[derive] Don't support IntoBytes on repr(Rust) types (#1783)

Closes #1764

zerocopy-derive/src/lib.rs

@@ -769,12 +769,11 @@ fn derive_into_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> Result<Tok
         // no padding unless #[repr(align)] explicitly adds padding, which we
         // check for in this branch's condition.
         (None, false)
-    } else if ast.generics.params.is_empty() {
-        // Since there are no generics, we can emit a padding check. This is
-        // more permissive than the next case, which requires that all field
-        // types implement `Unaligned`.
-        //
-        // TODO(#1764): This is probably unsound! Fix it.
+    } else if is_c && ast.generics.params.is_empty() {
+        // Since there are no generics, we can emit a padding check. `repr(C)`
+        // guarantees that fields won't overlap, so the padding check is sound.
+        // This is more permissive than the next case, which requires that all
+        // field types implement `Unaligned`.
         (Some(PaddingCheck::Struct), false)
     } else if is_c && !repr.is_align_gt_1() {
         // We can't use a padding check since there are generic type arguments.
@@ -787,7 +786,7 @@ fn derive_into_bytes_struct(ast: &DeriveInput, strct: &DataStruct) -> Result<Tok
         // structs without requiring `Unaligned`.
         (None, true)
     } else {
-        return Err(Error::new(Span::call_site(), "must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout"));
+        return Err(Error::new(Span::call_site(), "must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout"));
     };
 
     let field_bounds = if require_unaligned_fields {

zerocopy-derive/tests/struct_to_bytes.rs

@@ -153,7 +153,7 @@ util_assert_impl_all!(WithParams<'static, 'static, u8, 42>: imp::IntoBytes);
 // Test for the failure reported in #1182.
 
 #[derive(imp::IntoBytes)]
-#[repr(packed)]
+#[repr(C, packed)]
 pub struct IndexEntryFlags(u8);
 
 #[derive(imp::IntoBytes)]

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

@@ -4,94 +4,110 @@ error: this conflicts with another representation hint
 133 | #[repr(C, C)] // zerocopy-derive conservatively treats these as conflicting reprs
     |           ^
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-msrv/struct.rs:151:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-msrv/struct.rs:140:10
     |
-151 | #[derive(IntoBytes)]
+140 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-msrv/struct.rs:156:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-msrv/struct.rs:150:10
     |
-156 | #[derive(IntoBytes)]
+150 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-msrv/struct.rs:159:10
+    |
+159 | #[derive(IntoBytes)]
+    |          ^^^^^^^^^
+    |
+    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
    --> tests/ui-msrv/struct.rs:164:10
     |
 164 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-msrv/struct.rs:187:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-msrv/struct.rs:172:10
+    |
+172 | #[derive(IntoBytes)]
+    |          ^^^^^^^^^
+    |
+    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-msrv/struct.rs:195:10
     |
-187 | #[derive(IntoBytes)]
+195 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: cannot derive `Unaligned` on type with alignment greater than 1
-   --> tests/ui-msrv/struct.rs:198:11
+   --> tests/ui-msrv/struct.rs:206:11
     |
-198 | #[repr(C, align(2))]
+206 | #[repr(C, align(2))]
     |           ^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-msrv/struct.rs:202:8
+   --> tests/ui-msrv/struct.rs:210:8
     |
-202 | #[repr(transparent, align(2))]
+210 | #[repr(transparent, align(2))]
     |        ^^^^^^^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-msrv/struct.rs:208:16
+   --> tests/ui-msrv/struct.rs:216:16
     |
-208 | #[repr(packed, align(2))]
+216 | #[repr(packed, align(2))]
     |                ^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-msrv/struct.rs:212:18
+   --> tests/ui-msrv/struct.rs:220:18
     |
-212 | #[repr(align(1), align(2))]
+220 | #[repr(align(1), align(2))]
     |                  ^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-msrv/struct.rs:216:18
+   --> tests/ui-msrv/struct.rs:224:18
     |
-216 | #[repr(align(2), align(4))]
+224 | #[repr(align(2), align(4))]
     |                  ^^^^^
 
 error: must have #[repr(C)], #[repr(transparent)], or #[repr(packed)] attribute in order to guarantee this type's alignment
-   --> tests/ui-msrv/struct.rs:219:10
+   --> tests/ui-msrv/struct.rs:227:10
     |
-219 | #[derive(Unaligned)]
+227 | #[derive(Unaligned)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: must have #[repr(C)], #[repr(transparent)], or #[repr(packed)] attribute in order to guarantee this type's alignment
-   --> tests/ui-msrv/struct.rs:222:10
+   --> tests/ui-msrv/struct.rs:230:10
     |
-222 | #[derive(Unaligned)]
+230 | #[derive(Unaligned)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: this conflicts with another representation hint
-   --> tests/ui-msrv/struct.rs:232:8
+   --> tests/ui-msrv/struct.rs:240:8
     |
-232 | #[repr(C, packed(2))]
+240 | #[repr(C, packed(2))]
     |        ^
 
 error[E0692]: transparent struct cannot have other repr hints
-   --> tests/ui-msrv/struct.rs:202:8
+   --> tests/ui-msrv/struct.rs:210:8
     |
-202 | #[repr(transparent, align(2))]
+210 | #[repr(transparent, align(2))]
     |        ^^^^^^^^^^^  ^^^^^^^^
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time

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

@@ -135,17 +135,25 @@ struct IntoBytes5 {
     a: u8,
 }
 
-// TODO(#1764): This currently compiles, but maybe it shouldn't.
+// We don't emit a padding check unless there's a repr that can guarantee that
+// fields don't overlap.
 #[derive(IntoBytes)]
 struct IntoBytes6 {
     a: u8,
+    // Add a second field to avoid triggering the "repr(C) struct with one
+    // field" special case.
+    b: u8,
 }
 
-// TODO(#1764): This currently compiles, but maybe it shouldn't.
+// We don't emit a padding check unless there's a repr that can guarantee that
+// fields don't overlap. `repr(packed)` on its own doesn't guarantee this.
 #[derive(IntoBytes)]
 #[repr(packed(2))]
 struct IntoBytes7 {
     a: u8,
+    // Add a second field to avoid triggering the "repr(C) struct with one
+    // field" special case.
+    b: u8,
 }
 
 #[derive(IntoBytes)]

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

@@ -4,97 +4,113 @@ error: this conflicts with another representation hint
 133 | #[repr(C, C)] // zerocopy-derive conservatively treats these as conflicting reprs
     |        ^^^^
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-nightly/struct.rs:151:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-nightly/struct.rs:140:10
     |
-151 | #[derive(IntoBytes)]
+140 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-nightly/struct.rs:156:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-nightly/struct.rs:150:10
     |
-156 | #[derive(IntoBytes)]
+150 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-nightly/struct.rs:159:10
+    |
+159 | #[derive(IntoBytes)]
+    |          ^^^^^^^^^
+    |
+    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
    --> tests/ui-nightly/struct.rs:164:10
     |
 164 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: must have a non-align #[repr(...)] attribute or #[repr(packed)] in order to guarantee this type's memory layout
-   --> tests/ui-nightly/struct.rs:187:10
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-nightly/struct.rs:172:10
+    |
+172 | #[derive(IntoBytes)]
+    |          ^^^^^^^^^
+    |
+    = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: must have a non-align #[repr(...)] attribute in order to guarantee this type's memory layout
+   --> tests/ui-nightly/struct.rs:195:10
     |
-187 | #[derive(IntoBytes)]
+195 | #[derive(IntoBytes)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: cannot derive `Unaligned` on type with alignment greater than 1
-   --> tests/ui-nightly/struct.rs:198:11
+   --> tests/ui-nightly/struct.rs:206:11
     |
-198 | #[repr(C, align(2))]
+206 | #[repr(C, align(2))]
     |           ^^^^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-nightly/struct.rs:202:8
+   --> tests/ui-nightly/struct.rs:210:8
     |
-202 | #[repr(transparent, align(2))]
+210 | #[repr(transparent, align(2))]
     |        ^^^^^^^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-nightly/struct.rs:208:8
+   --> tests/ui-nightly/struct.rs:216:8
     |
-208 | #[repr(packed, align(2))]
+216 | #[repr(packed, align(2))]
     |        ^^^^^^^^^^^^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-nightly/struct.rs:212:8
+   --> tests/ui-nightly/struct.rs:220:8
     |
-212 | #[repr(align(1), align(2))]
+220 | #[repr(align(1), align(2))]
     |        ^^^^^^^^^^^^^^^^^^
 
 error: this conflicts with another representation hint
-   --> tests/ui-nightly/struct.rs:216:8
+   --> tests/ui-nightly/struct.rs:224:8
     |
-216 | #[repr(align(2), align(4))]
+224 | #[repr(align(2), align(4))]
     |        ^^^^^^^^^^^^^^^^^^
 
 error: must have #[repr(C)], #[repr(transparent)], or #[repr(packed)] attribute in order to guarantee this type's alignment
-   --> tests/ui-nightly/struct.rs:219:10
+   --> tests/ui-nightly/struct.rs:227:10
     |
-219 | #[derive(Unaligned)]
+227 | #[derive(Unaligned)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: must have #[repr(C)], #[repr(transparent)], or #[repr(packed)] attribute in order to guarantee this type's alignment
-   --> tests/ui-nightly/struct.rs:222:10
+   --> tests/ui-nightly/struct.rs:230:10
     |
-222 | #[derive(Unaligned)]
+230 | #[derive(Unaligned)]
     |          ^^^^^^^^^
     |
     = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: this conflicts with another representation hint
-   --> tests/ui-nightly/struct.rs:230:19
+   --> tests/ui-nightly/struct.rs:238:19
     |
-230 |   #[repr(packed(2), C)]
+238 |   #[repr(packed(2), C)]
     |  ___________________^
-231 | | #[derive(Unaligned)]
-232 | | #[repr(C, packed(2))]
+239 | | #[derive(Unaligned)]
+240 | | #[repr(C, packed(2))]
     | |________^
 
 error[E0692]: transparent struct cannot have other repr hints
-   --> tests/ui-nightly/struct.rs:202:8
+   --> tests/ui-nightly/struct.rs:210:8
     |
-202 | #[repr(transparent, align(2))]
+210 | #[repr(transparent, align(2))]
     |        ^^^^^^^^^^^  ^^^^^^^^
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
@@ -376,15 +392,15 @@ note: required by a bound in `macro_util::__size_of::size_of`
     |                             ^^^^^ required by this bound in `size_of`
 
 error[E0587]: type has conflicting packed and align representation hints
-   --> tests/ui-nightly/struct.rs:209:1
+   --> tests/ui-nightly/struct.rs:217:1
     |
-209 | struct Unaligned3;
+217 | struct Unaligned3;
     | ^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
-   --> tests/ui-nightly/struct.rs:182:28
+   --> tests/ui-nightly/struct.rs:190:28
     |
-182 |         is_into_bytes_11::<IntoBytes11<AU16>>();
+190 |         is_into_bytes_11::<IntoBytes11<AU16>>();
     |                            ^^^^^^^^^^^^^^^^^ the trait `Unaligned` is not implemented for `AU16`, which is required by `IntoBytes11<AU16>: zerocopy::IntoBytes`
     |
     = note: Consider adding `#[derive(Unaligned)]` to `AU16`
@@ -399,13 +415,13 @@ error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
               I16<O>
             and $N others
 note: required for `IntoBytes11<AU16>` to implement `zerocopy::IntoBytes`
-   --> tests/ui-nightly/struct.rs:171:10
+   --> tests/ui-nightly/struct.rs:179:10
     |
-171 | #[derive(IntoBytes)]
+179 | #[derive(IntoBytes)]
     |          ^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
 note: required by a bound in `is_into_bytes_11`
-   --> tests/ui-nightly/struct.rs:180:24
+   --> tests/ui-nightly/struct.rs:188:24
     |
-180 | fn is_into_bytes_11<T: IntoBytes>() {
+188 | fn is_into_bytes_11<T: IntoBytes>() {
     |                        ^^^^^^^^^ required by this bound in `is_into_bytes_11`
     = note: this error originates in the derive macro `IntoBytes` (in Nightly builds, run with -Z macro-backtrace for more info)