rustdoc-json: show leaked auto-trait definition for RPIT and async-fn

Currently, these two functions have the same type signature in rustdoc-json:

pub fn foo() -> impl Debug {
    0i32
}

pub fn bar() -> impl Debug {
    Cell::new(10)
}

 {
  "inputs": [],
  "is_c_variadic": false,
  "output": {
    "impl_trait": [
      {
        "trait_bound": {
          "generic_params": [],
          "modifier": "none",
          "trait": {"args": null, "id": 1, "path": "Debug"}
        }
      }
    ]
  }
}

However, in practice, they are different types:

pub fn foo() -> impl Debug {
    0i32
}

pub fn bar() -> impl Debug {
    Cell::new(10)
}

pub fn check_sync<T: Sync>(_: T) {}

fn main() {
    check_sync(foo()); // Ok
    check_sync(bar()); // Errors
}

playground

This is because impl-trait's in return position implement the auto-traits of the underling type:

The type would not be known to implement any other trait, with the exception of OIBITS (aka “auto traits”) and default traits like Sized.

-- RFC 1522: conservative_impl_trait

A similar behavior occurs for the auto-traits of the opaque Future type returned by async fn's:

pub async fn foo() -> i32 {
    0
}

pub async fn bar() -> i32 {
    let x = Cell::new(10);
    foo().await;
    x.get()
}

pub fn check_sync<T: Sync>(_: T) {}

fn main() {
    check_sync(foo()); // Ok
    check_sync(bar()); // Errors
}

playground

Again, there's information about the type of this function that rustdoc-json doesn't currently capture.

It'd be nice to capture this in rustdoc-json. The motivating use case comes from cargo-semver-checks, which would like to be able to warn users if their RPIT's (or async-fn futures) are no longer Send/Sync when they used to be.

I think doing this is going to be difficult to implement, as it requires being able to run the typechecker, which rustdoc currently doesn't do: https://hackmd.io/@fhcjVaPtST635ujGv6RF4w/SkulRNn93. Maybe we could seperatly land that for JSON before HTML, as JSON isn't stable.

Start by reading the rustdoc-json representation of RPIT and async-fn return types, then investigate the typechecker work described in the linked HackMD. Compare the JSON output for the Send/Sync examples and consider the separate JSON-before-HTML path mentioned in the issue. Done means rustdoc-json preserves the relevant leaked auto-trait information for cargo-semver-checks.