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clarify when reading uninititalized memory is allowed #852

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Sep 2, 2020
Merged

clarify when reading uninititalized memory is allowed #852

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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 that uninitialized memory is also implicitly invalid for any type that has
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a restricted set of valid values. In other words, the only cases in which
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reading uninitialized memory is permitted is inside `union`s, and between the
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You can probably just minimize the second part to just "padding". We also don't define "compound type" anywhere, so it's probably best not to use it. We do define "user defined type" for struct/enum/union though.

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It's all "types with fields" though that I mean by "compound type" -- so, including tuples and arrays.

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fields of a compound type (in the "padding").

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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