perf(lambda-rs): Cache missing per-instance vertex buffer slots

[vmarcella]

Summary

Cache missing per-instance vertex buffer slots in the render encoder so
instancing validation no longer rescans every pipeline slot on every draw.
This reduces validation overhead in instanced render passes from O(D * S) to
O(S + B + D), where D is draw calls, S is per-instance slots, and B is
vertex buffer binds.

Related Issues

Changes

• Cache missing per-instance slots when RenderPassEncoder::set_pipeline is called.
• Update the cache incrementally in RenderPassEncoder::set_vertex_buffer.
• Replace per-draw full scans with a constant-time cached missing-slot check in
  draw and draw_indexed.
• Centralize the missing-instance-binding error message in render validation helpers.
• Add encoder tests covering both post-pipeline binding and prebound-slot cases.
• Add rustdoc for cached instancing validation behavior and the instanced test helper.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (updates to docs, specs, tutorials, or comments)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Performance (change that improves performance)
• Test (adding or updating tests)
• Build/CI (changes to build process or CI configuration)

Affected Crates

• lambda-rs
• lambda-rs-platform
• lambda-rs-args
• lambda-rs-logging
• Other:

Checklist

• Code follows the repository style guidelines (cargo +nightly fmt --all)
• Code passes clippy (cargo clippy --workspace --all-targets -- -D warnings)
• Tests pass (cargo test --workspace)
• New code includes appropriate documentation
• Public API changes are documented
• Breaking changes are noted in this PR description

Testing

Commands run:

cargo fmt --package lambda-rs
cargo test -p lambda-rs render_pass_encoder_ -- --nocapture
cargo test -p lambda-rs validate_instance_ -- --nocapture

Manual verification steps (if applicable):

1. Confirmed instanced draws fail with the existing validation error until the
   required per-instance slot is bound.
2. Confirmed binding the required instance slot after set_pipeline allows the
   draw to proceed.
3. Confirmed prebinding the instance slot before set_pipeline is respected by
   the cached validation state.

Screenshots/Recordings

Not applicable.

Platform Testing

• macOS
• Windows
• Linux

Additional Notes

[github-actions]

✅ Coverage Report

📊 View Full HTML Report (download artifact)

Overall Coverage

Metric	Value
Total Line Coverage	76.88%
Lines Covered	12676 / 16487

Changed Files in This PR

File	Coverage	Lines
crates/lambda-rs/src/render/encoder.rs	85.77%	639/745
crates/lambda-rs/src/render/validation.rs	100.00%	131/131

PR Files Coverage: 87.89% (770/876 lines)

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Generated by cargo-llvm-cov · Latest main coverage

_{Last updated: 2026-03-08 22:17:02 UTC · Commit: 4c72dfe}

[Copilot]

Pull request overview

This PR optimizes instanced draw validation in lambda-rs by caching which per-instance vertex buffer slots are missing at set_pipeline time and updating that cache incrementally on set_vertex_buffer, avoiding per-draw rescans.

Changes:

• Cache missing per-instance vertex buffer slots during RenderPassEncoder::set_pipeline and update the cache in set_vertex_buffer.
• Replace per-draw full scans with a cached missing-slot check in draw / draw_indexed, and centralize the missing-binding error message.
• Add tests and rustdoc covering cached instancing validation behavior, including “bind-after-pipeline” and “prebound slot” cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
crates/lambda-rs/src/render/validation.rs	Centralizes the missing per-instance binding error message and documents the full-scan helper vs cached paths; adds a small formatter test.
crates/lambda-rs/src/render/encoder.rs	Implements cached missing-slot tracking in the render pass encoder and adds targeted tests for incremental cache updates and prebound behavior.

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