[derive] Improve IntoBytes error messages (#1712)

On Rust toolchains 1.78.0 or later, use
`#[diagnostic::on_unimplemented]` to improve the error message emitted
when `IntoBytes` is derived on a type with inter-field padding.

src/lib.rs

@@ -3852,25 +3852,26 @@ fn mut_from_prefix_suffix<T: FromBytes + KnownLayout + ?Sized>(
 ///
 /// # Error Messages
 ///
-/// Due to the way that the custom derive for `IntoBytes` is implemented, you
-/// may get an error like this:
+/// On Rust toolchains prior to 1.78.0, due to the way that the custom derive
+/// for `IntoBytes` is implemented, you may get an error like this:
 ///
 /// ```text
-/// error[E0277]: the trait bound `HasPadding<Foo, true>: ShouldBe<false>` is not satisfied
+/// error[E0277]: the trait bound `(): PaddingFree<Foo, true>` is not satisfied
 ///   --> lib.rs:23:10
 ///    |
 ///  1 | #[derive(IntoBytes)]
-///    |          ^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<Foo, true>`
+///    |          ^^^^^^^^^ the trait `PaddingFree<Foo, true>` is not implemented for `()`
 ///    |
-///    = help: the trait `ShouldBe<VALUE>` is implemented for `HasPadding<T, VALUE>`
+///    = help: the following implementations were found:
+///                   <() as PaddingFree<T, false>>
 /// ```
 ///
 /// This error indicates that the type being annotated has padding bytes, which
 /// is illegal for `IntoBytes` types. Consider reducing the alignment of some
-/// fields by using types in the [`byteorder`] module, adding explicit struct
-/// fields where those padding bytes would be, or using `#[repr(packed)]`. See
-/// the Rust Reference's page on [type layout] for more information about type
-/// layout and padding.
+/// fields by using types in the [`byteorder`] module, wrapping field types in
+/// [`Unalign`], adding explicit struct fields where those padding bytes would
+/// be, or using `#[repr(packed)]`. See the Rust Reference's page on [type
+/// layout] for more information about type layout and padding.
 ///
 /// [type layout]: https://doc.rust-lang.org/reference/type-layout.html
 ///

src/util/macro_util.rs

@@ -17,10 +17,7 @@
 
 #![allow(missing_debug_implementations)]
 
-use core::{
-    marker::PhantomData,
-    mem::{self, ManuallyDrop},
-};
+use core::mem::{self, ManuallyDrop};
 
 // TODO(#29), TODO(https://github.com/rust-lang/rust/issues/69835): Remove this
 // `cfg` when `size_of_val_raw` is stabilized.
@@ -35,13 +32,18 @@ use crate::{
     Immutable, IntoBytes, Ptr, TryFromBytes, Unalign, ValidityError,
 };
 
-/// A compile-time check that should be one particular value.
-pub trait ShouldBe<const VALUE: bool> {}
-
-/// A struct for checking whether `T` contains padding.
-pub struct HasPadding<T: ?Sized, const VALUE: bool>(PhantomData<T>);
-
-impl<T: ?Sized, const VALUE: bool> ShouldBe<VALUE> for HasPadding<T, VALUE> {}
+#[cfg_attr(
+    zerocopy_diagnostic_on_unimplemented,
+    diagnostic::on_unimplemented(
+        message = "`{T}` has inter-field padding",
+        label = "types with padding cannot implement `IntoBytes`",
+        note = "consider using `zerocopy::Unalign` to lower the alignment of individual fields",
+        note = "consider adding explicit fields where padding would be",
+        note = "consider using `#[repr(packed)]` to remove inter-field padding"
+    )
+)]
+pub trait PaddingFree<T: ?Sized, const HAS_PADDING: bool> {}
+impl<T: ?Sized> PaddingFree<T, false> for () {}
 
 /// A type whose size is equal to `align_of::<T>()`.
 #[repr(C)]

zerocopy-derive/Cargo.toml

@@ -28,7 +28,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0.1"
 quote = "1.0.10"
-syn = "2.0.46"
+syn = { version = "2.0.46", features = ["full"] }
 
 [dev-dependencies]
 dissimilar = "1.0.9"

zerocopy-derive/src/lib.rs

@@ -36,7 +36,6 @@ mod repr;
 
 use proc_macro2::{TokenStream, TokenTree};
 use quote::ToTokens;
-use syn::{punctuated::Punctuated, token::Colon, PredicateType};
 
 use {
     proc_macro2::Span,
@@ -1222,27 +1221,15 @@ fn impl_block<D: DataExt>(
             let validator_context = check.validator_macro_context();
             let validator_macro = check.validator_macro_ident();
             let t = tag.iter();
-            // We have to manually construct a bound here because the validator
-            // context may include type definitions and syn cannot parse type
-            // definitions in const exprs without the `full` feature enabled.
-            // Constructing these bounds "verbatim" gets around this issue.
-            let bounded_ty = Type::Verbatim(quote! {
-                ::zerocopy::util::macro_util::HasPadding<
+            parse_quote! {
+                (): ::zerocopy::util::macro_util::PaddingFree<
                     #type_ident,
                     {
                         #validator_context
                         ::zerocopy::#validator_macro!(#type_ident, #(#t,)* #(#variant_types),*)
                     }
                 >
-            });
-            let mut bounds = Punctuated::new();
-            bounds.push(parse_quote! { ::zerocopy::util::macro_util::ShouldBe<false> });
-            WherePredicate::Type(PredicateType {
-                lifetimes: None,
-                bounded_ty,
-                colon_token: Colon::default(),
-                bounds,
-            })
+            }
         });
 
     let self_bounds: Option<WherePredicate> = match self_type_trait_bounds {

zerocopy-derive/src/output_tests.rs

@@ -234,6 +234,29 @@ fn test_into_bytes() {
             }
         } no_build
     }
+
+    test! {
+        IntoBytes {
+            #[repr(C)]
+            struct Foo {
+                a: u8,
+                b: u8,
+            }
+        } expands to {
+            #[allow(deprecated)]
+            unsafe impl ::zerocopy::IntoBytes for Foo
+            where
+                u8: ::zerocopy::IntoBytes,
+                u8: ::zerocopy::IntoBytes,
+                (): ::zerocopy::util::macro_util::PaddingFree<
+                    Foo,
+                    { ::zerocopy::struct_has_padding!(Foo, [u8, u8]) },
+                >,
+            {
+                fn only_derive_is_allowed_to_implement_this_trait() {}
+            }
+        } no_build
+    }
 }
 
 #[test]

zerocopy-derive/tests/ui-msrv/enum.stderr

@@ -343,35 +343,35 @@ error[E0277]: the trait bound `bool: FromBytes` is not satisfied
     = help: see issue #48214
     = note: this error originates in the derive macro `FromBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `HasPadding<IntoBytes1, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes1, true>` is not satisfied
    --> tests/ui-msrv/enum.rs:533:10
     |
 533 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes1, true>`
+    |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes1, true>` is not implemented for `()`
     |
     = help: the following implementations were found:
-              <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+              <() as PaddingFree<T, false>>
     = 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 trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes2, true>` is not satisfied
    --> tests/ui-msrv/enum.rs:544:10
     |
 544 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+    |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
     |
     = help: the following implementations were found:
-              <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+              <() as PaddingFree<T, false>>
     = 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 trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes3, true>` is not satisfied
    --> tests/ui-msrv/enum.rs:550:10
     |
 550 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes3, true>`
+    |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes3, true>` is not implemented for `()`
     |
     = help: the following implementations were found:
-              <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+              <() as PaddingFree<T, false>>
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

zerocopy-derive/tests/ui-msrv/struct.stderr

@@ -110,25 +110,25 @@ error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
     = 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 trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes2, true>` is not satisfied
    --> tests/ui-msrv/struct.rs:107:10
     |
 107 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+    |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
     |
     = help: the following implementations were found:
-              <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+              <() as PaddingFree<T, false>>
     = 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 trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes3, true>` is not satisfied
    --> tests/ui-msrv/struct.rs:114:10
     |
 114 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes3, true>`
+    |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes3, true>` is not implemented for `()`
     |
     = help: the following implementations were found:
-              <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+              <() as PaddingFree<T, false>>
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 

zerocopy-derive/tests/ui-msrv/union.stderr

@@ -40,13 +40,13 @@ error[E0277]: the trait bound `UnsafeCell<()>: zerocopy::Immutable` is not satis
    = help: see issue #48214
    = note: this error originates in the derive macro `Immutable` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0277]: the trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: the trait bound `(): PaddingFree<IntoBytes2, true>` is not satisfied
   --> tests/ui-msrv/union.rs:39:10
    |
 39 | #[derive(IntoBytes)]
-   |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+   |          ^^^^^^^^^ the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
    |
    = help: the following implementations were found:
-             <HasPadding<T, VALUE> as ShouldBe<VALUE>>
+             <() as PaddingFree<T, false>>
    = help: see issue #48214
    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)

zerocopy-derive/tests/ui-nightly/enum.stderr

@@ -413,41 +413,53 @@ help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 9   + #![feature(trivial_bounds)]
     |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes1, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes1` has inter-field padding
    --> tests/ui-nightly/enum.rs:533:10
     |
 533 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes1, true>`
+    |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
     |
-    = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes1, true>`
+    = help: the trait `PaddingFree<IntoBytes1, true>` is not implemented for `()`
+    = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+    = note: consider adding explicit fields where padding would be
+    = note: consider using `#[repr(packed)]` to remove inter-field padding
+    = help: the trait `PaddingFree<IntoBytes1, false>` is implemented for `()`
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
     |
 9   + #![feature(trivial_bounds)]
     |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes2` has inter-field padding
    --> tests/ui-nightly/enum.rs:544:10
     |
 544 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+    |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
     |
-    = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes2, true>`
+    = help: the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
+    = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+    = note: consider adding explicit fields where padding would be
+    = note: consider using `#[repr(packed)]` to remove inter-field padding
+    = help: the trait `PaddingFree<IntoBytes2, false>` is implemented for `()`
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
     |
 9   + #![feature(trivial_bounds)]
     |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes3` has inter-field padding
    --> tests/ui-nightly/enum.rs:550:10
     |
 550 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes3, true>`
+    |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
     |
-    = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes3, true>`
+    = help: the trait `PaddingFree<IntoBytes3, true>` is not implemented for `()`
+    = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+    = note: consider adding explicit fields where padding would be
+    = note: consider using `#[repr(packed)]` to remove inter-field padding
+    = help: the trait `PaddingFree<IntoBytes3, false>` is implemented for `()`
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable

zerocopy-derive/tests/ui-nightly/struct.stderr

@@ -241,27 +241,35 @@ help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 9   + #![feature(trivial_bounds)]
     |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes2` has inter-field padding
    --> tests/ui-nightly/struct.rs:107:10
     |
 107 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+    |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
     |
-    = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes2, true>`
+    = help: the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
+    = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+    = note: consider adding explicit fields where padding would be
+    = note: consider using `#[repr(packed)]` to remove inter-field padding
+    = help: the trait `PaddingFree<IntoBytes2, false>` is implemented for `()`
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
     |
 9   + #![feature(trivial_bounds)]
     |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes3, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes3` has inter-field padding
    --> tests/ui-nightly/struct.rs:114:10
     |
 114 | #[derive(IntoBytes)]
-    |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes3, true>`
+    |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
     |
-    = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes3, true>`
+    = help: the trait `PaddingFree<IntoBytes3, true>` is not implemented for `()`
+    = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+    = note: consider adding explicit fields where padding would be
+    = note: consider using `#[repr(packed)]` to remove inter-field padding
+    = help: the trait `PaddingFree<IntoBytes3, false>` is implemented for `()`
     = help: see issue #48214
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable

zerocopy-derive/tests/ui-nightly/union.stderr

@@ -55,13 +55,17 @@ help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 9  + #![feature(trivial_bounds)]
    |
 
-error[E0277]: the trait bound `HasPadding<IntoBytes2, true>: ShouldBe<false>` is not satisfied
+error[E0277]: `IntoBytes2` has inter-field padding
   --> tests/ui-nightly/union.rs:39:10
    |
 39 | #[derive(IntoBytes)]
-   |          ^^^^^^^^^ the trait `ShouldBe<false>` is not implemented for `HasPadding<IntoBytes2, true>`
+   |          ^^^^^^^^^ types with padding cannot implement `IntoBytes`
    |
-   = help: the trait `ShouldBe<true>` is implemented for `HasPadding<IntoBytes2, true>`
+   = help: the trait `PaddingFree<IntoBytes2, true>` is not implemented for `()`
+   = note: consider using `zerocopy::Unalign` to lower the alignment of individual fields
+   = note: consider adding explicit fields where padding would be
+   = note: consider using `#[repr(packed)]` to remove inter-field padding
+   = help: the trait `PaddingFree<IntoBytes2, false>` is implemented for `()`
    = help: see issue #48214
    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: add `#![feature(trivial_bounds)]` to the crate attributes to enable