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Update nomicon on drop flags #38555

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Update nomicon on drop flags #38555

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17 changes: 8 additions & 9 deletions src/doc/nomicon/drop-flags.md
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Expand Up @@ -79,15 +79,14 @@ if condition {
}
```

As of Rust 1.0, the drop flags are actually not-so-secretly stashed in a hidden
field of any type that implements Drop. Rust sets the drop flag by overwriting
the entire value with a particular bit pattern. This is pretty obviously Not
The Fastest and causes a bunch of trouble with optimizing code. It's legacy from
a time when you could do much more complex conditional initialization.

As such work is currently under way to move the flags out onto the stack frame
where they more reasonably belong. Unfortunately, this work will take some time
as it requires fairly substantial changes to the compiler.
In Rust 1.0, the drop flags were actually not-so-secretly stashed in a hidden
field of any type that implements Drop. Rust set the drop flag by overwriting
the entire value with a particular bit pattern. This was pretty obviously Not
The Fastest and caused a bunch of trouble with optimizing code. It was legacy
from a time when you could do much more complex conditional initialization.

Since Rust 1.13, the flags has been moved to the stack frame where they more
reasonably belong, and types implementing Drop no more take up any extra space.
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@pnkfelix pnkfelix Jan 3, 2017 •
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nit: this should say "no longer take up any ...", not "no more take up any ..."


Regardless, Rust programs don't need to worry about uninitialized values on
the stack for correctness. Although they might care for performance. Thankfully,
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