Optimize string concatenation in hot paths

[Copilot]

Identified and fixed inefficient string concatenation patterns causing unnecessary allocations in frequently-executed code paths.

Changes

• compile_boolean_matcher.rs: Pre-allocate string capacity and use push_str() instead of += operator when building single-character regex patterns
• dependencies_block.rs: Replace fold with += with idiomatic map().collect::<String>() pattern for dependency ID generation
• eval/mod.rs: Replace manual loop with iterator-based collection for template string evaluation

Example

Before:

let mut single_char_items: String = String::new();
for item in items {
  if item.chars().count() == 1 {
    single_char_items += &item;  // Repeated allocations
  }
}

After:

let mut single_char_items = String::with_capacity(count_of_single_char_items);
for item in items {
  if item.chars().count() == 1 {
    single_char_items.push_str(&item);  // No reallocation
  }
}

Additional Observations

Minor optimization opportunities remain but are out of scope for this focused change:

• .chars().count() == 1 could use early termination
• Some HashMap/HashSet instances lack capacity hints

Original prompt

Identify and suggest improvements to slow or inefficient code

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[netlify]

✅ Deploy Preview for rspack canceled.

Name	Link
🔨 Latest commit	be810f7
🔍 Latest deploy log	https://app.netlify.com/projects/rspack/deploys/6905e143e1046600089f403c

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

📦 Binary Size-limit

Comparing be810f7 to chore(deps): update dependency @biomejs/biome to ^2.3.2 (#12055) by renovate[bot]

🙈 Size remains the same at 47.85MB

[codspeed-hq]

CodSpeed Performance Report

Merging #12058 will not alter performance

_{Comparing copilot/identify-inefficient-code (be810f7) with main (d808219)}

Summary

✅ 17 untouched