impr: Error messages for invalid array and tgpu.const usage

packages/typegpu/src/tgsl/wgslGenerator.ts

@@ -35,6 +35,7 @@ import {
   numericLiteralToSnippet,
 } from './generationHelpers.ts';
 import type { ShaderGenerator } from './shaderGenerator.ts';
+import { constant } from '../core/constant/tgpuConstant.ts';
 
 const { NodeTypeCatalog: NODE } = tinyest;
 
@@ -370,7 +371,7 @@ ${this.ctx.pre}}`;
         }
 
         throw new Error(
-          `Cannot index value ${targetStr} of unknown type with index ${propertyStr}`,
+          `Unable to index a value of unknown type with index ${propertyStr}. If the value is an array, to address this, consider one of the following approaches: (1) declare the array using 'tgpu.const', (2) store the array in a buffer, or (3) define the array within the GPU function scope.`,
         );
       }
 
@@ -441,6 +442,12 @@ ${this.ctx.pre}}`;
         );
       }
 
+      if (callee.value === constant) {
+        throw new Error(
+          'Constants cannot be defined within TypeGPU function scope. To address this, move the constant definition outside the function scope.',
+        );
+      }
+
       if (callee.value instanceof InfixDispatch) {
         // Infix operator dispatch.
         if (!argNodes[0]) {

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

@@ -1147,4 +1147,31 @@ describe('wgslGenerator', () => {
       }"
     `);
   });
+
+  it('throws a descriptive error when accessing an external array with a runtime known index', () => {
+    const myArray = [9, 8, 7, 6];
+
+    const testFn = tgpu.fn([d.u32], d.u32)((i) => {
+      return myArray[i] as number;
+    });
+
+    expect(() => asWgsl(testFn)).toThrowErrorMatchingInlineSnapshot(`
+      [Error: Resolution of the following tree failed:
+      - <root>
+      - fn:testFn: Unable to index a value of unknown type with index i. If the value is an array, to address this, consider one of the following approaches: (1) declare the array using 'tgpu.const', (2) store the array in a buffer, or (3) define the array within the GPU function scope.]
+    `);
+  });
+
+  it('throws a descriptive error when declaring a const inside TGSL', () => {
+    const testFn = tgpu.fn([d.u32], d.u32)((i) => {
+      const myArray = tgpu.const(d.arrayOf(d.u32, 4), [9, 8, 7, 6]);
+      return myArray.$[i] as number;
+    });
+
+    expect(() => asWgsl(testFn)).toThrowErrorMatchingInlineSnapshot(`
+      [Error: Resolution of the following tree failed:
+      - <root>
+      - fn:testFn: Constants cannot be defined within TypeGPU function scope. To address this, move the constant definition outside the function scope.]
+    `);
+  });
 });