fix: tgpu.unroll removing ranges ending at zero

packages/typegpu/src/tgsl/wgslGenerator.ts

@@ -1728,22 +1728,20 @@ ${this.ctx.pre}else ${alternate}`,
           if (!isKnownAtComptime(range.end)) {
             throw new Error('Cannot unroll loop. Length of iterable is unknown at comptime.');
           }
-
-          const length = range.end.value as number;
-          if (length === 0) {
-            return { code: '', definesInNearestScope: false };
-          }
-
           const { value } = iterableSnippet;
 
           const elements = isTgpuRange(value)
             ? value.map((i) => coerceToSnippet(i))
             : value instanceof ArrayExpression
               ? value.elements
-              : Array.from({ length }, (_, i) =>
+              : Array.from({ length: range.end.value as number }, (_, i) =>
                   forOfUtils.getElementSnippet(iterableSnippet, snip(i, u32, 'constant')),
                 );
 
+          if (elements.length === 0) {
+            return { code: '', definesInNearestScope: false };
+          }
+
           const firstElement = elements[0] as Snippet;
           if (!isAlias(firstElement) && !wgsl.isNaturallyEphemeral(firstElement.dataType)) {
             throw new WgslTypeError(

packages/typegpu/tests/tgsl/wgslGenerator.test.ts

@@ -2347,4 +2347,49 @@ describe('WgslGenerator', () => {
         `);
     });
   });
+
+  it('unrolls a descending range ending at zero', () => {
+    const main = () => {
+      'use gpu';
+      let sum = 0;
+      for (const i of tgpu.unroll(std.range(3, 0, -1))) {
+        sum += i;
+      }
+      return sum;
+    };
+
+    expect(main()).toBe(6);
+    expect(tgpu.resolve([main])).toMatchInlineSnapshot(`
+      "fn main() -> i32 {
+        var sum = 0;
+        // unrolled iteration #0
+        sum += 3i;
+        // unrolled iteration #1
+        sum += 2i;
+        // unrolled iteration #2
+        sum += 1i;
+        // ---
+        return sum;
+      }"
+    `);
+  });
+
+  it('unrolls an empty range with a nonzero endpoint', () => {
+    const main = () => {
+      'use gpu';
+      let sum = 0;
+      for (const i of tgpu.unroll(std.range(3, 3))) {
+        sum += i;
+      }
+      return sum;
+    };
+
+    expect(main()).toBe(0);
+    expect(tgpu.resolve([main])).toMatchInlineSnapshot(`
+      "fn main() -> i32 {
+        let sum = 0;
+        return sum;
+      }"
+    `);
+  });
 });