl ResolveMessage {
     pub fn get_import_kind(this: &Self, global: &JSGlobalObject) -> JsResult<JSValue> {
         Ok(match &this.msg.metadata {
             bun_ast::Metadata::Resolve(resolve) => {
-                ZigString::init(import_kind_label(resolve.import_kind)).to_js(global)
+                ZigString::init(resolve.import_kind.label()).to_js(global)
             }
             _ => ZigString::init(b"").to_js(global),
         })

test/bundler/bun-build-api.test.ts

@@ -214,6 +214,53 @@ describe("Bun.build", () => {
     }
   });
 
+  test("css @import resolve errors report importKind 'import-rule' with and without import conditions", async () => {
+    using dir = tempDir("bun-build-api-css-import-kind", {
+      "entry.css": `
+        @import "./missing-plain.css";
+        @import "./missing-conditional.css" supports(display: grid);
+        @import "bun:sqlite";
+        @import "bun:sqlite" supports(display: grid);
+      `,
+    });
+
+    const build = await Bun.build({
+      entrypoints: [join(String(dir), "entry.css")],
+      target: "browser",
+      throw: false,
+    });
+
+    expect(build.success).toBe(false);
+    for (const log of build.logs) expect(log).toBeInstanceOf(ResolveMessage);
+    const logs = (build.logs as ResolveMessage[]).toSorted((a, b) => a.position!.line - b.position!.line);
+    expect(logs.map(({ specifier, importKind, message }) => ({ specifier, importKind, message }))).toEqual([
+      {
+        specifier: "./missing-plain.css",
+        importKind: "import-rule",
+        message: 'Could not resolve: "./missing-plain.css"',
+      },
+      {
+        specifier: "./missing-conditional.css",
+        importKind: "import-rule",
+        message: 'Could not resolve: "./missing-conditional.css"',
+      },
+      {
+        specifier: "bun:sqlite",
+        importKind: "import-rule",
+        message: `Browser build cannot @import Bun builtin: "bun:sqlite". When bundling for Bun, set target to 'bun'`,
+      },
+      {
+        specifier: "bun:sqlite",
+        importKind: "import-rule",
+        message: `Browser build cannot @import Bun builtin: "bun:sqlite". When bundling for Bun, set target to 'bun'`,
+      },
+    ]);
+    expect(JSON.parse(JSON.stringify(logs[1]))).toMatchObject({
+      specifier: "./missing-conditional.css",
+      importKind: "import-rule",
+    });
+  });
+
   test("returns output files", async () => {
     Bun.gc(true);
     const build = await Bun.build({

test/bundler/bundler_plugin.test.ts

@@ -544,6 +544,56 @@ describe("bundler", () => {
       },
     };
   });
+  // Every import kind an onResolve callback can receive. Not reachable from a plugin: entry-point-run
+  // (runtime only), html_manifest (the record is retagged after resolution) and internal (never created).
+  itBundled("plugin/ResolveKindForEveryImportKind", () => {
+    const resolved: [string, string][] = [];
+    return {
+      files: {
+        "entry.js": /* js */ `
+          import "./styles.css";
+          require("./required.js");
+          import("./dynamic.js");
+          console.log(require.resolve("./resolved.js"));
+        `,
+        "required.js": `module.exports = 1;`,
+        "dynamic.js": `export const dynamic = 1;`,
+        "resolved.js": `export const resolved = 1;`,
+        "styles.css": /* css */ `
+          @import "./plain.css";
+          @import "./conditional.css" supports(display: grid);
+          @import "./a.module.css";
+          .bg { background: url("./image.png"); }
+        `,
+        "plain.css": `.plain { color: blue; }`,
+        "conditional.css": `.conditional { display: grid; }`,
+        "a.module.css": `.a { composes: b from "./b.module.css"; padding: 1px; }`,
+        "b.module.css": `.b { color: green; }`,
+        "image.png": "not really a png",
+      },
+      outdir: "/out",
+      plugins(builder) {
+        builder.onResolve({ filter: /.*/ }, args => {
+          resolved.push([path.basename(args.path), args.kind]);
+          return undefined;
+        });
+      },
+      onAfterBundle() {
+        expect(resolved.sort()).toEqual([
+          ["a.module.css", "import-rule"],
+          ["b.module.css", "composes"],
+          ["conditional.css", "import-rule"],
+          ["dynamic.js", "dynamic-import"],
+          ["entry.js", "entry-point-build"],
+          ["image.png", "url-token"],
+          ["plain.css", "import-rule"],
+          ["required.js", "require-call"],
+          ["resolved.js", "require-resolve"],
+          ["styles.css", "import-statement"],
+        ]);
+      },
+    };
+  });
   itBundled("plugin/ManyFiles", ({ root }) => {
     const FILES = process.platform === "win32" ? 50 : 200; // windows is slower at this
     const create = (fn: (i: number) => string) => new Array(FILES).fill(0).map((_, i) => fn(i));

test/bundler/metafile.test.ts

@@ -458,6 +458,40 @@ describe("bundler metafile", () => {
     expect(outputPaths).toContain(dynamicImport!.path);
   });
 
+  test("metafile tracks css @import imports as import-rule, with or without import conditions", async () => {
+    using dir = tempDir("metafile-css-import-rule-test", {
+      // Reached through a JS entry because a CSS entrypoint currently contributes nothing to metafile.inputs.
+      "entry.js": `import "./styles.css";`,
+      "styles.css": `
+        @import "./plain.css";
+        @import "./conditional.css" supports(display: grid);
+        @import "./media.css" screen;
+        @import "./layered.css" layer(base) supports(display: grid) screen;
+        .foo { color: red; }
+      `,
+      "plain.css": `.plain { color: blue; }`,
+      "conditional.css": `.conditional { display: grid; }`,
+      "media.css": `.media { color: green; }`,
+      "layered.css": `.layered { color: black; }`,
+    });
+
+    const result = await Bun.build({
+      entrypoints: [`${dir}/entry.js`],
+      metafile: true,
+    });
+
+    expect(result.success).toBe(true);
+    const inputs = (result.metafile as Metafile).inputs;
+    const stylesKey = Object.keys(inputs).find(path => path.endsWith("styles.css"))!;
+    expect(stylesKey).toBeDefined();
+    expect(inputs[stylesKey].imports.map(({ kind, original }) => ({ kind, original }))).toEqual([
+      { kind: "import-rule", original: "./plain.css" },
+      { kind: "import-rule", original: "./conditional.css" },
+      { kind: "import-rule", original: "./media.css" },
+      { kind: "import-rule", original: "./layered.css" },
+    ]);
+  });
+
   test("metafile includes cssBundle for CSS outputs", async () => {
     using dir = tempDir("metafile-css-bundle-test", {
       "entry.js": `import "./styles.css"; console.log("styled");`,