Support derive(KnownLayout) on DSTs (#643)

DSTs must be marked with `repr(C)`. The expansion requires the final
field implement `KnownLayout`.

Makes progress towards #29.

Author: jswrenn

src/lib.rs

@@ -168,7 +168,7 @@ define_kind_specific_repr!(
 );
 
 // All representations known to Rust.
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
 pub enum Repr {
     U8,
     U16,
@@ -183,40 +183,58 @@ pub enum Repr {
     C,
     Transparent,
     Packed,
+    PackedN(u64),
     Align(u64),
 }
 
 impl Repr {
     fn from_meta(meta: &Meta) -> Result<Repr, Error> {
-        match meta {
-            Meta::Path(path) => {
-                let ident = path
-                    .get_ident()
-                    .ok_or_else(|| Error::new_spanned(meta, "unrecognized representation hint"))?;
-                match format!("{}", ident).as_str() {
-                    "u8" => return Ok(Repr::U8),
-                    "u16" => return Ok(Repr::U16),
-                    "u32" => return Ok(Repr::U32),
-                    "u64" => return Ok(Repr::U64),
-                    "usize" => return Ok(Repr::Usize),
-                    "i8" => return Ok(Repr::I8),
-                    "i16" => return Ok(Repr::I16),
-                    "i32" => return Ok(Repr::I32),
-                    "i64" => return Ok(Repr::I64),
-                    "isize" => return Ok(Repr::Isize),
-                    "C" => return Ok(Repr::C),
-                    "transparent" => return Ok(Repr::Transparent),
-                    "packed" => return Ok(Repr::Packed),
-                    _ => {}
-                }
+        let (path, list) = match meta {
+            Meta::Path(path) => (path, None),
+            Meta::List(list) => (&list.path, Some(list)),
+            _ => return Err(Error::new_spanned(meta, "unrecognized representation hint")),
+        };
+
+        let ident = path
+            .get_ident()
+            .ok_or_else(|| Error::new_spanned(meta, "unrecognized representation hint"))?;
+
+        Ok(match (ident.to_string().as_str(), list) {
+            ("u8", None) => Repr::U8,
+            ("u16", None) => Repr::U16,
+            ("u32", None) => Repr::U32,
+            ("u64", None) => Repr::U64,
+            ("usize", None) => Repr::Usize,
+            ("i8", None) => Repr::I8,
+            ("i16", None) => Repr::I16,
+            ("i32", None) => Repr::I32,
+            ("i64", None) => Repr::I64,
+            ("isize", None) => Repr::Isize,
+            ("C", None) => Repr::C,
+            ("transparent", None) => Repr::Transparent,
+            ("packed", None) => Repr::Packed,
+            ("packed", Some(list)) => {
+                Repr::PackedN(list.parse_args::<LitInt>()?.base10_parse::<u64>()?)
             }
-            Meta::List(list) => {
-                return Ok(Repr::Align(list.parse_args::<LitInt>()?.base10_parse::<u64>()?))
+            ("align", Some(list)) => {
+                Repr::Align(list.parse_args::<LitInt>()?.base10_parse::<u64>()?)
             }
-            _ => {}
-        }
+            _ => return Err(Error::new_spanned(meta, "unrecognized representation hint")),
+        })
+    }
+}
 
-        Err(Error::new_spanned(meta, "unrecognized representation hint"))
+impl KindRepr for Repr {
+    fn is_align(&self) -> bool {
+        false
+    }
+
+    fn is_align_gt_one(&self) -> bool {
+        false
+    }
+
+    fn parse(meta: &Meta) -> syn::Result<Self> {
+        Self::from_meta(meta)
     }
 }
 
@@ -225,6 +243,9 @@ impl Display for Repr {
         if let Repr::Align(n) = self {
             return write!(f, "repr(align({}))", n);
         }
+        if let Repr::PackedN(n) = self {
+            return write!(f, "repr(packed({}))", n);
+        }
         write!(
             f,
             "repr({})",
@@ -248,7 +269,7 @@ impl Display for Repr {
     }
 }
 
-fn reprs<R: KindRepr>(attrs: &[Attribute]) -> Result<Vec<(Meta, R)>, Vec<Error>> {
+pub(crate) fn reprs<R: KindRepr>(attrs: &[Attribute]) -> Result<Vec<(Meta, R)>, Vec<Error>> {
     let mut reprs = Vec::new();
     let mut errors = Vec::new();
     for attr in attrs {

zerocopy-derive/src/lib.rs

@@ -9,7 +9,10 @@ mod util;
 
 use std::{marker::PhantomData, option::IntoIter};
 
-use {static_assertions::assert_impl_all, zerocopy::KnownLayout};
+use {
+    static_assertions::assert_impl_all,
+    zerocopy::{DstLayout, KnownLayout},
+};
 
 use crate::util::AU16;
 

zerocopy-derive/src/repr.rs

@@ -1,25 +1,33 @@
+warning: unused import: `zerocopy::KnownLayout`
+  --> tests/ui-msrv/late_compile_pass.rs:16:5
+   |
+16 | use zerocopy::KnownLayout;
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(unused_imports)]` on by default
+
 error[E0277]: the trait bound `NotZerocopy: FromZeroes` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:27:10
+  --> tests/ui-msrv/late_compile_pass.rs:28:10
    |
-27 | #[derive(FromZeroes)]
+28 | #[derive(FromZeroes)]
    |          ^^^^^^^^^^ the trait `FromZeroes` is not implemented for `NotZerocopy`
    |
    = help: see issue #48214
    = note: this error originates in the derive macro `FromZeroes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the trait bound `NotZerocopy: FromBytes` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:36:10
+  --> tests/ui-msrv/late_compile_pass.rs:37:10
    |
-36 | #[derive(FromBytes)]
+37 | #[derive(FromBytes)]
    |          ^^^^^^^^^ the trait `FromBytes` is not implemented for `NotZerocopy`
    |
    = 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 `FromBytes1: FromZeroes` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:36:10
+  --> tests/ui-msrv/late_compile_pass.rs:37:10
    |
-36 | #[derive(FromBytes)]
+37 | #[derive(FromBytes)]
    |          ^^^^^^^^^ the trait `FromZeroes` is not implemented for `FromBytes1`
    |
 note: required by a bound in `FromBytes`
@@ -30,36 +38,36 @@ note: required by a bound in `FromBytes`
    = 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 `NotZerocopy: AsBytes` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:45:10
+  --> tests/ui-msrv/late_compile_pass.rs:46:10
    |
-45 | #[derive(AsBytes)]
+46 | #[derive(AsBytes)]
    |          ^^^^^^^ the trait `AsBytes` is not implemented for `NotZerocopy`
    |
    = help: see issue #48214
    = note: this error originates in the derive macro `AsBytes` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:55:10
+  --> tests/ui-msrv/late_compile_pass.rs:56:10
    |
-55 | #[derive(Unaligned)]
+56 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `AU16`
    |
    = help: see issue #48214
    = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:63:10
+  --> tests/ui-msrv/late_compile_pass.rs:64:10
    |
-63 | #[derive(Unaligned)]
+64 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `AU16`
    |
    = help: see issue #48214
    = note: this error originates in the derive macro `Unaligned` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: the trait bound `AU16: Unaligned` is not satisfied
-  --> tests/ui-msrv/late_compile_pass.rs:70:10
+  --> tests/ui-msrv/late_compile_pass.rs:71:10
    |
-70 | #[derive(Unaligned)]
+71 | #[derive(Unaligned)]
    |          ^^^^^^^^^ the trait `Unaligned` is not implemented for `AU16`
    |
    = help: see issue #48214

zerocopy-derive/tests/struct_known_layout.rs

@@ -0,0 +1 @@
+../ui-nightly/mid_compile_pass.rs

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

@@ -0,0 +1,104 @@
+error[E0277]: the trait bound `T: KnownLayout` is not satisfied
+  --> tests/ui-msrv/mid_compile_pass.rs:59:26
+   |
+59 | fn test_kl13<T>(t: T) -> impl KnownLayout {
+   |                          ^^^^^^^^^^^^^^^^ the trait `KnownLayout` is not implemented for `T`
+   |
+note: required because of the requirements on the impl of `KnownLayout` for `KL13<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:55:10
+   |
+55 | #[derive(KnownLayout)]
+   |          ^^^^^^^^^^^
+   = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider restricting type parameter `T`
+   |
+59 | fn test_kl13<T: zerocopy::KnownLayout>(t: T) -> impl KnownLayout {
+   |               +++++++++++++++++++++++
+
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> tests/ui-msrv/mid_compile_pass.rs:31:15
+   |
+30 | fn test_kl04<T: ?Sized>(kl: &KL04<T>) {
+   |              - this type parameter needs to be `std::marker::Sized`
+31 |     assert_kl(kl);
+   |     --------- ^^ doesn't have a size known at compile-time
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required because it appears within the type `KL04<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:28:8
+   |
+28 | struct KL04<T: ?Sized>(u8, T);
+   |        ^^^^
+note: required because of the requirements on the impl of `KnownLayout` for `KL04<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:27:10
+   |
+27 | #[derive(KnownLayout)]
+   |          ^^^^^^^^^^^
+note: required by a bound in `assert_kl`
+  --> tests/ui-msrv/mid_compile_pass.rs:23:26
+   |
+23 | fn assert_kl<T: ?Sized + KnownLayout>(_: &T) {}
+   |                          ^^^^^^^^^^^ required by this bound in `assert_kl`
+   = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+30 - fn test_kl04<T: ?Sized>(kl: &KL04<T>) {
+30 + fn test_kl04<T>(kl: &KL04<T>) {
+   |
+
+error[E0277]: the size for values of type `T` cannot be known at compilation time
+  --> tests/ui-msrv/mid_compile_pass.rs:40:15
+   |
+39 | fn test_kl06<T: ?Sized + KnownLayout>(kl: &KL06<T>) {
+   |              - this type parameter needs to be `std::marker::Sized`
+40 |     assert_kl(kl);
+   |     --------- ^^ doesn't have a size known at compile-time
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required because it appears within the type `KL06<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:37:8
+   |
+37 | struct KL06<T: ?Sized + KnownLayout>(u8, T);
+   |        ^^^^
+note: required because of the requirements on the impl of `KnownLayout` for `KL06<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:36:10
+   |
+36 | #[derive(KnownLayout)]
+   |          ^^^^^^^^^^^
+note: required by a bound in `assert_kl`
+  --> tests/ui-msrv/mid_compile_pass.rs:23:26
+   |
+23 | fn assert_kl<T: ?Sized + KnownLayout>(_: &T) {}
+   |                          ^^^^^^^^^^^ required by this bound in `assert_kl`
+   = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+39 - fn test_kl06<T: ?Sized + KnownLayout>(kl: &KL06<T>) {
+39 + fn test_kl06<T: KnownLayout>(kl: &KL06<T>) {
+   |
+
+error[E0277]: the trait bound `T: KnownLayout` is not satisfied
+  --> tests/ui-msrv/mid_compile_pass.rs:50:15
+   |
+50 |     assert_kl(kl)
+   |     --------- ^^ the trait `KnownLayout` is not implemented for `T`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required because of the requirements on the impl of `KnownLayout` for `KL12<T>`
+  --> tests/ui-msrv/mid_compile_pass.rs:45:10
+   |
+45 | #[derive(KnownLayout)]
+   |          ^^^^^^^^^^^
+note: required by a bound in `assert_kl`
+  --> tests/ui-msrv/mid_compile_pass.rs:23:26
+   |
+23 | fn assert_kl<T: ?Sized + KnownLayout>(_: &T) {}
+   |                          ^^^^^^^^^^^ required by this bound in `assert_kl`
+   = note: this error originates in the derive macro `KnownLayout` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider further restricting this bound
+   |
+49 | fn test_kl12<T: ?Sized + zerocopy::KnownLayout>(kl: &KL12<T>) {
+   |                        +++++++++++++++++++++++