Summary

Fixes #1252 by adding validation to reject field-level @assert and @check attributes on test blocks. These attributes were previously accepted by the parser but silently ignored at runtime, causing confusion.

Problem

• BAML tests incorrectly accept @assert (single @) syntax without warnings
• Field-level assertions on test fields are silently ignored at runtime
• Only block-level @@assert (double @) assertions actually work for tests
• This leads to users writing non-functional tests that appear to work

Solution

Added semantic validation in the test validator to:

1. Check all fields in test blocks for @assert or @check attributes
2. Emit clear error messages directing users to use @@assert or @@check at the block level instead
3. Point to the exact location of the invalid attribute

Changes

• Modified: engine/baml-lib/baml-core/src/validate/validation_pipeline/validations/tests.rs
  • Added validation logic to detect field-level assertion attributes
  • Provides helpful error messages with the correct syntax
• Added: Test files to verify the validation works correctly
  • field_level_assertions_v2.baml - validates @Assert rejection
  • field_level_check.baml - validates @check rejection

Testing

• ✅ All existing tests pass
• ✅ New validation tests pass with expected error messages
• ✅ No clippy warnings
• ✅ Error messages point to correct locations

Example Error

error: @assert is not allowed on test fields. Use @@assert at the test block level instead.
  -->  functions_v2/tests/field_level_assertions_v2.baml:9
   |
 8 | test MyTest {
 9 |   functions [TestFunction] @assert({{ true }})
   |

🤖 Generated with Claude Code