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@ -15,7 +15,7 @@ reinterpreting values as a different type.
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We strongly recommend using [rust-bindgen][] and/or [cbindgen][] to manage your FFI
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We strongly recommend using [rust-bindgen][] and/or [cbindgen][] to manage your FFI
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boundaries for you. The Rust team works closely with those projects to ensure
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boundaries for you. The Rust team works closely with those projects to ensure
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that they work robustly and are compatible with current and future guarantees
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that they work robustly and are compatible with current and future guarantees
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about type layouts and reprs.
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about type layouts and `repr`s.
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The interaction of `repr(C)` with Rust's more exotic data layout features must be
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The interaction of `repr(C)` with Rust's more exotic data layout features must be
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kept in mind. Due to its dual purpose as "for FFI" and "for layout control",
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kept in mind. Due to its dual purpose as "for FFI" and "for layout control",
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@ -92,7 +92,7 @@ manipulate its tag and fields. See [the RFC][really-tagged] for details.
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Adding an explicit `repr` to an enum suppresses the null-pointer
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Adding an explicit `repr` to an enum suppresses the null-pointer
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optimization.
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optimization.
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These reprs have no effect on a struct.
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These `repr`s have no effect on a struct.
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## repr(packed)
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## repr(packed)
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@ -107,7 +107,8 @@ compiler might be able to paper over alignment issues with shifts and masks.
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However if you take a reference to a packed field, it's unlikely that the
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However if you take a reference to a packed field, it's unlikely that the
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compiler will be able to emit code to avoid an unaligned load.
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compiler will be able to emit code to avoid an unaligned load.
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**[As of Rust 2018, this still can cause undefined behavior.][ub loads]**
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[As this can cause undefined behavior][ub loads], the lint has been implemented
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and it will become a hard error.
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`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
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`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
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this should not be used.
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this should not be used.
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