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Add some hints to the docs for cfg() targets #6990

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May 28, 2019
Merged

Add some hints to the docs for cfg() targets #6990

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03294f8
Add some hints to the docs for `cfg()` targets
notriddle May 27, 2019
f2663ba
Update specifying-dependencies.md
notriddle May 28, 2019
69f9adb
Link cfg, remove old note, fix link.
ehuss May 28, 2019
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13 changes: 13 additions & 0 deletions src/doc/src/reference/specifying-dependencies.md
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Expand Up @@ -461,6 +461,19 @@ Like with Rust, the syntax here supports the `not`, `any`, and `all` operators
to combine various cfg name/value pairs. Note that the `cfg` syntax has only
been available since Cargo 0.9.0 (Rust 1.8.0).

If you want to know which cfg targets are available on your platform, run
`rustc --print=cfg` from the command line. If you want to know which `cfg`
targets are available for another platform, such as 64-bit Windows,
run `rustc --print=cfg --target=x86_64-pc-windows-msvc`.
If you want to know which cfg targets are used by the release profile,
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add the `-O` parameter.

This same set of values is used throughout the build for all crates in your project,
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I'm a little confused by how this is worded. Instead of trying to explain why it works this way, I would just be direct and say cfg(feature = "foo") cannot be used in the [target] table, with a link to the [features] section. Similarly test, proc-macro, and debug-assertions do not work either, though I'm not sure if it is worthwhile to list everything that does or does not work.

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@notriddle notriddle May 28, 2019 •
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Not wanting to list all of the configuration options is the reason I tried to explain why it works this way. Between giving a mechanical explanation of why feature flags don't work, and pointing the user at --print=cfg, I figured those would allow the user to figure out which options work and which options don't.

I stripped it out, though. Let me know if you can think of a better way to do this.

so, unlike in your Rust source code,
you cannot use `[target.'cfg(feature = "my_crate")'.dependencies]` to add
dependencies based on optional crate features.
Use [the `[features]` section](manifest.md#the-features-section) instead.

In addition to `#[cfg]` syntax, Cargo also supports listing out the full target
the dependencies would apply to:

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