Blanket implementation for `der::asn1::SetOfVec` not recognized by compiler? - rust

Question

I want to DER encode a nested data structure using the der crate of the RustCrypto project. I have a Vec of SetOfVecs. There should be a blanket implementation of der::Encode for SetOfVec, but the compiler does not recognize it:
// Add this to Cargo.toml:
// ...
// [dependencies]
// der = { version = "0.6", features = ["alloc", "oid"]}
// main.rs:
use der::{DecodeValue, Encode, Header, Reader, Sequence};
use der::asn1::{ObjectIdentifier, SetOfVec};
struct NestedType(Vec<SetOfVec<ObjectIdentifier>>);
impl<'a> DecodeValue<'a> for NestedType {
fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> der::Result<Self> {
Ok(reader.decode()?)
}
}
impl<'a> Sequence<'a> for NestedType {
fn fields<F, T>(&self, field_encoder: F) -> der::Result<T>
where
F: FnOnce(&[&dyn Encode]) -> der::Result<T>,
{
// This works
// (the following 3 lines are just for comparison with the failing case)
let encoder1 = SetOfVec::<ObjectIdentifier>::new();
let encoder2 = SetOfVec::<ObjectIdentifier>::new();
field_encoder(&[&encoder1, &encoder2])?;
// This doesn't work
let mut refs: Vec<&SetOfVec<ObjectIdentifier>> = Vec::new();
for rdn in self.0.iter() {
refs.push(rdn);
}
field_encoder(refs.as_slice())
}
}
fn main() {}
The compiler error:
error[E0308]: mismatched types
--> src\main.rs:33:23
|
33 | field_encoder(refs.as_slice())
| ^^^^^^^^^^^^^^^ expected trait object `dyn Encode`, found struct `SetOfVec`
|
= note: expected reference `&[&dyn Encode]`
found reference `&[&SetOfVec<der::asn1::ObjectIdentifier>]`
As the Encode trait is implemented for SetOfVec, there should be no error. What's the problem here?

Answer 1

Looks like the compiler needs more info. The following code creates a Vec<&dyn Encode> with the references pointing to the SetOfVecs in the original Vec. This should also solve the memory allocation issue, that #Chayim Friedman mentioned (please correct me, if I'm wrong):
impl<'a> Sequence<'a> for NestedType {
fn fields<F, T>(&self, field_encoder: F) -> der::Result<T>
where
F: FnOnce(&[&dyn Encode]) -> der::Result<T>,
{
let mut refs: Vec<&dyn Encode> = Vec::new();
for rdn in self.0.iter() {
refs.push(rdn);
}
field_encoder(refs.as_slice())
}
}