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rust/tests/ui/derives/deriving-with-repr-packed.stderr

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Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
warning: byte slice in a packed struct that derives a built-in trait
make unaligned_reference a hard error
2022-09-30 07:54:30 -05:00
--> $DIR/deriving-with-repr-packed.rs:31:5
limit packed copy-out to non-generic Copy structs
2017-11-19 09:04:24 -06:00
|
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
LL | #[derive(Debug)]
| ----- in this derive macro expansion
...
LL | data: [u8],
| ^^^^^^^^^^
limit packed copy-out to non-generic Copy structs
2017-11-19 09:04:24 -06:00
|
bless ui tests
2022-09-18 10:55:36 -05:00
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
= note: for more information, see issue #107457 <https://github.com/rust-lang/rust/issues/107457>
= help: consider implementing the trait by hand, or remove the `packed` attribute
= note: `#[warn(byte_slice_in_packed_struct_with_derive)]` on by default
= note: this warning originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
limit packed copy-out to non-generic Copy structs
2017-11-19 09:04:24 -06:00
Extend `BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE`. To temporarily allow a `str` field in a packed struct using `derive`, along with `[u8]`.
2023-02-05 19:58:39 -06:00
warning: string slice in a packed struct that derives a built-in trait
--> $DIR/deriving-with-repr-packed.rs:41:5
|
LL | #[derive(Debug)]
| ----- in this derive macro expansion
...
LL | data: str,
| ^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #107457 <https://github.com/rust-lang/rust/issues/107457>
= help: consider implementing the trait by hand, or remove the `packed` attribute
= note: this warning originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
error[E0507]: cannot move out of `self` which is behind a shared reference
make unaligned_reference a hard error
2022-09-30 07:54:30 -05:00
--> $DIR/deriving-with-repr-packed.rs:22:10
improve error messages
2017-11-26 11:01:19 -06:00
|
Improve error messages involving `derive` and `packed`. There are two errors involving `derive` and `packed`. ``` `#[derive]` can't be derived on a `#[repr(packed)]` struct with type or const parameters `#[derive]` can't be derived on a `#[repr(packed)]` struct that does not derive Copy ``` The second one overstates things. It is possible to use derive on a repr(packed) struct that doesn't derive Copy in two cases. - If all the fields within the struct meet the required alignment: 1 for `repr(packed)`, or `N` for `repr(packed(N))`. - If `Default` is the only trait derived. This commit improves things in a few ways. - Changes the errors to say `$TRAIT can't be derived on this ...`. This is more accurate, because it's just $TRAIT and *this* packed struct that are a problem, not *all* derived traits on *all* packed structs. - Adds more details to the "ERROR" lines in the test case, enough to distinguish between the two error messages. - Adds more cases to the test case that don't cause errors, e.g. `Default` derives. - Uses a wider variety of builtin traits in the test case, for better coverage.
2022-07-21 19:04:37 -05:00
LL | #[derive(Debug, Default)]
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
| ----- in this derive macro expansion
LL | #[repr(packed)]
LL | struct X(Y);
| ^ move occurs because `self.0` has type `Y`, which does not implement the `Copy` trait
improve error messages
2017-11-26 11:01:19 -06:00
|
Improve error messages involving `derive` and `packed`. There are two errors involving `derive` and `packed`. ``` `#[derive]` can't be derived on a `#[repr(packed)]` struct with type or const parameters `#[derive]` can't be derived on a `#[repr(packed)]` struct that does not derive Copy ``` The second one overstates things. It is possible to use derive on a repr(packed) struct that doesn't derive Copy in two cases. - If all the fields within the struct meet the required alignment: 1 for `repr(packed)`, or `N` for `repr(packed(N))`. - If `Default` is the only trait derived. This commit improves things in a few ways. - Changes the errors to say `$TRAIT can't be derived on this ...`. This is more accurate, because it's just $TRAIT and *this* packed struct that are a problem, not *all* derived traits on *all* packed structs. - Adds more details to the "ERROR" lines in the test case, enough to distinguish between the two error messages. - Adds more cases to the test case that don't cause errors, e.g. `Default` derives. - Uses a wider variety of builtin traits in the test case, for better coverage.
2022-07-21 19:04:37 -05:00
= note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
improve error messages
2017-11-26 11:01:19 -06:00
Extend `BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE`. To temporarily allow a `str` field in a packed struct using `derive`, along with `[u8]`.
2023-02-05 19:58:39 -06:00
error: aborting due to previous error; 2 warnings emitted
limit packed copy-out to non-generic Copy structs
2017-11-19 09:04:24 -06:00
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
For more information about this error, try `rustc --explain E0507`.
make unaligned_reference warning visible in future compat report
2022-04-14 20:18:19 -05:00
Future incompatibility report: Future breakage diagnostic:
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
warning: byte slice in a packed struct that derives a built-in trait
make unaligned_reference a hard error
2022-09-30 07:54:30 -05:00
--> $DIR/deriving-with-repr-packed.rs:31:5
make unaligned_reference warning visible in future compat report
2022-04-14 20:18:19 -05:00
|
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
LL | #[derive(Debug)]
| ----- in this derive macro expansion
...
LL | data: [u8],
| ^^^^^^^^^^
make unaligned_reference warning visible in future compat report
2022-04-14 20:18:19 -05:00
|
bless ui tests
2022-09-18 10:55:36 -05:00
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
Allow more deriving on packed structs. Currently, deriving on packed structs has some non-trivial limitations, related to the fact that taking references on unaligned fields is UB. The current approach to field accesses in derived code: - Normal case: `&self.0` - In a packed struct that derives `Copy`: `&{self.0}` - In a packed struct that doesn't derive `Copy`: `&self.0` Plus, we disallow deriving any builtin traits other than `Default` for any packed generic type, because it's possible that there might be misaligned fields. This is a fairly broad restriction. Plus, we disallow deriving any builtin traits other than `Default` for most packed types that don't derive `Copy`. (The exceptions are those where the alignments inherently satisfy the packing, e.g. in a type with `repr(packed(N))` where all the fields have alignments of `N` or less anyway. Such types are pretty strange, because the `packed` attribute is not having any effect.) This commit introduces a new, simpler approach to field accesses: - Normal case: `&self.0` - In a packed struct: `&{self.0}` In the latter case, this requires that all fields impl `Copy`, which is a new restriction. This means that the following example compiles under the old approach and doesn't compile under the new approach. ``` #[derive(Debug)] struct NonCopy(u8); #[derive(Debug) #[repr(packed)] struct MyType(NonCopy); ``` (Note that the old approach's support for cases like this was brittle. Changing the `u8` to a `u16` would be enough to stop it working. So not much capability is lost here.) However, the other constraints from the old rules are removed. We can now derive builtin traits for packed generic structs like this: ``` trait Trait { type A; } #[derive(Hash)] #[repr(packed)] pub struct Foo<T: Trait>(T, T::A); ``` To allow this, we add a `T: Copy` bound in the derived impl and a `T::A: Copy` bound in where clauses. So `T` and `T::A` must impl `Copy`. We can now also derive builtin traits for packed structs that don't derive `Copy`, so long as the fields impl `Copy`: ``` #[derive(Hash)] #[repr(packed)] pub struct Foo(u32); ``` This includes types that hand-impl `Copy` rather than deriving it, such as the following, that show up in winapi-0.2: ``` #[derive(Clone)] #[repr(packed)] struct MyType(i32); impl Copy for MyType {} ``` The new approach is simpler to understand and implement, and it avoids the need for the `unsafe_derive_on_repr_packed` check. One exception is required for backwards-compatibility: we allow `[u8]` fields for now. There is a new lint for this, `byte_slice_in_packed_struct_with_derive`.
2022-11-20 21:40:32 -06:00
= note: for more information, see issue #107457 <https://github.com/rust-lang/rust/issues/107457>
= help: consider implementing the trait by hand, or remove the `packed` attribute
= note: `#[warn(byte_slice_in_packed_struct_with_derive)]` on by default
= note: this warning originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
make unaligned_reference warning visible in future compat report
2022-04-14 20:18:19 -05:00
Extend `BYTE_SLICE_IN_PACKED_STRUCT_WITH_DERIVE`. To temporarily allow a `str` field in a packed struct using `derive`, along with `[u8]`.
2023-02-05 19:58:39 -06:00
Future breakage diagnostic:
warning: string slice in a packed struct that derives a built-in trait
--> $DIR/deriving-with-repr-packed.rs:41:5
|
LL | #[derive(Debug)]
| ----- in this derive macro expansion
...
LL | data: str,
| ^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #107457 <https://github.com/rust-lang/rust/issues/107457>
= help: consider implementing the trait by hand, or remove the `packed` attribute
= note: `#[warn(byte_slice_in_packed_struct_with_derive)]` on by default
= note: this warning originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
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