Deprecation warning emitted from derive without a span

[bluss]

Reproducing code:

#[derive(Clone)]
#[deprecated(note="This is no longer useful")]
pub struct MyType { }

fn main() { }

Emits (without line numbers)

warning: use of deprecated item 'MyType': This is no longer useful
  |
  = note: #[warn(deprecated)] on by default

---

(Old report)

In this branch / commit of itertools, running cargo build shows a deprecation error without location in the code.

• repo https://github.com/bluss/rust-itertools
• branch std-deprecations-bug (commit fe7ee14276c68ec1de225c70e80da7535565d607)

Running `rustc --crate-name itertools src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="default"' --cfg 'feature="use_std"' -C metadata=ba59252761bab077 -C extra-filename=-ba59252761bab077 --out-dir /home/travis/build/bluss/rust-itertools/target/debug/deps -L dependency=/home/travis/build/bluss/rust-itertools/target/debug/deps --extern either=/home/travis/build/bluss/rust-itertools/target/debug/deps/libeither-298d84de5004c8fc.rlib`
warning: use of deprecated item: Use std .step_by() instead
  |
  = note: #[warn(deprecated)] on by default
warning: use of deprecated item: Use std .step_by() instead

Version: rustc 1.32.0-nightly (1f57e48 2018-11-23)

[bluss]

Culprit found: the derives on top of the deprecated type.

#[deprecated(note="Use std .step_by() instead", since="0.8")]
#[derive(Clone, Debug)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct Step<I> {
    iter: Fuse<I>,
    skip: usize,
}

Inserting an#[allow(deprecated)] on the deprecated struct silences the mystery warning.

[joshlf]

Same as #38832?