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Fix note about using proc_macro with Cargo. #815

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Jun 10, 2020
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Fix note about using proc_macro with Cargo. #815

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19 changes: 8 additions & 11 deletions src/items/extern-crates.md
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Original file line number Diff line number Diff line change
Expand Up @@ -71,21 +71,19 @@ prelude lookups within that module.
> Beginning in the 2018 edition, [use declarations] can reference crates in
> the extern prelude, so it is considered unidiomatic to use `extern crate`.

> **Note**: Additional crates that ship with `rustc`, such as [`proc_macro`],
> [`alloc`], and [`test`], are not automatically included with the `--extern`
> flag when using Cargo. They must be brought into scope with an `extern
> crate` declaration, even in the 2018 edition.
> **Note**: Additional crates that ship with `rustc`, such as [`alloc`], and
> [`test`], are not automatically included with the `--extern` flag when using
> Cargo. They must be brought into scope with an `extern crate` declaration,
> even in the 2018 edition.
>
> ```rust
> extern crate proc_macro;
> use proc_macro::TokenStream;
> extern crate alloc;
> use alloc::rc::Rc;
> ```

<!--
The proc_macro/alloc/test limitation may be lifted if the `--extern`
flag is stabilized and used. See tracking issue
https://github.com/rust-lang/rust/issues/57288 and the unstable
`--extern` flag added in https://github.com/rust-lang/rust/pull/54116.
See https://github.com/rust-lang/rust/issues/57288 for more about the
alloc/test limitation.
-->

## Underscore Imports
Expand All @@ -110,6 +108,5 @@ crate to access only its macros.
[`alloc`]: https://doc.rust-lang.org/alloc/
[`no_implicit_prelude`]: modules.md#prelude-items
[`no_std`]: ../crates-and-source-files.md#preludes-and-no_std
[`proc_macro`]: https://doc.rust-lang.org/proc_macro/
[`test`]: https://doc.rust-lang.org/test/
[use declarations]: use-declarations.md