Note that we sort of made a "new" kind of unsafe with the UnwindSafe trait: https://doc.rust-lang.org/std/panic/trait.UnwindSafe.html

That's probably how we intend to solve these kinds of problem in the future.

Re: aliasing -- if it's a serious enough problem, one of two things will happen:

* Someone will develop a version of asan/ubsan for Rust.

* The Rust devs will be forced to reduce the extent to which they apply alias analysis by default (possibly with a flag to opt into it). At least temporarily.

The rust devs have backed off optimizations in the past when they break stuff in the ecosystem (struct layout optimization). But they also work with the affected devs to fix those bugs so they can turn the optimization on.

This already happened (japaric [1]). But ASan won't save you from a bug due to optimization-because-I-assumed-these-locations-dont-alias (maybe TSan might?).

[1] https://users.rust-lang.org/t/howto-sanitize-your-rust-code/...

https://news.ycombinator.com/item?id=14553679

It is definitely an important missing piece to bridge the gap to ASan/TSan, but it's still just a proposal/work in progress, not least because, AIUI, the precise rules it needs to enforce aren't yet entirely clear.

You are correct: it's possible to write nefarious code inside an 'unsafe' block then only suffer its effects outside of it, and Rust has to document that fact. The Nomicon, mentioned in the blog post a whole bunch, points this out early on:

> 'unsafe' does more than pollute a whole function: it pollutes a whole module. Generally, the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy.

[ https://doc.rust-lang.org/nightly/nomicon/working-with-unsaf... ]

I sometimes feel the same way, but remember that the `unsafe` keyword only unlocks four additional features:

1. Dereferencing a raw pointer

2. Calling an unsafe function or method

3. Accessing or modifying a mutable static variable (and this might conceivably even be removed entirely someday)

4. Implementing an unsafe trait

It's unclear to me how to make this any more fine-grained such that annotating the "kind" of unsafe you're using would be useful and enforceable by the compiler (which is crucial, because otherwise why not just use a comment?).

In practice I think really the only "distinction" in unsafe Rust that I want is the ability to distinguish unsafe blocks that exist only to call external C code.

Mutable static variables removed or the unsafety of accessing them? Didn't Rust, at one early point, not allow mutable global variables?

As part of the unsafe code guidelines effort, there's been talk of adding an 'unsafe checker' mode to rustc, analogous to valgrind or Clang's AddressSanitizer, which would alter code generation to add checks that could catch many classes of incorrect behavior at runtime. (This would have a high performance cost and would be intended as a debugging tool.) One of the things it would probably do is keep a global map of all live references, and complain if references are created that break the rules, e.g. a mutable reference is created to something that already has a (mutable or immutable) reference somewhere else in the program. Thus it could catch the kind of bug you mentioned.

Of course, you would have to remember to run the checker, and as a dynamic rather than static analysis it would only catch errors that are actually exhibited at runtime (so it probably wouldn't catch the MutexGuard example from the original blog post, unless there was some real code that raced on a MutexGuard). Still, in practice it should help a lot with ensuring that unsafe code doesn't break the rules.

Edit: Niko talked about this in a blog post in February. He proposes a somewhat more complex tracking system than the global list of references I mentioned:

http://smallcultfollowing.com/babysteps/blog/2017/02/01/unsa...

if implemented it would probably outperform haskell (given that monad transformers in haskell have runtime overhead, unfortunately).