perf: fast path array self append

[larimonious]

Summary

• Adds a narrow fast path for loop/control-flow statement contexts shaped like arr = arr + [item] / [items...].
• Mutates the resolved array binding in place instead of cloning the growing array on every append.
• Preserves generic + behavior for non-target shapes, including call-containing RHS arrays, assignment-expression results, RHS error behavior, and alias/copy semantics.
• Updates DD-061 with PR 6 completion notes and measured benchmark results.

Benchmarks

• Before: 20k append CLI fixture: 7.11s
• After: 20k median: 0.0122s
• After: 50k median: 0.0238s
• After: 100k median: 0.0439s

Verification

• cargo fmt -- --check
• cargo build --profile dev-release
• cargo test --lib -q — 1153 passed
• cargo test --test language_features_tests --test type_checker_tests --test cli_tests -q
• ./target/dev-release/ntnt docs --generate
• git diff --check

[greptile-apps]

Greptile Summary

This PR introduces a narrow in-place fast path for the arr = arr + [item] idiom inside loop bodies, eliminating the O(n²) full-array-clone cliff. It also fixes a latent bug in eval_block where statement errors skipped deferred statement execution and left the interpreter's scope pointing at the child environment — the fix is unified into the new eval_block_inner helper and applied to both the new control-flow path and the existing non-control path.

• Fast path (try_eval_array_self_append_statement): activated only when the exact assignment shape matches, all RHS elements pass a conservative expression_is_side_effect_free check, and the binding is confirmed to be an array; mutates the stored binding in place, skipping the intermediate clone.
• eval_block deferred-statement fix: eval_block_inner now captures errors, drains and executes deferred statements (LIFO), restores the previous environment, then re-propagates — matching the correct behaviour already implemented in the for-loop env-restore path.
• DD-061 update: renumbers PRs 6–10 on the roadmap and records measured benchmark results (20k appends: 7.11 s → 12.2 ms).

Confidence Score: 5/5

Safe to merge. The fast path is narrow and well-guarded: it only fires when the exact assignment shape matches, all RHS elements pass an exhaustive side-effect-free check, and the binding is confirmed to be an array before any mutation. Errors during element evaluation abort before any in-place append.

The fast path guards are conservative and exhaustive — all 27 Expression variants are handled in expression_is_side_effect_free, the TOCTOU window between binding_is_array and append_to_array_binding is sealed by the side-effect-free predicate, and partial mutation on RHS error is prevented. The eval_block deferred-statement fix is backward-compatible and directly tested. The 1153-test suite, including new alias, error-abort, and deferred-drain tests, gives high confidence no regressions were introduced.

No files require special attention.

Important Files Changed

Filename	Overview
src/interpreter.rs	Core changes: new binding_is_array/append_to_array_binding Environment helpers, try_eval_array_self_append_statement fast path, exhaustive expression_is_side_effect_free predicate (all 27 Expression variants covered), unified eval_block_inner that correctly runs deferred statements on error in both control and non-control contexts, and eval_block_for_control wiring into while/loop/for statement handlers. Logic is sound; alias semantics, partial-append prevention, and scope restoration are all tested.
design-docs/dd-061-interpreter-performance-roadmap.md	Documentation-only: PR 6 rewritten to describe the array self-append fast path, PR 7 added for string self-concat, existing PRs 6–8 renumbered to 8–10, priority table updated, benchmark deltas recorded. No code impact.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["eval_block_for_control(block)"] --> B["eval_block_inner(block, control=true)"]
    B --> C["for each stmt"]
    C --> D["eval_statement_for_control(stmt)"]
    D --> E{Statement kind}
    E -- "Located{stmt}" --> F["set line/col, recurse"]
    F --> D
    E -- "Expression(expr)" --> G["try_eval_array_self_append_statement(expr)"]
    G --> H{Shape match?}
    H -- "Not Assign / lhs≠rhs / no array binding or elements have side effects" --> I["eval_expression(expr) — generic path"]
    H -- "arr = arr + literal + binding_is_array passes" --> J["eval each element"]
    J --> K{Any element error?}
    K -- "Err" --> L["propagate error, no mutation"]
    K -- "Ok(items)" --> M["append_to_array_binding() — mutate in-place"]
    M --> N["return Ok(Value::Unit)"]
    E -- "other statement" --> O["eval_statement(stmt) + control_flow_or_unit"]
    B --> P{stmt error?}
    P -- "Err captured" --> Q["drain & run deferred stmts, restore env, propagate Err"]
    P -- "Break/Continue/Return" --> R["break loop, run deferred stmts, restore env, return control value"]

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flowchart TD
    A["eval_block_for_control(block)"] --> B["eval_block_inner(block, control=true)"]
    B --> C["for each stmt"]
    C --> D["eval_statement_for_control(stmt)"]
    D --> E{Statement kind}
    E -- "Located{stmt}" --> F["set line/col, recurse"]
    F --> D
    E -- "Expression(expr)" --> G["try_eval_array_self_append_statement(expr)"]
    G --> H{Shape match?}
    H -- "Not Assign / lhs≠rhs / no array binding or elements have side effects" --> I["eval_expression(expr) — generic path"]
    H -- "arr = arr + literal + binding_is_array passes" --> J["eval each element"]
    J --> K{Any element error?}
    K -- "Err" --> L["propagate error, no mutation"]
    K -- "Ok(items)" --> M["append_to_array_binding() — mutate in-place"]
    M --> N["return Ok(Value::Unit)"]
    E -- "other statement" --> O["eval_statement(stmt) + control_flow_or_unit"]
    B --> P{stmt error?}
    P -- "Err captured" --> Q["drain & run deferred stmts, restore env, propagate Err"]
    P -- "Break/Continue/Return" --> R["break loop, run deferred stmts, restore env, return control value"]

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_{Reviews (2): Last reviewed commit: "fix: restore block scope on errors" | Re-trigger Greptile}

[larimonious]

Addressed Greptile's feedback in ebedea5:

• unified normal/control block evaluation through a shared eval_block_inner path
• eval_block now restores the previous environment and drains deferred statements before returning statement errors
• added regression tests for scope restoration and deferred execution on block errors
• made the array self-append fast path fail closed if its post-guard append invariant ever breaks, avoiding accidental generic fallthrough/double evaluation

Local verification passed: cargo fmt -- --check, cargo build --profile dev-release, targeted tests, full lib tests, integration tests, docs generation, and git diff --check.