Polymorphic code generation experiment

[nxsaken]

Metadata
Contact	@camelid
Team(s)	compiler (@camelid), types
Goal document	2026/polymorphization
Funding contact	Funding team

Summary

Implement experimental support in the Rust compiler for polymorphic code generation. Instead of monomorphizing generic code, the compiler will generate type-agnostic instances and add hidden vtable parameters where needed, which should reduce compile times and generated binary sizes.

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[nxsaken]

This issue is intended for status updates only.

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[camelid]

Current status: I have a proof of concept implementation (rust-lang/rust#160300) that can successfully compile std and other real-world code and reduce the number of generated instances. However, it's very restricted in which parameters and parameter values it can actually polymorphize (e.g. the generic type argument cannot have a custom Drop implementation). And it also has some issues, like it doesn't handle reflection (TypeId) correctly. I have designs for how to fix these limitations, and I'll be restarting work on implementing those designs this month.