Duplicate `extern crate` in doc tests (2021-11-21 Nightly Regression)

[TheBlueMatt]

We have a Rust-2015 doc test which has an extern crate $CRATE_DOC_TEST_IS_IN which started failing to build on nightly 2021-11-21 (but passes on stable/2021-11-20).

It now fails with

---- src/chain/transaction.rs - chain::transaction::TransactionData (line 22) stdout ----
error[E0259]: the name `lightning` is defined multiple times
 --> src/chain/transaction.rs:24:1
  |
3 | extern crate lightning;
  | ----------------------- previous import of the extern crate `lightning` here
4 | 
5 | extern crate r#lightning;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ `lightning` reimported here
  |
  = note: `lightning` must be defined only once in the type namespace of this module

error: aborting due to previous error

[mathstuf]

Probably caused by 331465a (#91066; no rollup?). @camelid

[jyn514]

That seems unlikely, #91026 is more plausible. #91066 doesn't affect doctests.

[mathstuf]

Oh, I think I got misled by the "rustdoc" bit.

[jyn514]

I can't reproduce this. I tried

/// ```
/// extern crate inner;
/// ```
pub trait Trait {
    type Item;
}

and running cargo test --doc, but all the tests passed fine. @TheBlueMatt can you link the original code?

[TheBlueMatt]

The failure is https://github.com/rust-bitcoin/rust-lightning/blob/main/lightning/src/chain/transaction.rs#L24 You can see the first failure in CI at https://github.com/rust-bitcoin/rust-lightning/runs/4288660534?check_suite_focus=true

[ehuss]

The following seems like a minimal reproduction when used with 2015:

/// ```
/// extern crate mycrate;
/// mycrate::foo();
/// ```
pub fn foo() {}

Bisected to #91026.

[jyn514]

cc @notriddle , I think it might make sense to revert #91026 temporarily unless you already have an idea for a fix.