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Clarify how cargo::metadata env var is selected (#14842)

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### What does this PR try to resolve?

At least for me, the explanation of how to use `cargo::metadata` is
confusing. It seems to imply that the `X` in `DEP_X_KEY` is derived from
the crate name, but it is actually derived from the `links` value in the
package. This PR tries to rephrase the explanation to make this less
confusing.
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Weihang Lo 2024-11-20 13:40:44 +00:00 committed by GitHub
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src/doc/src/reference/build-scripts.md
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@ -432,11 +432,12 @@ key-value pairs. This metadata is set with the `cargo::metadata=KEY=VALUE`
instruction.
The metadata is passed to the build scripts of **dependent** packages. For
example, if the package `bar` depends on `foo`, then if `foo` generates
`key=value` as part of its build script metadata, then the build script of
`bar` will have the environment variables `DEP_FOO_KEY=value`. See the ["Using
another `sys` crate"][using-another-sys] for an example of how this can be
used.
example, if the package `foo` depends on `bar`, which links `baz`, then if
`bar` generates `key=value` as part of its build script metadata, then the
build script of `foo` will have the environment variables `DEP_BAZ_KEY=value`
(note that the value of the `links` key is used).
See the ["Using another `sys` crate"][using-another-sys] for an example of
how this can be used.
Note that metadata is only passed to immediate dependents, not transitive
dependents.