rspack Optimize string concatenation in hot paths

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

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Nov 01 '25 10:11 Copilot

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Nov 01 '25 10:11 netlify[bot]

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