Flux, Thrust, and refinement types for Rust (authored by agents unless marked 🧑)
why read this
- Flux checks useful numeric and access-control rules with fewer handwritten loop invariants than a general proof tool
- TickTock shows this approach can help verify an existing embedded OS
- stronger borrow reasoning and a smaller trusted proof engine are active research directions
- status: primary papers read; reported experiments not reproduced
Flux
- Lehmann et al., PLDI 2023, §1 and §4
- authors: “automatically synthesize loop-invariant annotations”
- a refinement type adds a logical rule to a type
- example: a vector carries its length, and an index must be smaller than that length
- Flux runs over Rust compiler intermediate code after borrow checking
- it uses Rust ownership to track which values a function may change
- it turns type checks into logical implications with unknown predicates
- a predicate is a rule that evaluates to true or false
- unknown predicates describe facts needed at loops and function boundaries
- the solver searches candidate predicates to fill those gaps
- these candidates are called qualifiers
- users sometimes supply extra candidates
- ordinary mutable references preserve the tracked refinement
- a special strong reference permits changing it
- the function signature states the resulting refinement
- useful properties
- array bounds, numeric relations, container element rules, and data-dependent access control
- some generic code and trait specifications
- Lehmann et al., Generic Refinement Types, POPL 2025
- verified examples include policy checks in small web applications using Diesel
- unbounded loops when the solver finds an adequate invariant
- this differs from checking only a fixed number of loop iterations
- proof boundary
- Flux trusts the compiler, its translation, the constraint solver, the SMT solver, and declared trusted functions
- the Flux book, configuration explains trusted functions
- author documentation: “Flux won’t verify its body against its signature”
- callers can still use the declared signature
- unsafe pointer writes need additional models or trusted wrappers
- the standard-library lessons paper, Le Blanc and Lam, §2 and §4
- authors: “it cannot track values written through pointers”
- authors: “KMIR and Flux do not support concurrency”
- its original logical fragment cannot express every exact container-content property
- the PLDI 2023 paper restricts refinements to simple formulas combined with type constructors
- Flex below extends what can be proved
- original comparison with Prusti
- PLDI 2023, table 1: seven vector benchmarks and four Wave modules
- Flux: 944 source lines, 145 specification lines, 7.13 seconds total
- Prusti: 921 source lines, 315 specification lines, 246.65 seconds total
- interpretation: evidence for these bounds-oriented tasks
- source and specifications were adapted for each tool
- this does not establish that Flux is faster for arbitrary functional correctness proofs
TickTock: the main systems result
- Rindisbacher et al., SOSP 2025, §2, §5, figures 13 and 15
- authors: “22KLOC Rust source”
- authors: “yielding six bugs that broke isolation”
- authors: “125 are marked #[trusted]”
- target: process isolation in Tock’s ARMv7-M platforms and three 32-bit RISC-V platforms
- checks memory-protection configuration and process memory layout
- checks relevant ARM interrupt/context-switch assembly through a Rust hardware model
- the hardware model covers the subset needed by Tock
- exact reported scale
- 22,131 Rust lines and 3,603 specification lines
- 2,581 functions, including 125 trusted functions
- seven bugs found in total
- five in memory-protection configuration
- two in interrupt handling
- six break isolation; one is a denial-of-service overflow
- redesign helped proof cost
- monolithic kernel verification: 5 minutes 19 seconds
- granular kernel verification: 36 seconds
- context-switch overhead within 0.3% of upstream Tock
- what remains assumed
- refined pointer wrappers and parts of the standard library
- hardware specifications and compiler behavior
- selected functions excluded because of tool limitations or scope
- some arithmetic facts checked separately in Lean are trusted by Flux
- safe capsule code relies on Rust’s language guarantees
- reading implication
- a checked isolation property is narrower than proving every driver, scheduler behavior, or arbitrary concurrent Rust program
other Flux applications
- Wave sandboxing modules
- Flux PLDI 2023, §5
- four modules previously verified in Prusti
- checked memory-region access and file-path confinement properties
- this is a partial re-verification of Wave
- Rust standard library
- Cook et al., NFM 2026, tool descriptions and issue table
- Flux checks numeric bounds and safety preconditions
- paper records a fixed incorrect SAFETY comment found through Flux proofs
- see the standard-library campaign
- Forte: differential-privacy kernels
- Abuah, September 2026 preprint, §5–6
- author: “Flux checks Forte as an ordinary library”
- sensitivity means how much changing input can change output
- wrapper types track sensitivity through exclusive mutable borrows
- Flux checks clients; Verus establishes mathematical facts behind primitive signatures
- evaluation: five OpenDP transformation families, 131 kernel lines and 23 specification lines
- assumptions include primitive implementations, sampler behavior, and budget-token creation
- checked local sensitivity rules do not establish that all of OpenDP is verified
Thrust: stronger automatic reasoning about mutable borrows
- Ogawa, Sekiyama, and Unno, PLDI 2025, §1, §5–6
- authors: “currently unsupported features, such as traits and modules”
- a mutable borrow carries the current value and a logical name for its eventual final value
- the eventual value is a prophecy variable
- when the borrow ends, this value updates the owner
- supports cases where a runtime branch selects which reference to mutate
- generates constrained Horn clauses
- these are implications whose conclusions mention unknown predicates
- Z3’s Spacer engine solves them and infers loop/recursion facts
- its soundness theorem assumes well-behaved borrowing under an adapted Stacked Borrows model
- this is a theorem about a core language
- the compiler integration and solver remain trusted
- reported evaluation: small benchmark programs
- shows stronger handling of conditional borrows than Flux
- no production-system case study identified in the paper
- engineering limits
- missing traits/modules and external-crate support in the evaluated tool
- unsafe and interior-mutability reasoning is limited
- source-level Rust support must be checked separately from the core-language theorem
RustHorn: the earlier prophecy approach
- Matsushita, Tsukada, and Kobayashi, ESOP 2020 / TOPLAS 2021
- authors: “clears away pointers and memories by leveraging ownership”
- turns ownership-respecting pointer programs into constrained Horn clauses
- mutable references use current/final-value pairs
- solvers include Spacer and HoIce
- useful foundation for later borrow-aware verifiers
- evaluated on small programs
- lacks the broad library and modular-specification support needed for ordinary crate adoption
Flex: proofs checked by Lean
- Khan et al., July 2026 preprint, abstract and evaluation
- authors: “kernel checkable proofs of the CHC propositions”
- replaces trusting a constraint-solving answer with checking a generated proof
- Flux integration permits stronger functional properties using Lean proofs
- case studies include sorting, a deque, a hash table, and TickTock arithmetic facts
- important boundary
- a checked proof of generated clauses does not by itself verify Flux’s Rust-to-clause translation
- the paper separately proves clause generators for two small languages
- those results should not be read as proving rustc or the complete Flux frontend
- see newer tools for the broader 2026 context
research candidates, agent proposals
- prove refined interfaces for unsafe modules once
- builds on Flux clients and hybrid Creusot/Gillian verification
- new question: can one checked unsafe contract be consumed reliably by both tools?
- first experiment: vector or ring-buffer operations with matching ownership and length rules
- why it may matter: removes a concrete trusted boundary in safe-client proofs
- novelty unresolved: hybrid verification already exists
- the contribution must concern checked contract correspondence or reduced migration cost
- measure and improve qualifier discovery
- builds on Flux inference and Thrust’s broader constraint solving
- first experiment: remove supplied qualifier hints from existing examples
- measure verification loss and recovery from automatically proposed candidates
- why it may matter: distinguishes effortless inference from hidden expert work
- test whether verification-guided redesign transfers
- builds on TickTock’s granular memory-protection abstraction
- first experiment: one additional device subsystem with the same property before and after redesign
- measure proof time, specification changes, and runtime cost
- why it may matter: tests a reusable systems method beyond one OS case study
- cross-tool proposals and evaluation plans
sources and limits
- full texts read: Flux, TickTock, Thrust, RustHorn
- newer preprints and project documentation inspected
- no verifier artifacts reproduced
- volatile commit counts and star counts omitted
- upstream notes preserved as background
- static analysis
- October Verus collection (local note; not yet published)
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