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Document unstable --extern options #98226

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Jun 22, 2022
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Document unstable --extern options #98226

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22 changes: 22 additions & 0 deletions src/doc/unstable-book/src/compiler-flags/extern-options.md
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# `--extern` Options

The behavior of the `--extern` flag can be modified with `noprelude`, `priv` or `nounused` options.

This is unstable feature, so you have to provide `-Zunstable-options` to enable it.

## Examples

Use your own build of the `core` crate.

`rustc main.rs -Z unstable-options --extern noprelude:core=libcore.rlib`

To use multiple options, separate them with a comma:

`rustc main.rs -Z unstable-options --extern noprelude,priv,nounused:mydep=mydep.rlib`

## Options

* `noprelude`: Do not add the crate to the external prelude. If used, it will need to be imported using `extern crate`.
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This is used by the [build-std project](https://github.com/rust-lang/wg-cargo-std-aware/) to simulate compatibility with sysroot-only crates.
* `priv`: Mark the crate as a private dependency for the [`exported_private_dependencies`](../../rustc/lints/listing/warn-by-default.html#exported-private-dependencies) lint.
* `nounused`: Suppress [`unused-crate-dependencies`](../../rustc/lints/listing/allowed-by-default.html#unused-crate-dependencies) warnings for the crate.