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Meta-Monomorphizing Specializations

Federico Bruzzone, Walter Cazzola

TL;DR

Meta-monomorphizing specializations is introduced, a novel framework that achieves specialization by repurposing monomorphization through compile-time metaprogramming, and demonstrates that meta-monomorphization enables expressive specialization patterns -- previously rejected by the compiler -- while maintaining full compatibility with standard optimization pipelines.

Abstract

Achieving zero-cost specialization remains a fundamental challenge in programming language and compiler design. It often necessitates trade-offs between expressive power and type system soundness, as the interaction between conditional compilation and static dispatch can easily lead to unforeseen coherence violations and increased complexity in the formal model. This paper introduces meta-monomorphizing specializations, a novel framework that achieves specialization by repurposing monomorphization through compile-time metaprogramming. Instead of modifying the host compiler, our approach generates meta-monomorphized traits and implementations that encode specialization constraints directly into the type structure, enabling deterministic, coherent dispatch without overlapping instances. We formalize this method for first-order, predicate-based, and higher-ranked polymorphic specialization, also in presence of lifetime parameters. Our evaluation, based on a Rust implementation using only existing macro facilities, demonstrates that meta-monomorphization enables expressive specialization patterns -- previously rejected by the compiler -- while maintaining full compatibility with standard optimization pipelines. We show that specialization can be realized as a disciplined metaprogramming layer, offering a practical, language-agnostic path to high-performance abstraction. A comprehensive study of public Rust codebases further validates our approach, revealing numerous workarounds that meta-monomorphization can eliminate, leading to more idiomatic and efficient code.

Meta-Monomorphizing Specializations

TL;DR

Meta-monomorphizing specializations is introduced, a novel framework that achieves specialization by repurposing monomorphization through compile-time metaprogramming, and demonstrates that meta-monomorphization enables expressive specialization patterns -- previously rejected by the compiler -- while maintaining full compatibility with standard optimization pipelines.

Abstract

Achieving zero-cost specialization remains a fundamental challenge in programming language and compiler design. It often necessitates trade-offs between expressive power and type system soundness, as the interaction between conditional compilation and static dispatch can easily lead to unforeseen coherence violations and increased complexity in the formal model. This paper introduces meta-monomorphizing specializations, a novel framework that achieves specialization by repurposing monomorphization through compile-time metaprogramming. Instead of modifying the host compiler, our approach generates meta-monomorphized traits and implementations that encode specialization constraints directly into the type structure, enabling deterministic, coherent dispatch without overlapping instances. We formalize this method for first-order, predicate-based, and higher-ranked polymorphic specialization, also in presence of lifetime parameters. Our evaluation, based on a Rust implementation using only existing macro facilities, demonstrates that meta-monomorphization enables expressive specialization patterns -- previously rejected by the compiler -- while maintaining full compatibility with standard optimization pipelines. We show that specialization can be realized as a disciplined metaprogramming layer, offering a practical, language-agnostic path to high-performance abstraction. A comprehensive study of public Rust codebases further validates our approach, revealing numerous workarounds that meta-monomorphization can eliminate, leading to more idiomatic and efficient code.
Paper Structure (14 sections, 10 equations, 10 figures)

This paper contains 14 sections, 10 equations, 10 figures.

Table of Contents

  1. Introduction
  2. The Rust Programming Language
  3. Meta-Monomorphizing Specializations by Examples
  4. First-Order Programs with Equality Bounds
  5. Predicate Polymorphism with Trait Bounds
  6. Polymorphic $\sum$- and $\prod$-type Constructors
  7. Lifetime Polymorphism with Reference Types
  8. Higher-Ranked Polymorphism with Higher-Order Functions
  9. Limitations
  10. Implementation Details
  11. Validation
  12. Threats to Validity
  13. Related Work
  14. Conclusion

Figures (10)

  • Figure 1: A motivating example.
  • Figure 2: The compilation error of \ref{['lst:motivation']}.
  • Figure 3: Specialization solved through meta-monomorphization.
  • Figure 4: A first-order program with trait specializations.
  • Figure 5: Predicate polymorphism and trait specializations.
  • ...and 5 more figures