Walkthrough

This PR introduces three new bytecode instructions (DictMerge, ListExtend, SetUpdate) with corresponding VM frame handlers and compiler codegen support. Additionally, it adds stdlib directory path tracking throughout the VM's runtime configuration and sys module, refactors collection construction in the code generator to optimize starred/unstarred elements via threshold-based branching, and improves Windows path resolution robustness.

Changes

Sequence Diagram(s)

sequenceDiagram
    participant Compiler as Compiler<br/>(codegen)
    participant Codegen as Codegen Helper<br/>(codegen_subkwargs)
    participant Bytecode as Bytecode<br/>(DictMerge instr)
    participant VM as VM Frame<br/>(Execution)
    participant Dict as Target Dict

    Compiler->>Codegen: compile dict merge<br/>with **kwargs
    Codegen->>Codegen: choose fast or<br/>streaming path
    alt Streaming Path (stars present)
        Codegen->>Codegen: emit MapAdd ops
    else Fast Path (no stars)
        Codegen->>Codegen: build single map
    end
    Codegen->>Bytecode: emit DictMerge<br/>{index}
    Bytecode->>Bytecode: stack effect: -1
    VM->>VM: execute DictMerge
    VM->>VM: validate source<br/>is mapping
    VM->>Dict: iterate keys & merge<br/>with validation
    VM-->>VM: return Ok(None)<br/>on success

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~50 minutes

Possibly related PRs

• Replace custom opcodes with CPython standard sequences #6794 — Implements the same codegen changes and VM instruction additions (DictMerge, ListExtend, SetUpdate, and codegen_subkwargs streaming patterns) for collection handling.
• Add sys._stdlib_dir #6798 — Adds and wires sys._stdlib_dir across build.rs, getpath.rs, vm settings, sys.rs, and interpreter state initialization.
• nt junction #6407 — Improves Windows Lib path handling by stripping the extended-length prefix and handling symlinks more robustly.

Suggested reviewers

• ShaharNaveh

Poem

🐰 A hop through bytecode, paths aligned!
DictMerge, ListExtend, SetUpdate designed,
Stdlib paths now tracked with care,
Fast paths chosen, streaming paths fair,
Windows paths robust, no more despair! 🌟

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

• 📝 Generate docstrings

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