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update aliasing rules section of the reference #1290

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merged 5 commits into from
Nov 25, 2022
Merged

update aliasing rules section of the reference #1290

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66754d6
update aliasing rules section of the reference
RalfJung Oct 31, 2022
7110c8e
add back link
RalfJung Oct 31, 2022
3657967
clarify liveness
RalfJung Oct 31, 2022
e05229d
clarify "upper bound"
RalfJung Nov 1, 2022
2af818d
forward reference to 'dangling'
RalfJung Nov 1, 2022
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11 changes: 9 additions & 2 deletions src/behavior-considered-undefined.md
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Expand Up @@ -26,8 +26,15 @@ code.
* Evaluating a [dereference expression] (`*expr`) on a raw pointer that is
[dangling] or unaligned, even in [place expression context]
(e.g. `addr_of!(&*expr)`).
* Breaking the [pointer aliasing rules]. `&mut T` and `&T` follow LLVM’s scoped
[noalias] model, except if the `&T` contains an [`UnsafeCell<U>`].
* Breaking the [pointer aliasing rules]. `Box<T>`, `&mut T` and `&T` follow LLVM’s
scoped [noalias] model, except if the `&T` contains an [`UnsafeCell<U>`].
References must not be dangling while they are live. (The exact liveness
duration is not specified, but it is certainly upper-bounded by the syntactic
lifetime assigned by the borrow checker. When a reference is passed to a
function, it is live at least as long as that function call, again except if
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... Does this apply to Box? (The intent is obviously no, but what about for what we're telling LLVM?)

Because it's obviously safe to deallocate a Box allocation within a function, 
fn example(x: Box<Data>) {
    drop(x);
    // here -- is `x` still considered `dereferencable`?
}

does this mean that Box needs to be marked with the hypothetical LLVM-dereferenceable_on_entry instead of just LLVM-dereferenceable? I suspect this is a nonissue in practice (memory allocation is either a very side-effectful built-in or an opaque environment call preventing code motion over it) but it's definitely relevant.

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@RalfJung RalfJung Oct 31, 2022 •
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See rust-lang/rust#66600 (linked in the PR description) :)

But I will clarify the text, thanks!

the `&T` contains an [`UnsafeCell<U>`].) All this also applies when values of
these types are passed in a (nested) field of a compound type, but not behind
pointer indirections.
* Mutating immutable data. All data inside a [`const`] item is immutable. Moreover, all
data reached through a shared reference or data owned by an immutable binding
is immutable, unless that data is contained within an [`UnsafeCell<U>`].
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