cargo doc: shows different return types for re-exported methods

[bheesham]

Running cargo doc for the async-net and smol repositories show different return types for async fn methods.

(Links to docs.rs provided below as a quick preview of the issue.)

git clone https://github.com/smol-rs/async-net.git && cd async-net && cargo doc --open && cd ..
git clone https://github.com/smol-rs/smol.git && cd smol && cargo doc --open && cd ..

Then browsing to the documentation for UdpSocket::bind in each, we see (using links from docs.rs):

async_net::UdpSocket::bind:

pub async fn bind<A: AsyncToSocketAddrs>(addr: A) -> Result<UdpSocket>

smol::UdpSocket::bind:

pub async fn bind<A>(addr: A) -> impl Future<Output = Result<UdpSocket, Error>> where
    A: AsyncToSocketAddrs,

I expected to see this happen: the same types should have been displayed.

Instead, this happened: smol's type is an async fn which returns a Future, which seems to conflict with async-net's definition.

Meta

rustc --version --verbose:

rustc 1.74.0-nightly (030e4d382 2023-09-10)
binary: rustc
commit-hash: 030e4d382f1df30240408540f25cd1ccc8dbbf50
commit-date: 2023-09-10
host: x86_64-unknown-linux-gnu
release: 1.74.0-nightly
LLVM version: 17.0.0

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Some discussion: https://users.rust-lang.org/t/cargo-doc-shows-different-return-for-re-exported-methods/99687

[steffahn]

FYI, an IMO easier way to reproduce:

• cargo new async_doc_repro; cd async_doc_repro; cargo add smol@=1.3.0; cargo doc --open
• put UdpSocket::bind into the search bar
• open at the first two results

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It probably also would be nice to test if this can be reproduced using a minimal setup. E.g.: are multiple crates needed? Is publicly re-exporting (with doc(inline) of a whole crate essential, the way smol does for async_net or does re-exporting a module or a function do the same, too? Etc…

[fmease]

Minimized:

// ––– a.rs –––   rustc a.rs --crate-type=lib --edition=2021
pub async fn f() -> i32 { 0 }

// ––– b.rs –––   rustdoc b.rs --edition=2021 --extern=a=liba.rlib
pub use a::*;

Rendered as pub async fn f() -> impl Future<Output = i32> instead of pub async fn f() -> i32 or pub fn f() -> impl Future<Output = i32>.