[error] Implement std::error::Error on errors

While we're here, also relax `Dispaly for AlignmentError<Src, Dst>` to
permit `Dst: ?Sized` in exchange for `Dst: KnownLayout`. This is an
important relaxation since our APIs permit performing conversions into
unsized destination types with runtime alignment checking.

Makes progress on #1297

Cargo.toml

@@ -60,10 +60,11 @@ alloc = []
 derive = ["zerocopy-derive"]
 simd = []
 simd-nightly = ["simd"]
+std-error = ["alloc"]
 # This feature depends on all other features that work on the stable compiler.
 # We make no stability guarantees about this feature; it may be modified or
 # removed at any time.
-__internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd"]
+__internal_use_only_features_that_work_on_stable = ["alloc", "derive", "simd", "std-error"]
 
 [dependencies]
 zerocopy-derive = { version = "=0.8.0-alpha.14", path = "zerocopy-derive", optional = true }

src/error.rs

@@ -25,9 +25,9 @@
 
 use core::{convert::Infallible, fmt, marker::PhantomData, ops::Deref};
 
-use crate::TryFromBytes;
 #[cfg(doc)]
 use crate::{FromBytes, Ref};
+use crate::{KnownLayout, TryFromBytes};
 
 /// Zerocopy's generic error type.
 ///
@@ -82,6 +82,15 @@ impl<A: fmt::Display, S: fmt::Display, V: fmt::Display> fmt::Display for Convert
     }
 }
 
+#[cfg(feature = "std-error")]
+impl<A, S, V> std::error::Error for ConvertError<A, S, V>
+where
+    A: fmt::Display + fmt::Debug,
+    S: fmt::Display + fmt::Debug,
+    V: fmt::Display + fmt::Debug,
+{
+}
+
 /// The error emitted if the conversion source is improperly aligned.
 #[derive(PartialEq, Eq)]
 pub struct AlignmentError<Src, Dst: ?Sized> {
@@ -126,9 +135,10 @@ impl<Src, Dst: ?Sized> fmt::Debug for AlignmentError<Src, Dst> {
 // The bounds on this impl are intentionally conservative, and can be relaxed
 // either once a `?Sized` alignment accessor is stabilized, or by storing the
 // alignment as a runtime value.
-impl<Src, Dst> fmt::Display for AlignmentError<Src, Dst>
+impl<Src, Dst: ?Sized> fmt::Display for AlignmentError<Src, Dst>
 where
     Src: Deref,
+    Dst: KnownLayout,
 {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -139,14 +149,22 @@ where
         f.write_str("the conversion failed because the address of the source (a multiple of ")?;
         addr_align.fmt(f)?;
         f.write_str(") is not a multiple of the alignment (")?;
-        core::mem::align_of::<Dst>().fmt(f)?;
+        <Dst as KnownLayout>::LAYOUT.align.get().fmt(f)?;
         f.write_str(") of the destination type: ")?;
         f.write_str(core::any::type_name::<Dst>())?;
         Ok(())
     }
 }
 
-impl<Src, Dst, S, V> From<AlignmentError<Src, Dst>>
+#[cfg(feature = "std-error")]
+impl<Src, Dst: ?Sized> std::error::Error for AlignmentError<Src, Dst>
+where
+    Src: Deref,
+    Dst: KnownLayout,
+{
+}
+
+impl<Src, Dst: ?Sized, S, V> From<AlignmentError<Src, Dst>>
     for ConvertError<AlignmentError<Src, Dst>, S, V>
 {
     #[inline]
@@ -216,7 +234,10 @@ where
     }
 }
 
-impl<Src, Dst, A, V> From<SizeError<Src, Dst>> for ConvertError<A, SizeError<Src, Dst>, V> {
+#[cfg(feature = "std-error")]
+impl<Src, Dst: ?Sized> std::error::Error for SizeError<Src, Dst> where Src: Deref {}
+
+impl<Src, Dst: ?Sized, A, V> From<SizeError<Src, Dst>> for ConvertError<A, SizeError<Src, Dst>, V> {
     #[inline]
     fn from(err: SizeError<Src, Dst>) -> Self {
         Self::Size(err)
@@ -274,6 +295,9 @@ where
     }
 }
 
+#[cfg(feature = "std-error")]
+impl<Src, Dst: ?Sized + TryFromBytes> std::error::Error for ValidityError<Src, Dst> where Src: Deref {}
+
 impl<Src, Dst: ?Sized + TryFromBytes, A, S> From<ValidityError<Src, Dst>>
     for ConvertError<A, S, ValidityError<Src, Dst>>
 {

src/lib.rs

@@ -255,7 +255,7 @@
     clippy::arithmetic_side_effects,
     clippy::indexing_slicing,
 ))]
-#![cfg_attr(not(test), no_std)]
+#![cfg_attr(not(any(test, feature = "std-error")), no_std)]
 #![cfg_attr(
     all(feature = "simd-nightly", any(target_arch = "x86", target_arch = "x86_64")),
     feature(stdarch_x86_avx512)