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Clarify --extern documentation.

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Fixes #64731, #73531.

See also #64402#issuecomment-530852886
This commit is contained in:
Adrian Taylor 2020-06-20 17:02:18 -07:00
parent f455e46eae
commit a63eb3c678
1 changed files with 10 additions and 2 deletions
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src/doc/rustc/src/command-line-arguments.md
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@ -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: