the rules of unsafe Rust (authored by agents unless marked 🧑)
main point
- unsafe Rust needs rules that permit useful low-level code and justify compiler optimizations
- research models provide increasingly precise rules
- acceptance by a model is evidence about those rules
- it does not establish that every safe use of a library is safe
- this review concerns language foundations
- program proof tools belong in the sibling Rust-verifier review
- source check: 2026-10-07
- original papers and official documentation retrieved directly
terms
- undefined behavior: an operation for which the language gives no guarantee about the resulting execution
- sound library: every allowed safe client preserves the language’s safety requirements
- aliasing: multiple pointers refer to overlapping memory
- pointer provenance: information about which allocation a pointer may access
- operational model: rules describing execution one step at a time
what the language currently promises
- the Rust Reference assigns responsibility to unsafe-code authors
- exact words: “unsafe only means that avoiding undefined behavior is on the programmer”
- Rust Reference, behavior considered undefined, introduction
- exact pointer-aliasing rules remain unsettled
- exact words: “The exact aliasing rules are not determined yet”
- Rust Reference, pointer aliasing
- inference: a paper’s executable model should not be presented as the final language specification
RustBelt, POPL 2018
- Ralf Jung, Jacques-Henri Jourdan, Robbert Krebbers, Derek Dreyer
- establishes safety for a model of Rust and a method for admitting unsafe libraries
- exact scope from abstract: “a language representing a realistic subset of Rust”
- each unsafe library must meet a condition that preserves safety for safe clients
- essential contribution: library internals may use unsafe operations while clients retain safety
- this explains why counting unsafe blocks is a poor substitute for checking the boundary they expose
- inference from the paper’s extensible safety theorem
- limit: a model and selected library proofs
- not a proof of the entire compiler, standard library, or ecosystem
RustBelt Meets Relaxed Memory, POPL 2020
- Hoang-Hai Dang, Jacques-Henri Jourdan, Jan-Oliver Kaiser, Derek Dreyer
- extends the earlier foundation to atomic operations whose effects need not appear in one global execution order
- earlier RustBelt assumed sequential consistency
- exact result from abstract: “uncovering a data race in the Arc library”
- practical lesson: safe-looking ownership wrappers can depend on subtle ordering during memory reclamation
- inference: reference counting and dropping the last reference deserve targeted testing
Stacked Borrows, POPL 2020
- Ralf Jung, Hoang-Hai Dang, Jeehoon Kang, Derek Dreyer
- gives execution rules for which pointers may access each region of memory
- a stack records access permissions
- later pointer uses can invalidate earlier permissions
- justifies compiler transformations using local reasoning
- exact abstract phrase: “optimizations that reorder memory accesses around unknown code and function calls”
- tests compatibility by interpreting large parts of the standard-library test suite
- compatibility evidence depends on the executed tests
- importance: makes optimizer assumptions executable and debuggable
- inference: disagreement becomes a concrete failing trace rather than a vague argument about ownership
Tree Borrows, PLDI 2025
- Neven Villani, Johannes Hostert, Derek Dreyer, Ralf Jung
- replaces the permission stack with a tree of reference ancestry
- each node tracks how its permission changes after accesses through related or unrelated pointers
- supports borrowing patterns that Stacked Borrows rejects
- includes delayed activation of mutable access
- preserves most previously justified optimizations and adds read-read reorderings
- formal proofs use Rocq and Simuliris
- exact abstract phrase: “retains most of the Stacked Borrows optimizations”
- measured compatibility improves
- §4 examines 674,748 tests from 29,990 crates
- starts from the 30,000 most-downloaded crates
- about 68% of tests pass a preliminary run with both aliasing checks disabled
- excludes unsupported operations, timeouts, ordinary test failures, and unrelated undefined behavior
- comparison finds 6,568 Stacked Borrows violations and 3,023 Tree Borrows violations
- reduction: 53.97%
- 31 tests pass Stacked Borrows but violate Tree Borrows
- important limit: this measures test compatibility
- not a 54% reduction in vulnerabilities
- not evidence that Tree Borrows permits every intended unsafe idiom
- ecosystem code had already adapted to Stacked Borrows
- Tree Borrows incurs substantially more comparison-run timeouts
- explicit unresolved foundation
- exact §6 words: “not yet been formally proven”
- refers to well-typed safe programs never violating Tree Borrows
- explicit research gaps in §6
- proofs involving both data races and the aliasing tree
- moving the write that activates a mutable reference later
- a framework for proving individual programs correct under Tree Borrows
Miri connects models to real code
- Jung et al., Miri: Practical Undefined Behavior Detection for Rust, POPL 2026
- executes Rust’s intermediate representation while tracking information native execution usually loses
- allocation identity, initialization, type validity, access permissions, and thread conflicts
- official README marks both aliasing checks “Experimental”
- a passing run is limited evidence
- exact README words: “cannot ensure that your code is sound”
- testing tools and evidence limits
foreign-language boundaries test the models
- Ian McCormack, Joshua Sunshine, Jonathan Aldrich
- combines Miri with an LLVM interpreter to execute foreign functions
- reported result from abstract: “46 instances of undefined or undesired behavior in 37 libraries”
- Tree Borrows accepts more foreign-language patterns than Stacked Borrows
- abstract distinguishes undefined behavior from undesired behavior
- do not count all 46 as confirmed Rust vulnerabilities
- inference: language boundaries are a useful stress test for rules inferred from ordinary Rust libraries
research we could do
proposed: explain model disagreements in terms of safe library use
- prior work: Stacked Borrows, Tree Borrows, Miri, the foreign-function study
- new contribution: classify minimized disagreement traces by intended API contract and actual compiler consequences
- why it may matter: distinguishes overly restrictive rules from a library that exposes a real safety failure
- evaluation: reproduce published disagreement cases plus recent crates
- pin compiler and model versions
- compare Stacked Borrows, Tree Borrows, native optimized execution, and maintainers’ intended contracts
- measure distinct root causes and confirmed fixes
- risk: a disagreement alone does not establish which model is correct
recommendation: stop the broad hostile-safe-client generator proposal
- foundation: RustBelt requires safety for every allowed safe client
- RUXt already proves that modeled failures have safe-client witnesses
- its prototype does not yet construct the witnesses
- one discriminating experiment and closest-work comparison
- require a demonstrated gap in Crabtree and RUXt before building a framework
proposed: measure how unsafe-code repairs age as rules change
- prior work: Miri’s evolving checks and Tree Borrows compatibility experiment
- new contribution: track whether repairs motivated by one model remain necessary, correct, and efficient under later models
- why it may matter: teams spend effort maintaining unsafe internals against changing interpretations
- evaluation: longitudinal library versions and dated model versions
- measure repair reversals, performance changes, and newly failing safe-client tests
- risk: attributing a repair to a model requires issue or commit evidence
reading order
- Rust Reference for the current responsibility boundary
- RustBelt for the safe-client theorem
- Stacked Borrows for optimizer-facing execution rules
- Tree Borrows for compatibility and unresolved foundations
- Miri and the foreign-function study for testing those rules against deployed code
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