Enables GATs iteration in the Element API (#442)

This PR replaces all uses of `Box<dyn Iterator<Item=_> + 'a>`
in the `Element` and `ElementRef` APIs with generic associated
types. This means that heap allocations are no longer required
to iterate over annotations, sequence elements, or struct fields.

Because this change uses generic associated types, it also
configures the CI tests to use the beta channel of cargo. GATs will
be available in the next release of Rust (v1.65, out in November);
once that happens, we can and should switch back to 
stable. (#443)

This change also removes the recently added dependency
on the `linkhash` crate. That dependency was originally pulled
in so structs could remember the order in which their fields were
inserted for later iteration. However, it only worked if the struct
did not contain duplicate field names. This PR changes the field
storage for `Struct` and `StructRef` to store their fields in a
`Vec` and also maintain a `HashMap<Symbol, IndexVec>`
that remembers all of the indexes at which values for a given
field can be found.

Further, it moves `Sequence` elements and `Struct` fields into
an `Rc` to make clone() operations cheaper. A future PR will
modify iterators over collection types to clone the `Rc` and
thus remove one of their lifetime constraints, making it much
easier to write a recursive iterator over an element tree without
constant cloning.

.github/workflows/coverage.yml

@@ -23,7 +23,7 @@ jobs:
         with:
           profile: minimal
           # nightly can be very volatile--pin this to a version we know works well...
-          toolchain: nightly-2022-05-01
+          toolchain: nightly-2022-10-12
           override: true
       - name: Cargo Test
         uses: actions-rs/cargo@v1

.github/workflows/rust.yml

@@ -34,7 +34,11 @@ jobs:
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          # We're temporarily testing against `beta` to have access to GATs. When Rust v1.65 is released in
+          # 2022-11, we should change this back to `stable`.
+          # See: https://github.com/amzn/ion-rust/issues/443
+          toolchain: beta
+          components: rustfmt, clippy
           override: true
       - name: Cargo Build 
         uses: actions-rs/cargo@v1

Cargo.toml

@@ -38,7 +38,6 @@ num-bigint = "0.3"
 num-integer = "0.1.44"
 num-traits = "0.2"
 arrayvec = "0.7"
-hashlink = "0.8.1"
 smallvec = "1.9.0"
 
 [dev-dependencies]

examples/read_all_values.rs

@@ -62,7 +62,7 @@ fn read_all_values<R: RawReader>(reader: &mut R) -> IonResult<usize> {
                 match ion_type {
                     Struct | List | SExpression => reader.step_in()?,
                     String => {
-                        let _text = reader.map_string(|_s| ())?;
+                        reader.map_string(|_s| ())?;
                     }
                     Symbol => {
                         let _symbol_id = reader.read_symbol()?;
@@ -83,10 +83,10 @@ fn read_all_values<R: RawReader>(reader: &mut R) -> IonResult<usize> {
                         let _boolean = reader.read_bool()?;
                     }
                     Blob => {
-                        let _blob = reader.map_blob(|_b| ())?;
+                        reader.map_blob(|_b| ())?;
                     }
                     Clob => {
-                        let _clob = reader.map_clob(|_c| ())?;
+                        reader.map_clob(|_c| ())?;
                     }
                     Null => {}
                 }

src/binary/int.rs

@@ -125,7 +125,7 @@ impl DecodedInt {
         let empty_leading_bytes: u32 = (magnitude.leading_zeros() - 1) >> 3;
         let first_occupied_byte = empty_leading_bytes as usize;
 
-        let mut magnitude_bytes: [u8; mem::size_of::<u64>()] = (magnitude as u64).to_be_bytes();
+        let mut magnitude_bytes: [u8; mem::size_of::<u64>()] = magnitude.to_be_bytes();
         let bytes_to_write: &mut [u8] = &mut magnitude_bytes[first_occupied_byte..];
         if value < 0 {
             bytes_to_write[0] |= 0b1000_0000;

src/binary/non_blocking/raw_binary_reader.rs

@@ -86,7 +86,7 @@ struct EncodedValue {
 impl EncodedValue {
     /// Returns the offset of the current value's type descriptor byte.
     fn header_offset(&self) -> usize {
-        self.header_offset as usize
+        self.header_offset
     }
 
     /// Returns the length of this value's header, including the type descriptor byte and any
@@ -734,7 +734,7 @@ impl<A: AsRef<[u8]>> IonReader for RawBinaryBufferReader<A> {
         let coefficient_size_in_bytes =
             encoded_value.value_length() - exponent_var_int.size_in_bytes();
 
-        let exponent = exponent_var_int.value() as i64;
+        let exponent = exponent_var_int.value();
         let coefficient = buffer.read_int(coefficient_size_in_bytes)?;
 
         if coefficient.is_negative_zero() {

src/binary/raw_binary_reader.rs

@@ -91,7 +91,7 @@ struct EncodedValue {
 impl EncodedValue {
     /// Returns the offset of the current value's type descriptor byte.
     fn header_offset(&self) -> usize {
-        self.header_offset as usize
+        self.header_offset
     }
 
     /// Returns the length of this value's header, including the type descriptor byte and any
@@ -526,7 +526,7 @@ impl<R: IonDataSource> IonReader for RawBinaryReader<R> {
         let coefficient_size_in_bytes =
             self.cursor.value.value_length - exponent_var_int.size_in_bytes();
 
-        let exponent = exponent_var_int.value() as i64;
+        let exponent = exponent_var_int.value();
         let coefficient = self.read_int(coefficient_size_in_bytes)?;
 
         if coefficient.is_negative_zero() {
@@ -938,7 +938,7 @@ where
     #[inline(always)]
     fn read_var_int(&mut self) -> IonResult<VarInt> {
         let var_int = VarInt::read(&mut self.data_source)?;
-        self.cursor.bytes_read += var_int.size_in_bytes() as usize;
+        self.cursor.bytes_read += var_int.size_in_bytes();
         Ok(var_int)
     }
 

src/binary/var_uint.rs

@@ -117,7 +117,7 @@ impl VarUInt {
     /// unsigned integer
     #[inline(always)]
     pub fn size_in_bytes(&self) -> usize {
-        self.size_in_bytes as usize
+        self.size_in_bytes
     }
 }
 

src/symbol_ref.rs

@@ -75,6 +75,21 @@ impl AsSymbolRef for Symbol {
     }
 }
 
+impl AsSymbolRef for &Symbol {
+    fn as_symbol_ref(&self) -> SymbolRef {
+        self.text()
+            .map(SymbolRef::with_text)
+            .unwrap_or_else(SymbolRef::with_unknown_text)
+    }
+}
+
+impl<'borrow, 'data> AsSymbolRef for &'borrow SymbolRef<'data> {
+    fn as_symbol_ref(&self) -> SymbolRef<'data> {
+        // This is essentially free; the only data inside is an Option<&str>
+        (*self).clone()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;