[server sdk] Nested list shapes with constraints can result in unbuildable code

[nated0g]

We ran into an issue when modelling an existing API (that we do not control the shape of) as smithy and trying to generate a rust server sdk with smithy-rs. The following smithy model:

$version: "2"

namespace example

use aws.api#service
use aws.protocols#restJson1
use smithy.framework#ValidationException

/// Bars Quuxes into Foos.
@service(sdkId: "Foo")
@restJson1
service FooService {
    version: "2006-03-01"
    operations: [
        BarOperation
    ]
}

/// Performs a bar operation.
@http(uri: "/bar", method: "POST")
operation BarOperation {
    input: BarInput
    output: BarOutput
    errors: [
        ValidationException
    ]
}

string Foo

/// Output for the BarOperation.
structure BarOutput {
    /// The result of the bar operation.
    result: Foo
}

/// Input for the BarOperation.
structure BarInput {
    @required
    quuxes: QuuxList
}

@length(min: 0, max: 2)
list QuuxList {
    member: SubQuuxList
}

list SubQuuxList {
    member: Quux
}

structure Quux {
    @required
    quux: String
}

Will generate code that can't compile. After generating a server SDK and running cargo build on it, we encounter the following error:

   Compiling foo-rs v0.0.1 (/Users/nathaniel/workplace/smithy-openapi-issue-test/build/smithy/source/rust-server-codegen)
error[E0277]: the trait bound `QuuxList: TryFrom<Vec<SubQuuxListConstrained>>` is not satisfied
  --> src/unconstrained.rs:37:13
   |
37 |             Self::try_from(inner)
   |             ^^^^ the trait `From<Vec<SubQuuxListConstrained>>` is not implemented for `QuuxList`, which is required by `QuuxList: TryFrom<Vec<SubQuuxListConstrained>>`
   |
   = help: the following other types implement trait `TryFrom<T>`:
             `QuuxList` implements `TryFrom<QuuxListUnconstrained>`
             `QuuxList` implements `TryFrom<Vec<Vec<Quux>>>`
   = note: required for `Vec<SubQuuxListConstrained>` to implement `Into<QuuxList>`
   = note: required for `QuuxList` to implement `TryFrom<Vec<SubQuuxListConstrained>>`

error[E0308]: mismatched types
  --> src/unconstrained.rs:37:13
   |
19 |         fn try_from(value: QuuxListUnconstrained) -> Result<Self, Self::Error> {
   |                                                      ------------------------- expected `Result<QuuxList, quux_list::ConstraintViolation>` because of return type
...
37 |             Self::try_from(inner)
   |             ^^^^^^^^^^^^^^^^^^^^^ expected `Result<QuuxList, ConstraintViolation>`, found `Result<QuuxList, Infallible>`
   |
   = note: expected enum `Result<_, quux_list::ConstraintViolation>`
              found enum `Result<_, Infallible>`

error[E0277]: the trait bound `QuuxList: From<Vec<SubQuuxListConstrained>>` is not satisfied
  --> src/unconstrained.rs:37:13
   |
37 |             Self::try_from(inner)
   |             ^^^^^^^^^^^^^^^^^^^^^ the trait `From<Vec<SubQuuxListConstrained>>` is not implemented for `QuuxList`, which is required by `QuuxList: TryFrom<Vec<SubQuuxListConstrained>>`
   |
   = help: the following other types implement trait `TryFrom<T>`:
             `QuuxList` implements `TryFrom<QuuxListUnconstrained>`
             `QuuxList` implements `TryFrom<Vec<Vec<Quux>>>`
   = note: required for `Vec<SubQuuxListConstrained>` to implement `Into<QuuxList>`
   = note: required for `QuuxList` to implement `TryFrom<Vec<SubQuuxListConstrained>>`

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `foo-rs` (lib) due to 3 previous errors

Adding a length constraint to SubQuuxList,

@length(min: 1)
list SubQuuxList {
    member: Quux
}

or removing the @required trait from Quux:

structure Quux {
    quux: String
}

will generate valid code.

---

Setting aside how sane it is or isn't to allow entry of:

{
  "quuxes": [[]]
}

, this seems like a bug. I think valid smithy should generate valid code, or fail while running smithy build, if there is some limitation that makes it impossible to generate valid code.