Towards #426

Discussion

Below is a list of problems affecting our test coverage and possible mitigation steps:

1. Solidity code used in sol! macros (events, errors, storage) is included in test coverage (effect: high).
   • Mitigation: group Solidity code in an internal module annotated with #[cfg_attr(coverage, coverage(off))] (see example, and pseudo code is below).

   #[cfg_attr(coverage, coverage(off))]
   mod sol_defs {
       sol! {
           event SomeEvent();
           // ...
       }
   
       sol! {
           error SomeError();
           // ...
       }
   
       // NOTE: has to be included, as `SolidityError` affects coverage,
       // so has to be ignored
       #[derive(SolidityError, Debug)]
       pub enum Error {
           // ...
       }
   
       sol_storage! {
           pub struct State {
               // ...
           }
       }
   }
   
   // use in the contract
   use sol_defs::*;
   // reexport the `State` struct to make visible outside the crate
   pub use sol_defs::State;

2. Missing unit tests for functions that interact with other contracts (effect: high).
   • Mitigation 1: will be possible to add these after feat(test): unit test external call mock POC #423.
   • Mitigation 2: including integration tests in the report might solve this issue.
3. Re-exported functions missing unit tests (effect: medium/high).
   • Mitigation 1: duplicate missing unit tests.
   • Mitigation 2: annotate relevant functions with #[cfg_attr(coverage, coverage(off))].
4. Missing unit tests for certain flows, and even whole functions in certain contracts and in contracts/src/utils (effect: medium).
   • Mitigation: add missing unit tests.
5. Using internal functions instead of public ones (e.g. IErc20::transfer is tested by calling _update, causing code coverage to register transfer as "unconvered") (effect: medium).
   • Mitigation: test publicly exposed fns, not the internal ones.
6. IErc165-related tests exist verifying the generated INTERFACE_ID value is correct, but no tests exist that call <ContractName>::supports_interface (effect: low).
   • Mitigation: add missing supports_interface tests.
7. MethodError impls uncovered (effect: low).
   • Mitigation: annotate with #[cfg_attr(coverage, coverage(off))].
8. #[public] attr is included in coverage (effect: low).
   • Mitigation: open to suggestions.
9. #[cfg(test)] mod tests (unit tests) are included in coverage (effect: none). NOTE: technically not a problem atm, as the effect of this is positive or neutral. If we still wanted to exclude them, see mitigation steps below.
   • Mitigation 1: annotate tests module with #[cfg_attr(coverage, coverage(off))]
   • Mitigation 2: move all unit tests to separate tests.rs files, which would be automatically ignored (see Ignore all #[test]-related code automatically taiki-e/cargo-llvm-cov#123 (comment)) (this ties into our previous discussion on unit test organization where this was one of the proposals https://openzeppelin.slack.com/archives/C06R5AGFE9G/p1732625889805619?thread_ts=1732618832.030739&cid=C06R5AGFE9G)
   • Mitigation 3: Integrate https://github.com/taiki-e/coverage-helper into motsu