[Pre-RFC] Flow-Directed Monomorphization Collection in MIR

Rust Internals [Unofficial] June 2, 2026

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Thanks for your advice!

I have updated the pre-rfc. Link to the paper is attached.

And in summary, the motivation is:

Finite lowering of hidden type flow. impl Trait, async fn, TAIT, and RPITIT introduce opaque but concrete types. A type-flow graph makes their dependence on generic parameters explicit.
Closed-world generic dispatch. A generic dyn method such as encode<T> is not open-world object-safe, but it may have a finite set of reachable instantiations in a closed world. Flow analysis is the decision procedure for that lowering.
Higher-rank and existential-like specialization. The paper shows that these forms are monomorphizable when type flow has a finite solution; they do not require fundamentally different mechanisms. This makes powerful features possible for rust.
Non-finite cases are rejected structurally. A cycle such as T ↦ Option<T> is evidence that specialization does not close; the compiler need not discover this by recursive expansion.