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Sep 21, 2024
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Improve IntoBytes derive error message #1713

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36 changes: 31 additions & 5 deletions src/util/macro_util.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -285,6 +285,32 @@ mod size_to_tag {
#[doc(hidden)]
pub type SizeToTag<const SIZE: usize> = <() as size_to_tag::SizeToTag<SIZE>>::Tag;

// We put `Sized` in its own module so it can have the same name as the standard
// library `Sized` without shadowing it in the parent module.
#[cfg(zerocopy_diagnostic_on_unimplemented)]
mod __size_of {
#[diagnostic::on_unimplemented(
message = "`{Self}` is unsized",
label = "`IntoBytes` needs all field types to be `Sized` in order to determine whether there is inter-field padding",
note = "consider using `#[repr(packed)]` to remove inter-field padding",
note = "`IntoBytes` does not require the fields of `#[repr(packed)]` types to be `Sized`"
)]
pub trait Sized: core::marker::Sized {}
impl<T: core::marker::Sized> Sized for T {}

#[inline(always)]
#[must_use]
#[allow(clippy::needless_maybe_sized)]
pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
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This is such a cursed bound.

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I just stumbled across this randomly: why is that bound there? Wouldn't any T automatically also be core::mem::Sized due to the trait bound of __size_of::Sized? Thanks!

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From a correctness perspective, you're right. However, without ?core::marker::Sized, when Rust encounters a type that is unsized that is passed to this size_of, it emits a Sized error first and bails, so it never gets to the point of discovering that the Sized bound (our bespoke Sized trait) isn't satisfied, and so it never emits our custom diagnostic. Adding ?core::marker::Sized "uncovers" our unsatisfied trait bound so that the desired diagnostic actually gets emitted.

It is, as all things in proc macro land, mildly cursed 😛

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@joshlf thanks for the explanation!

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np!

core::mem::size_of::<T>()
}
}

#[cfg(zerocopy_diagnostic_on_unimplemented)]
pub use __size_of::size_of;
#[cfg(not(zerocopy_diagnostic_on_unimplemented))]
pub use core::mem::size_of;

/// Does the struct type `$t` have padding?
///
/// `$ts` is the list of the type of every field in `$t`. `$t` must be a
Expand All @@ -301,7 +327,7 @@ pub type SizeToTag<const SIZE: usize> = <() as size_to_tag::SizeToTag<SIZE>>::Ta
#[macro_export]
macro_rules! struct_has_padding {
($t:ty, [$($ts:ty),*]) => {
::zerocopy::util::macro_util::core_reexport::mem::size_of::<$t>() > 0 $(+ ::zerocopy::util::macro_util::core_reexport::mem::size_of::<$ts>())*
::zerocopy::util::macro_util::size_of::<$t>() > 0 $(+ ::zerocopy::util::macro_util::size_of::<$ts>())*
};
}

Expand All @@ -321,7 +347,7 @@ macro_rules! struct_has_padding {
#[macro_export]
macro_rules! union_has_padding {
($t:ty, [$($ts:ty),*]) => {
false $(|| ::zerocopy::util::macro_util::core_reexport::mem::size_of::<$t>() != ::zerocopy::util::macro_util::core_reexport::mem::size_of::<$ts>())*
false $(|| ::zerocopy::util::macro_util::size_of::<$t>() != ::zerocopy::util::macro_util::size_of::<$ts>())*
};
}

Expand All @@ -346,10 +372,10 @@ macro_rules! union_has_padding {
macro_rules! enum_has_padding {
($t:ty, $disc:ty, $([$($ts:ty),*]),*) => {
false $(
|| ::zerocopy::util::macro_util::core_reexport::mem::size_of::<$t>()
|| ::zerocopy::util::macro_util::size_of::<$t>()
!= (
::zerocopy::util::macro_util::core_reexport::mem::size_of::<$disc>()
$(+ ::zerocopy::util::macro_util::core_reexport::mem::size_of::<$ts>())*
::zerocopy::util::macro_util::size_of::<$disc>()
$(+ ::zerocopy::util::macro_util::size_of::<$ts>())*
)
)*
}
Expand Down
56 changes: 44 additions & 12 deletions zerocopy-derive/tests/ui-msrv/struct.stderr
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Original file line number Diff line number Diff line change
@@ -1,37 +1,37 @@
error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-msrv/struct.rs:128:11
--> tests/ui-msrv/struct.rs:137:11
|
128 | #[repr(C, align(2))]
137 | #[repr(C, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-msrv/struct.rs:132:21
--> tests/ui-msrv/struct.rs:141:21
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-msrv/struct.rs:138:16
--> tests/ui-msrv/struct.rs:147:16
|
138 | #[repr(packed, align(2))]
147 | #[repr(packed, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-msrv/struct.rs:142:18
--> tests/ui-msrv/struct.rs:151:18
|
142 | #[repr(align(1), align(2))]
151 | #[repr(align(1), align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-msrv/struct.rs:146:8
--> tests/ui-msrv/struct.rs:155:8
|
146 | #[repr(align(2), align(4))]
155 | #[repr(align(2), align(4))]
| ^^^^^^^^

error[E0692]: transparent struct cannot have other repr hints
--> tests/ui-msrv/struct.rs:132:8
--> tests/ui-msrv/struct.rs:141:8
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^^^^ ^^^^^^^^

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
Expand Down Expand Up @@ -131,3 +131,35 @@ error[E0277]: the trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is
<HasPadding<T, VALUE> as ShouldBe<VALUE>>
= help: see issue #48214
= note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
--> tests/ui-msrv/struct.rs:125:10
|
125 | #[derive(IntoBytes)]
| ^^^^^^^^^ doesn't have a size known at compile-time
|
= help: within `IntoBytes4`, the trait `Sized` is not implemented for `[u8]`
note: required because it appears within the type `IntoBytes4`
--> tests/ui-msrv/struct.rs:127:8
|
127 | struct IntoBytes4 {
| ^^^^^^^^^^
note: required by a bound in `std::mem::size_of`
--> $RUST/core/src/mem/mod.rs
|
| pub const fn size_of<T>() -> usize {
| ^ required by this bound in `std::mem::size_of`
= note: this error originates in the macro `::zerocopy::struct_has_padding` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
--> tests/ui-msrv/struct.rs:129:8
|
129 | b: [u8],
| ^^^^ doesn't have a size known at compile-time
|
= help: the trait `Sized` is not implemented for `[u8]`
note: required by a bound in `std::mem::size_of`
--> $RUST/core/src/mem/mod.rs
|
| pub const fn size_of<T>() -> usize {
| ^ required by this bound in `std::mem::size_of`
9 changes: 9 additions & 0 deletions zerocopy-derive/tests/ui-nightly/struct.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -120,6 +120,15 @@ struct IntoBytes3 {
bar: u64,
}

// NOTE(#1708): This exists to ensure that our error messages are good when a
// field is unsized.
#[derive(IntoBytes)]
#[repr(C)]
struct IntoBytes4 {
a: u8,
b: [u8],
}

//
// Unaligned errors
//
Expand Down
63 changes: 49 additions & 14 deletions zerocopy-derive/tests/ui-nightly/struct.stderr
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Original file line number Diff line number Diff line change
@@ -1,37 +1,37 @@
error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-nightly/struct.rs:128:11
--> tests/ui-nightly/struct.rs:137:11
|
128 | #[repr(C, align(2))]
137 | #[repr(C, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-nightly/struct.rs:132:21
--> tests/ui-nightly/struct.rs:141:21
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-nightly/struct.rs:138:16
--> tests/ui-nightly/struct.rs:147:16
|
138 | #[repr(packed, align(2))]
147 | #[repr(packed, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-nightly/struct.rs:142:18
--> tests/ui-nightly/struct.rs:151:18
|
142 | #[repr(align(1), align(2))]
151 | #[repr(align(1), align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-nightly/struct.rs:146:8
--> tests/ui-nightly/struct.rs:155:8
|
146 | #[repr(align(2), align(4))]
155 | #[repr(align(2), align(4))]
| ^^^^^^^^

error[E0692]: transparent struct cannot have other repr hints
--> tests/ui-nightly/struct.rs:132:8
--> tests/ui-nightly/struct.rs:141:8
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^^^^ ^^^^^^^^

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
Expand Down Expand Up @@ -269,8 +269,43 @@ help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
9 + #![feature(trivial_bounds)]
|

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
--> tests/ui-nightly/struct.rs:127:8
|
127 | struct IntoBytes4 {
| ^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: within `IntoBytes4`, the trait `Sized` is not implemented for `[u8]`, which is required by `IntoBytes4: macro_util::__size_of::Sized`
note: required because it appears within the type `IntoBytes4`
--> tests/ui-nightly/struct.rs:127:8
|
127 | struct IntoBytes4 {
| ^^^^^^^^^^
= note: required for `IntoBytes4` to implement `macro_util::__size_of::Sized`
note: required by a bound in `macro_util::__size_of::size_of`
--> $WORKSPACE/src/util/macro_util.rs
|
| pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
| ^^^^^ required by this bound in `size_of`

error[E0277]: `[u8]` is unsized
--> tests/ui-nightly/struct.rs:129:8
|
129 | b: [u8],
| ^^^^ `IntoBytes` needs all field types to be `Sized` in order to determine whether there is inter-field padding
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NICE!

|
= help: the trait `Sized` is not implemented for `[u8]`, which is required by `[u8]: macro_util::__size_of::Sized`
= note: consider using `#[repr(packed)]` to remove inter-field padding
= note: `IntoBytes` does not require the fields of `#[repr(packed)]` types to be `Sized`
= note: required for `[u8]` to implement `macro_util::__size_of::Sized`
note: required by a bound in `macro_util::__size_of::size_of`
--> $WORKSPACE/src/util/macro_util.rs
|
| pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
| ^^^^^ required by this bound in `size_of`

error[E0587]: type has conflicting packed and align representation hints
--> tests/ui-nightly/struct.rs:139:1
--> tests/ui-nightly/struct.rs:148:1
|
139 | struct Unaligned3;
148 | struct Unaligned3;
| ^^^^^^^^^^^^^^^^^
63 changes: 49 additions & 14 deletions zerocopy-derive/tests/ui-stable/struct.stderr
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Original file line number Diff line number Diff line change
@@ -1,37 +1,37 @@
error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-stable/struct.rs:128:11
--> tests/ui-stable/struct.rs:137:11
|
128 | #[repr(C, align(2))]
137 | #[repr(C, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-stable/struct.rs:132:21
--> tests/ui-stable/struct.rs:141:21
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-stable/struct.rs:138:16
--> tests/ui-stable/struct.rs:147:16
|
138 | #[repr(packed, align(2))]
147 | #[repr(packed, align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-stable/struct.rs:142:18
--> tests/ui-stable/struct.rs:151:18
|
142 | #[repr(align(1), align(2))]
151 | #[repr(align(1), align(2))]
| ^^^^^^^^

error: cannot derive Unaligned with repr(align(N > 1))
--> tests/ui-stable/struct.rs:146:8
--> tests/ui-stable/struct.rs:155:8
|
146 | #[repr(align(2), align(4))]
155 | #[repr(align(2), align(4))]
| ^^^^^^^^

error[E0692]: transparent struct cannot have other repr hints
--> tests/ui-stable/struct.rs:132:8
--> tests/ui-stable/struct.rs:141:8
|
132 | #[repr(transparent, align(2))]
141 | #[repr(transparent, align(2))]
| ^^^^^^^^^^^ ^^^^^^^^

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
Expand Down Expand Up @@ -233,8 +233,43 @@ error[E0277]: the trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is
= help: see issue #48214
= note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
--> tests/ui-stable/struct.rs:127:8
|
127 | struct IntoBytes4 {
| ^^^^^^^^^^ doesn't have a size known at compile-time
|
= help: within `IntoBytes4`, the trait `Sized` is not implemented for `[u8]`, which is required by `IntoBytes4: macro_util::__size_of::Sized`
note: required because it appears within the type `IntoBytes4`
--> tests/ui-stable/struct.rs:127:8
|
127 | struct IntoBytes4 {
| ^^^^^^^^^^
= note: required for `IntoBytes4` to implement `macro_util::__size_of::Sized`
note: required by a bound in `macro_util::__size_of::size_of`
--> $WORKSPACE/src/util/macro_util.rs
|
| pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
| ^^^^^ required by this bound in `size_of`

error[E0277]: `[u8]` is unsized
--> tests/ui-stable/struct.rs:129:8
|
129 | b: [u8],
| ^^^^ `IntoBytes` needs all field types to be `Sized` in order to determine whether there is inter-field padding
|
= help: the trait `Sized` is not implemented for `[u8]`, which is required by `[u8]: macro_util::__size_of::Sized`
= note: consider using `#[repr(packed)]` to remove inter-field padding
= note: `IntoBytes` does not require the fields of `#[repr(packed)]` types to be `Sized`
= note: required for `[u8]` to implement `macro_util::__size_of::Sized`
note: required by a bound in `macro_util::__size_of::size_of`
--> $WORKSPACE/src/util/macro_util.rs
|
| pub const fn size_of<T: Sized + ?core::marker::Sized>() -> usize {
| ^^^^^ required by this bound in `size_of`

error[E0587]: type has conflicting packed and align representation hints
--> tests/ui-stable/struct.rs:139:1
--> tests/ui-stable/struct.rs:148:1
|
139 | struct Unaligned3;
148 | struct Unaligned3;
| ^^^^^^^^^^^^^^^^^