diff --git a/src/common/wasi/wasishim.ts b/src/common/wasi/wasishim.ts
index 6b03498d..460233f5 100644
--- a/src/common/wasi/wasishim.ts
+++ b/src/common/wasi/wasishim.ts
@@ -440,9 +440,9 @@ export class WASIRunner {
                 } else return WASIErrors.INVAL;
             }
         } else {
-            if (file == null) return WASIErrors.NOSYS;
+            if (file == null) return WASIErrors.NOENT;
             if (o_flags & FDOpenFlags.DIRECTORY) { // check type
-                if (file.type !== FDType.DIRECTORY) return WASIErrors.NOSYS;
+                if (file.type !== FDType.DIRECTORY) return WASIErrors.NOTDIR;
             }
             if (o_flags & FDOpenFlags.EXCL) return WASIErrors.INVAL; // already exists
             if (o_flags & FDOpenFlags.TRUNC) { // truncate
diff --git a/src/worker/tools/urust.ts b/src/worker/tools/urust.ts
index c9e24635..a30aa35b 100644
--- a/src/worker/tools/urust.ts
+++ b/src/worker/tools/urust.ts
@@ -27,8 +27,9 @@ export function compileUrust(step: BuildStep): BuildStepResult {
     wasi.initSync(wasiModule);
     populateWASIFiles(wasi, step);
     const args = ['urust', 'build', step.path, '--platform', platform, '--emit', 'bin', '-o', binpath,
-        '-Ob', '-fno-bounds-checks',
+        '-O1', '-fno-bounds-checks',
         '-Wall',
+        '--auto-mod',
         '--debug-out', dbgpath, '--error-format', 'json'];
     args.push.apply(args, extraArgsFor('urust', step.params && step.params.buildArgs));
     wasi.setArgs(args);
diff --git a/test/unit/testurust.ts b/test/unit/testurust.ts
index cbfa303b..563fb15c 100644
--- a/test/unit/testurust.ts
+++ b/test/unit/testurust.ts
@@ -54,7 +54,7 @@ describe('urust tool', function () {
     assert.strictEqual(urustPlatformName('c64'), 'c64');
     assert.strictEqual(urustPlatformName('atari8-800xl'), 'atari8');
     assert.strictEqual(urustPlatformName('vcs'), 'vcs');
-    assert.strictEqual(urustPlatformName('apple2'), undefined);
+    assert.strictEqual(urustPlatformName('apple2'), 'apple2');
   });
 
   it('parses JSON diagnostics; end column only on a single line', function () {
diff --git a/test/unit/testwasishim.ts b/test/unit/testwasishim.ts
index 25983e7e..9d4d24f8 100644
--- a/test/unit/testwasishim.ts
+++ b/test/unit/testwasishim.ts
@@ -1,5 +1,5 @@
 import assert from "assert";
-import { WASIRunner, WASIMemoryFilesystem, WASIErrors, normalizeWASIPath } from "../../src/common/wasi/wasishim";
+import { WASIRunner, WASIMemoryFilesystem, WASIErrors, FDOpenFlags, normalizeWASIPath } from "../../src/common/wasi/wasishim";
 import * as fs from "fs";
 import { loadWASIFilesystemZip, unzipWASIFilesystem } from "../../src/worker/wasiutils";
 import { parseOscar64Listing, parseOscar64Lbl, parseOscar64Map } from "../../src/worker/tools/oscar64parse";
@@ -168,6 +168,12 @@ describe('WASIMemoryFilesystem path handling', function () {
         assert.strictEqual(normalizeWASIPath('.'), '.');
         assert.strictEqual(normalizeWASIPath('/'), '/');
     });
+    it('reports a missing file as NOENT, not NOSYS', function () {
+        const wasi = new WASIRunner();
+        assert.strictEqual(wasi.openFile('./missing.bin', 0), WASIErrors.NOENT);
+        wasi.fs.putFile('./plain.bin', 'x');
+        assert.strictEqual(wasi.openFile('./plain.bin', FDOpenFlags.DIRECTORY), WASIErrors.NOTDIR);
+    });
     it('finds files written with a different leading ./ depth', function () {
         const fsys = new WASIMemoryFilesystem();
         fsys.putFile('././main.map', 'map');