fix: Better error message when assigning to a value defined outside TGSL

packages/typegpu/src/tgsl/wgslGenerator.ts

@@ -365,7 +365,7 @@ ${this.ctx.pre}}`;
           convLhs.origin === 'runtime-tgpu-const-ref'
         ) {
           throw new WgslTypeError(
-            `'${lhsStr} = ${rhsStr}' is invalid, because ${lhsStr} is a constant.`,
+            `'${lhsStr} = ${rhsStr}' is invalid, because ${lhsStr} is a constant. This error may also occur when assigning to a value defined outside of a TypeGPU function's scope.`,
           );
         }
 

packages/typegpu/tests/constant.test.ts

@@ -83,7 +83,7 @@ describe('tgpu.const', () => {
       [Error: Resolution of the following tree failed:
       - <root>
       - fn*:fn
-      - fn*:fn(): 'boid.pos = vec3f()' is invalid, because boid.pos is a constant.]
+      - fn*:fn(): 'boid.pos = vec3f()' is invalid, because boid.pos is a constant. This error may also occur when assigning to a value defined outside of a TypeGPU function's scope.]
     `);
 
     // Since we freeze the object, we cannot mutate when running the function in JS either
@@ -158,7 +158,7 @@ describe('tgpu.const', () => {
       [Error: Resolution of the following tree failed:
       - <root>
       - fn*:fn
-      - fn*:fn(): 'boid.pos = vec3f()' is invalid, because boid.pos is a constant.]
+      - fn*:fn(): 'boid.pos = vec3f()' is invalid, because boid.pos is a constant. This error may also occur when assigning to a value defined outside of a TypeGPU function's scope.]
     `);
 
     // Since we freeze the object, we cannot mutate when running the function in JS either

packages/typegpu/tests/tgslFn.test.ts

@@ -1007,4 +1007,17 @@ describe('tgsl fn when using plugin', () => {
       }"
     `);
   });
+
+  it('throws a readable error when assigning to a value defined outside of tgsl', () => {
+    let a = 0;
+    const f = tgpu.fn([])(() => {
+      a = 2;
+    });
+
+    expect(() => tgpu.resolve([f])).toThrowErrorMatchingInlineSnapshot(`
+      [Error: Resolution of the following tree failed:
+      - <root>
+      - fn:f: '0 = 2' is invalid, because 0 is a constant. This error may also occur when assigning to a value defined outside of a TypeGPU function's scope.]
+    `);
+  });
 });