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clarify when reading uninititalized memory is allowed #852

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Merged
merged 4 commits into from
Sep 2, 2020
Merged

clarify when reading uninititalized memory is allowed #852

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2826ff5
clarify when reading uninititalized memory is allowed
RalfJung Jul 17, 2020
2275cde
Apply suggestions from code review
RalfJung Sep 1, 2020
26211c4
explain 'padding' differently
RalfJung Sep 1, 2020
8aa6f0f
fix grammar
RalfJung Sep 2, 2020
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5 changes: 5 additions & 0 deletions src/behavior-considered-undefined.md
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Expand Up @@ -57,6 +57,11 @@ code.
> **Note**: `rustc` achieves this with the unstable
> `rustc_layout_scalar_valid_range_*` attributes.

**Note:** Uninitialized memory is also implicitly invalid for any type that has
a restricted set of valid values. In other words, the only cases in which
reading uninitialized memory is permitted are inside `union`s and in "padding"
(the gaps between the fields/elements of a type).

A reference/pointer is "dangling" if it is null or not all of the bytes it
points to are part of the same allocation (so in particular they all have to be
part of *some* allocation). The span of bytes it points to is determined by the
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