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Fix some blatant errors in ffi guide #16426

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Fix some blatant errors in ffi guide #16426

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4 changes: 2 additions & 2 deletions src/doc/guide-ffi.md
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Expand Up @@ -510,7 +510,7 @@ Rust guarantees that the layout of a `struct` is compatible with the platform's
A `#[packed]` attribute is available, which will lay out the struct members without padding.
However, there are currently no guarantees about the layout of an `enum`.

Rust's owned and managed boxes use non-nullable pointers as handles which point to the contained
Rust's boxes use non-nullable pointers as handles which point to the contained
object. However, they should not be manually created because they are managed by internal
allocators. References can safely be assumed to be non-nullable pointers directly to the
type. However, breaking the borrow checking or mutability rules is not guaranteed to be safe, so
Expand All @@ -527,7 +527,7 @@ the `libc` module, and Rust links against `libc` and `libm` by default.
# The "nullable pointer optimization"

Certain types are defined to not be `null`. This includes references (`&T`,
`&mut T`), owning pointers (`~T`), and function pointers (`extern "abi"
`&mut T`), boxes (`Box<T>`), and function pointers (`extern "abi"
fn()`). When interfacing with C, pointers that might be null are often used.
As a special case, a generic `enum` that contains exactly two variants, one of
which contains no data and the other containing a single field, is eligible
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