Incremental path: @N position syntax and MEAN with dynamic ranges unsupported

[bpowers]

Summary

Three array features are not yet supported on the salsa incremental compilation path and fall back to the monolithic path. The affected tests explicitly use assert_compiles() instead of assert_compiles_incremental() with comments documenting the limitation.

Affected features

1. @n position syntax (subscript element selection by position)

@N syntax allows selecting array elements by dimension position (e.g., arr[@1] for the first element, matrix[@2, @1] for dimension reordering). Two tests retain the monolithic path for this reason:

• dimension_position_single (line 341): arr[@1] -- scalar selection by position
• dimension_position_and_wildcard (line 1359): cube[@1, *, @3] -- combining position syntax with wildcards

Note: the dimension_position_reorder test (line 366) using matrix[@2, @1] does use assert_compiles_incremental(), so some @N cases may already work on the incremental path.

2. MEAN with dynamic ranges

MEAN(data[start_idx:end_idx]) where the range bounds are variable references rather than literal constants. One test retains the monolithic path:

• mean_with_dynamic_range (line 2022): MEAN(data[start_idx:end_idx]) with start_idx and end_idx as scalar constants

Why it matters

These tests silently fall back to the monolithic compilation path, which means they do not exercise the incremental pipeline. As monolithic-path code is eventually removed (per the salsa migration plan), these features will need incremental support or will regress.

Component(s) affected

• simlin-engine: src/simlin-engine/src/db.rs (incremental compilation pipeline)
• simlin-engine: src/simlin-engine/src/array_tests.rs (test assertions)

Possible approaches

• Investigate what specifically fails when switching these tests to assert_compiles_incremental() -- the error messages will indicate which incremental tracked function is missing the relevant array lowering logic.
• The @N syntax likely needs handling in the dimension resolution or subscript expansion phases of the incremental path.
• Dynamic ranges for MEAN likely need the range size computation to be available during incremental compilation.

Context

Identified during review of the finish-salsa-migration branch. Related to but distinct from #295 (which covers module expansion, builtin expansion, and variable layout gaps).