internal/shader: bug fix: panic when an assignment mismatch happens

Closes #2654

internal/shader/stmt.go

@@ -639,6 +639,15 @@ func (cs *compileState) assign(block *block, fname string, pos token.Pos, lhs, r
 			}
 			stmts = append(stmts, ss...)
 
+			if len(l) != len(r) {
+				if len(r) == 0 {
+					cs.addError(pos, fmt.Sprintf("right-hand side (no value) used as value"))
+				} else {
+					cs.addError(pos, fmt.Sprintf("assignment mismatch: %d variables but the right-hand side has %d values", len(l), len(r)))
+				}
+				return nil, false
+			}
+
 			if l[0].Type == shaderir.Blank {
 				continue
 			}

internal/shader/syntax_test.go

@@ -3122,3 +3122,19 @@ func Fragment(position vec4, texCoord vec2, color vec4) vec4 {
 		}
 	}
 }
+
+// Issue #2654
+func TestOmittedReturnType(t *testing.T) {
+	if _, err := compileToIR([]byte(`package main
+
+func foo(x vec2) {
+	x = bar(x)
+	_ = x
+}
+
+func bar(x vec2) {
+	return x
+}`)); err == nil {
+		t.Error("compileToIR must return an error but did not")
+	}
+}