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do not reference LLVM in our definition of UB #1750

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Mar 28, 2025
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do not reference LLVM in our definition of UB #1750

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70ae397
do not reference LLVM in our definition of UB
RalfJung Mar 5, 2025
cfd33b0
clarify liveness rules
RalfJung Mar 10, 2025
80e4268
finesse wording
RalfJung Mar 27, 2025
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31 changes: 12 additions & 19 deletions src/behavior-considered-undefined.md
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Original file line number Diff line number Diff line change
@@ -43,22 +43,18 @@ r[undefined.place-projection]
[array/slice index expression][project-slice].

r[undefined.alias]
* 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 and boxes must not be [dangling] while they are
live. The exact liveness duration is not specified, but some bounds exist:
* Breaking the pointer aliasing rules. The exact aliasing rules are not determined yet, but here is an outline of the general principles:
`&T` must point to memory that is not mutated while they are live (except for data inside an [`UnsafeCell<U>`]),
and `&mut T` must point to memory that is not read or written by any pointer not derived from the reference and that no other reference points to while they are live.
`Box<T>` is treated similar to `&'static mut T` for the purpose of these rules.
The exact liveness duration is not specified, but some bounds exist:
* For references, the liveness duration is upper-bounded by the syntactic
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We have a lower-bound too, right? e.g., if you have a use of some reference x at time T1 and then a subsequent use of x (or some reference derived from x) at time T2, then it is live from T1 ..= T2

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(Note that I didn't change this part in this PR.)

Yes, of course each time a reference is dereferenced, it must be live. That is a lower bound.

lifetime assigned by the borrow checker; it cannot be live any *longer* than
that lifetime.
* Each time a reference or box is passed to or returned from a function, it is
considered live.
* When a reference (but not a `Box`!) is passed to a function, it is live at
least as long as that function call, again except if 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.
lifetime assigned by the borrow checker; it cannot be live any *longer* than that lifetime.
* Each time a reference or box is dereferenced or reborrowed, it is considered live.
* Each time a reference or box is passed to or returned from a function, it is considered live.
* When a reference (but not a `Box`!) is passed to a function, it is live at least as long as that function call, again except if 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.

r[undefined.immutable]
* Mutating immutable bytes.
@@ -201,7 +197,7 @@ r[undefined.validity.never]

r[undefined.validity.scalar]
* An integer (`i*`/`u*`), floating point value (`f*`), or raw pointer must be
initialized, i.e., must not be obtained from [uninitialized memory][undef].
initialized, i.e., must not be obtained from uninitialized memory.

r[undefined.validity.str]
* A `str` value is treated like `[u8]`, i.e. it must be initialized.
@@ -248,10 +244,7 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"

[`bool`]: types/boolean.md
[`const`]: items/constant-items.md
[noalias]: http://llvm.org/docs/LangRef.html#noalias
[pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
[abi]: items/external-blocks.md#abi
[undef]: http://llvm.org/docs/LangRef.html#undefined-values
[`target_feature`]: attributes/codegen.md#the-target_feature-attribute
[`UnsafeCell<U>`]: std::cell::UnsafeCell
[Rustonomicon]: ../nomicon/index.html