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Auto merge of #86279 - JohnTitor:transparent-zero-size-fields, r=nikomatsakis

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Permit zero non-zero-field on transparent types

Fixes #77841

This makes the transparent fields meet the below:
> * A `repr(transparent)` type `T` must meet the following rules:
>   * It may have any number of 1-ZST fields
>   * In addition, it may have at most one other field of type U

r? `@nikomatsakis`
This commit is contained in:
bors 2021-06-24 07:29:59 +00:00
commit 456a03227e
5 changed files with 51 additions and 81 deletions
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10
compiler/rustc_error_codes/src/error_codes/E0690.md
View file

@ -1,19 +1,19 @@
A struct with the representation hint `repr(transparent)` had zero or more than
one fields that were not guaranteed to be zero-sized.
A struct with the representation hint `repr(transparent)` had two or more fields
that were not guaranteed to be zero-sized.
Erroneous code example:
```compile_fail,E0690
#[repr(transparent)]
struct LengthWithUnit<U> { // error: transparent struct needs exactly one
struct LengthWithUnit<U> { // error: transparent struct needs at most one
value: f32, // non-zero-sized field, but has 2
unit: U,
}
```
Because transparent structs are represented exactly like one of their fields at
run time, said field must be uniquely determined. If there is no field, or if
there are multiple fields, it is not clear how the struct should be represented.
run time, said field must be uniquely determined. If there are multiple fields,
it is not clear how the struct should be represented.
Note that fields of zero-sized types (e.g., `PhantomData`) can also exist
alongside the field that contains the actual data, they do not count for this
error. When generic types are involved (as in the above example), an error is