polyfill: Extend cold_exhaustive_error to struct types.

src/aead/overlapping/array.rs

@@ -14,6 +14,7 @@
 
 #![cfg_attr(not(test), allow(dead_code))]
 
+pub use self::len_mismatch_error::LenMismatchError;
 use super::Overlapping;
 use core::array::TryFromSliceError;
 
@@ -58,15 +59,6 @@ impl<T, const N: usize> Array<'_, T, N> {
     }
 }
 
-pub struct LenMismatchError {
-    #[allow(dead_code)]
-    len: usize,
-}
-
-impl LenMismatchError {
-    #[cold]
-    #[inline(never)]
-    fn new(len: usize) -> Self {
-        Self { len }
-    }
+cold_exhaustive_error! {
+    struct len_mismatch_error::LenMismatchError { len: usize }
 }

src/aead/overlapping/base.rs

@@ -12,6 +12,7 @@
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
+pub use self::index_error::IndexError;
 use super::{Array, LenMismatchError};
 use core::{mem, ops::RangeFrom};
 
@@ -130,12 +131,6 @@ impl<T> Overlapping<'_, T> {
     }
 }
 
-pub struct IndexError(#[allow(dead_code)] usize);
-
-impl IndexError {
-    #[cold]
-    #[inline(never)]
-    fn new(index: usize) -> Self {
-        Self(index)
-    }
+cold_exhaustive_error! {
+    struct index_error::IndexError { index: usize }
 }

src/polyfill/cold_error.rs

@@ -35,6 +35,29 @@
 /// work around the lack of the ability to mark an enum variant `#[cold]` and
 /// `#[inline(never)]`.
 macro_rules! cold_exhaustive_error {
+    // struct
+    {
+        struct $mod_name:ident::$Error:ident {
+            $field:ident: $ValueType:ty
+        }
+    } => {
+        mod $mod_name {
+            #[allow(unused_imports)]
+            use super::*; // So `$ValueType` is in scope.
+
+            pub struct $Error { #[allow(dead_code)] $field: $ValueType }
+
+            impl $Error {
+                #[cold]
+                #[inline(never)]
+                pub(super) fn new($field: $ValueType) -> Self {
+                    Self { $field }
+                }
+            }
+        }
+    };
+
+    // enum
     {
         enum $mod_name:ident::$Error:ident {
             $(