New reader API that allows providing the scratch buffer.

Signed-off-by: Itamar Turner-Trauring <[email protected]>

ciborium/src/de/mod.rs

@@ -815,17 +815,35 @@ where
     }
 }
 
-/// Deserializes as CBOR from a type with [`impl ciborium_io::Read`](ciborium_io::Read)
+/// Deserializes as CBOR from a type with [`impl
+/// ciborium_io::Read`](ciborium_io::Read) using a 4KB buffer on the stack.
+///
+/// If you want to deserialize faster at the cost of more memory, consider using
+/// [`from_reader_with_buffer`](from_reader_with_buffer) with a larger buffer,
+/// for example 64KB.
 #[inline]
 pub fn from_reader<T: de::DeserializeOwned, R: Read>(reader: R) -> Result<T, Error<R::Error>>
 where
     R::Error: core::fmt::Debug,
 {
     let mut scratch = [0; 4096];
+    from_reader_with_buffer(reader, &mut scratch)
+}
 
+/// Deserializes as CBOR from a type with [`impl
+/// ciborium_io::Read`](ciborium_io::Read), using a caller-specific buffer as a
+/// temporary scratch space.
+#[inline]
+pub fn from_reader_with_buffer<T: de::DeserializeOwned, R: Read>(
+    reader: R,
+    scratch_buffer: &mut [u8],
+) -> Result<T, Error<R::Error>>
+where
+    R::Error: core::fmt::Debug,
+{
     let mut reader = Deserializer {
         decoder: reader.into(),
-        scratch: &mut scratch,
+        scratch: scratch_buffer,
         recurse: 256,
     };
 

ciborium/src/lib.rs

@@ -100,6 +100,8 @@ pub mod value;
 // Re-export the [items recommended by serde](https://serde.rs/conventions.html).
 #[doc(inline)]
 pub use crate::de::from_reader;
+#[doc(inline)]
+pub use crate::de::from_reader_with_buffer;
 
 #[doc(inline)]
 pub use crate::ser::into_writer;

ciborium/tests/codec.rs

@@ -7,7 +7,7 @@ use std::convert::TryFrom;
 use std::fmt::Debug;
 
 use ciborium::value::Value;
-use ciborium::{cbor, de::from_reader, ser::into_writer};
+use ciborium::{cbor, de::from_reader, de::from_reader_with_buffer, ser::into_writer};
 
 use rstest::rstest;
 use serde::{de::DeserializeOwned, Deserialize, Serialize};
@@ -309,6 +309,11 @@ fn codec<'de, T: Serialize + Clone, V: Debug + PartialEq + DeserializeOwned, F:
     eprintln!("{:x?} == {:x?}", &value, &decoded);
     assert!(veq(&value, &decoded));
 
+    let mut scratch = vec![0; 65536];
+    let decoded: Value = from_reader_with_buffer(&bytes[..], &mut scratch).unwrap();
+    eprintln!("{:x?} == {:x?}", &value, &decoded);
+    assert!(veq(&value, &decoded));
+
     let decoded: V = value.deserialized().unwrap();
     let answer = equality(input);
     eprintln!("{:x?} == {:x?}", answer, decoded);