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Author: nikomatsakis

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+- Feature Name: N/A
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Incorporate a strike team dedicated to preparing rules and guidelines
+for writing unsafe code in Rust (commonly referred to as Rust's
+"memory model"), in cooperation with the lang team. The discussion
+will generally proceed in phases, starting with establishing
+high-level principles and gradually getting down to the nitty gritty
+details (though some back and forth is expected). The strike team will
+produce various intermediate documents that will be submitted as
+normal RFCs.
+
+# Motivation
+[motivation]: #motivation
+
+Rust's safe type system offers very strong aliasing information that
+promises to be a rich source of compiler optimization.  For example,
+in safe code, the compiler can infer that if a function takes two
+`&mut T` parameters, those two parameters must reference disjoint
+areas of memory (this allows optimizations similar to C99's `restrict`
+keyword, except that it is both automatic and fully enforced). The
+compiler also knows that given a shared reference type `&T`, the
+referent is immutable, except for data contained in an `UnsafeCell`.
+
+Unfortunately, there is a fly in the ointment. Unsafe code can easily
+be made to violate these sorts of rules. For example, using unsafe
+code, it is trivial to create two `&mut` references that both refer to
+the same memory (and which are simultaneously usable).  In that case,
+if the unsafe code were to (say) return those two points to safe code,
+that would undermine Rust's safety guarantees -- hence it's clear that
+this code would be "incorrect".
+
+But things become more subtle when we just consider what happens
+*within* the abstraction. For example, is unsafe code allowed to use
+two overlapping `&mut` references internally, without returning it to
+the wild? Is it all right to overlap with `*mut`? And so forth.
+
+It is the contention of this RFC that a complete guidelines for unsafe
+code are far too big a topic to be fruitfully addressed in a single
+RFC. Therefore, this RFC proposes the formation of a dedicated
+**strike team** (that is, a temporary, single-purpose team) that will
+work on hammering out the details over time. Precise membership of
+this team is not part of this RFC, but will be determined by the lang
+team as well as the strike team itself.
+
+The unsafe guidelines work will proceed in rough stages, described
+below. An initial goal is to produce a **high-level summary detailing
+the general approach of the guidelines.** Ideally, this summary should
+be sufficient to help guide unsafe authors in best practices that are
+most likely to be forwards compatible. Further work will then expand
+on the model to produce a more **detailed set of rules**, which may in
+turn require revisiting the high-level summary if contradictions are
+uncovered.
+
+This new "unsafe code" strike team is intended to work in
+collaboration with the existing lang team. Ultimately, whatever rules
+are crafted must be adopted with the **general consensus of both the
+strike team and the lang team**. It is expected that lang team members
+will be more involved in the early discussions that govern the overall
+direction and less involved in the fine details.
+
+#### History and recent discussions
+
+The history of optimizing C can be instructive. All code in C is
+effectively unsafe, and so in order to perform optimizations,
+compilers have come to lean heavily on the notion of "undefined
+behavior" as well as various ad-hoc rules about what programs ought
+not to do (see e.g. [these][cl1] [three][cl2] [posts][cl3] entitled
+"What Every C Programmer Should Know About Undefined Behavior", by
+Chris Lattner). This can cause some very surprising behavior (see e.g.
+["What Every Compiler Author Should Know About Programmers"][cap] or
+[this blog post by John Regehr][jr], which is quite humorous).  Note that
+Rust has a big advantage over C here, in that only the authors of
+unsafe code should need to worry about these rules.
+
+[cl1]: http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html
+[cl2]: http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html
+[cl3]: http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_21.html
+[cap]: http://www.complang.tuwien.ac.at/kps2015/proceedings/KPS_2015_submission_29.pdf
+[jr]: http://blog.regehr.org/archives/761
+
+In terms of Rust itself, there has been a large amount of discussion
+over the years. Here is a (non-comprehensive) set of relevant links,
+with a strong bias towards recent discussion:
+
+- [RFC Issue #1447](https://github.com/rust-lang/rfcs/issues/1447) provides
+  a general set of links as well as some discussion.
+- [RFC #1578](https://github.com/rust-lang/rfcs/pull/1578) is an initial
+  proposal for a Rust memory model by ubsan.
+- The
+  [Tootsie Pop](http://smallcultfollowing.com/babysteps/blog/2016/05/27/the-tootsie-pop-model-for-unsafe-code/)
+  blog post by nmatsakis proposed an alternative approach, building on
+  [background about unsafe abstractions](http://smallcultfollowing.com/babysteps/blog/2016/05/23/unsafe-abstractions/)
+  described in an earlir post. There is also a lot of valuable
+  discussion in
+  [the corresponding internals thread](http://smallcultfollowing.com/babysteps/blog/2016/05/23/unsafe-abstractions/).
+
+#### Other factors
+
+Another factor that must be considered is the interaction with weak
+memory models. Most of the links above focus purely on sequential
+code: Rust has more-or-less adopted the C++ memory model for governing
+interactions across threads. But there may well be subtle cases that
+arise we delve deeper. For more on the C++ memory model, see
+[Hans Boehm's excellent webpage](http://www.hboehm.info/c++mm/).
+
+# Detailed design
+[design]: #detailed-design
+
+## Scope
+
+Here are some of the issues that should be resolved as part of these
+unsafe code guidelines. The following list is not intended as
+comprehensive (suggestions for additions welcome):
+
+- Legal aliasing rules and patterns of memory accesses
+  - e.g., which of the patterns listed in [rust-lang/rust#19733](https://github.com/rust-lang/rust/issues/19733)
+    are legal?
+  - can unsafe code create (but not use) overlapping `&mut`?
+  - under what conditions is it legal to dereference a `*mut T`?
+  - when can an `&mut T` legally alias an `*mut T`?
+- Struct layout guarantees
+- Interactions around zero-sized types
+  - e.g., what pointer values can legally be considered a `Box<ZST>`?
+- Allocator dependencies
+
+One specific area that we can hopefully "outsource" is detailed rules
+regarding the interaction of different threads. Rust exposes atomics
+that roughly correspond to C++11 atomics, and the intention is that we
+can layer our rules for sequential execution atop those rules for
+parallel execution.
+
+## Time frame
+
+Working out a a set of rules for unsafe code is a detailed process and
+is expected to take months (or longer, depending on the level of
+detail we ultimately aim for). However, the intention is to publish
+preliminary documents as RFCs as we go, so hopefully we can be
+providing ever more specific guidance for unsafe code authors.
+
+Note that even once an initial set of guidelines is adopted, problems
+or inconsistencies may be found. If that happens, the guidelines will
+be adjusted as needed to correct the problem, naturally with an eye
+towards backwards compatibility. In other words, the unsafe
+guidelines, like the rules for Rust language itself, should be
+considered a "living document".
+
+As a note of caution, experience from other languages such as Java or
+C++ suggests that the work on memory models can take years. Moreover,
+even once a memory model is adopted, it can be unclear whether
+[common compiler optimizations are actually permitted](http://www.di.ens.fr/~zappa/readings/c11comp.pdf)
+under the model. The hope is that by focusing on sequential and
+Rust-specific issues we can sidestep some of these quandries.
+
+## Intermediate documents
+
+Because hammering out the finer points of the memory model is expected
+to possibly take some time, it is important to produce intermediate
+agreements. This section describes some of the documents that may be
+useful. These also serve as a rough guideline to the overall "phases"
+of discussion that are expected, though in practice discussion will
+likely go back and forth:
+
+- **Key examples and optimizations**: highlighting code examples that
+  ought to work, or optimizations we should be able to do, as well as
+  some that will not work, or those whose outcome is in doubt.
+- **High-level design**: describe the rules at a high-level. This
+  would likely be the document that unsafe code authors would read to
+  know if their code is correct in the majority of scenarios. Think of
+  this as the "user's guide".
+- **Detailed rules**: More comprehensive rules. Think of this as the
+  "reference manual".
+  
+Note that both the "high-level design" and "detailed rules", once
+considered complete, will be submitted as RFCs and undergo the usual
+final comment period.
+
+### Key examples and optimizations
+
+Probably a good first step is to agree on some key examples and
+overall principles. Examples would fall into several categories:
+
+- Unsafe code that we feel **must** be considered **legal** by any model
+- Unsafe code that we feel **must** be considered **illegal** by any model
+- Unsafe code that we feel **may or may not** be considered legal
+- Optimizations that we **must** be able to perform
+- Optimizations that we **should not** expect to be able to perform
+- Optimizations that it would be nice to have, but which may be sacrificed
+  if needed
+
+Having such guiding examples naturally helps to steer the effort, but
+it also helps to provide guidance for unsafe code authors in the
+meantime. These examples illustrate patterns that one can adopt with
+reasonable confidence.
+
+Deciding about these examples should also help in enumerating the
+guiding principles we would like to adhere to. The design of a memory
+model ultimately requires balancing several competing factors and it
+may be useful to state our expectations up front on how these will be
+weighed:
+
+- **Optimization.** The stricter the rules, the more we can optimize.
+  - on the other hand, rules that are overly strict may prevent people
+    from writing unsafe code that they would like to write, ultimately
+    leading to slower exeution.
+- **Comprehensibility.** It is important to strive for rules that end
+  users can readily understand. If learning the rules requires diving
+  into academic papers or using Coq, it's a non-starter.
+- **Effect on existing code.** No matter what model we adopt, existing
+  unsafe code may or may not comply. If we then proceed to optimize,
+  this could cause running code to stop working. While
+  [RFC 1122](https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md)
+  explicitly specified that the rules for unsafe code may change, we
+  will have to decide where to draw the line in terms of how much to
+  weight backwards compatibility.
+
+It is expected that the lang team will be **highly involved** in this discussion.
+
+It is also expected that we will gather examples in the following ways:
+
+- survey existing unsafe code;
+- solicit suggestions of patterns from the Rust-using public:
+  - scenarios where they would like an official judgement; 
+  - interesting questions involving the standard library.
+
+### High-level design
+
+The next document to produce is to settle on a high-level
+design. There have already been several approaches floated. This phase
+should build on the examples from before, in that proposals can be
+weighed against their effect on the examples and optimizations.
+
+There will likely also be some feedback between this phase and the
+previosu: as new proposals are considered, that may generate new
+examples that were not relevant previously.
+
+Note that even once a high-level design is adopted, it will be
+considered "tentative" and "unstable" until the detailed rules have
+been worked out to a reasonable level of confidence.
+
+Once a high-level design is adopted, it may also be used by the
+compiler team to inform which optimizations are legal or illegal.
+However, if changes are later made, the compiler will naturally have
+to be adjusted to match.
+
+It is expected that the lang team will be **highly involved** in this discussion.
+
+### Detailed rules
+
+Once we've settled on a high-level path -- and, no doubt, while in the
+process of doing so as well -- we can begin to enumerate more detailed
+rules. It is also expected that working out the rules may uncover
+contradictions or other problems that require revisiting the
+high-level design.
+
+### Lints and other checkers
+
+Ideally, the team will also consider whether automated checking for
+conformance is possible. It is not a responsibility of this strike
+team to produce such automated checking, but automated checking is
+naturally a big plus!
+
+## Repository
+
+In general, the memory model discussion will be centered on a specific
+repository (perhaps
+<https://github.com/nikomatsakis/rust-memory-model>, but perhaps moved
+to the rust-lang organization). This allows for multi-faced
+discussion: for example, we can open issues on particular questions,
+as well as storing the various proposals and litmus tests in their own
+directories. We'll work out and document the procedures and
+conventions here as we go.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The main drawback is that this discussion will require time and energy
+which could be spent elsewhere. The justification for spending time on
+developing the memory model instead is that it is crucial to enable
+the compiler to perform aggressive optimizations. Until now, we've
+limited ourselves by and large to conservative optimizations (though
+we do supply some LLVM aliasing hints that can be affected by unsafe
+code). As the transition to MIR comes to fruition, it is clear that we
+will be in a place to perform more aggressive optimization, and hence
+the need for rules and guidelines is becoming more acute. We can
+continue to adopt a conservative course, but this risks growing an
+ever larger body of code dependent on the compiler not performing
+aggressive optimization, which may close those doors forever.
+
+# Alternatives
+[alternatives]: #alternatives
+
+- Adopt a memory model in one fell swoop:
+  - considered too complicated
+- Defer adopting a memory model for longer:
+  - considered too risky
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.