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Clarify --extern documentation. #73567

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Jun 23, 2020
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Clarify --extern documentation. #73567

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12 changes: 10 additions & 2 deletions src/doc/rustc/src/command-line-arguments.md
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Expand Up @@ -273,10 +273,18 @@ This flag, when combined with other flags, makes them produce extra output.
This flag allows you to pass the name and location for an external crate of a
direct dependency. Indirect dependencies (dependencies of dependencies) are
located using the [`-L` flag](#option-l-search-path). The given crate name is
added to the [extern prelude], which is the same as specifying `extern crate`
within the root module. The given crate name does not need to match the name
added to the [extern prelude], similar to specifying `extern crate` within the
root module. The given crate name does not need to match the name
the library was built with.

Specifying `--extern` has one behavior difference from `extern crate`:
`--extern` merely makes the crate a _candidate_ for being linked; it does not
actually link it unless it's actively used. In rare occasions you may wish
to ensure a crate is linked even if you don't actively use it from your
code: for example, if it changes the global allocator or if it contains
`#[no_mangle]` symbols for use by other programming languages. In such
cases you'll need to use `extern crate`.

This flag may be specified multiple times. This flag takes an argument with
either of the following formats:

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