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Document the ordering behavior of crate cfgs #1369

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Jun 22, 2023
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Document the ordering behavior of crate cfgs #1369

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6 changes: 6 additions & 0 deletions src/conditional-compilation.md
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Expand Up @@ -254,6 +254,12 @@ It is written as `cfg`, `(`, a configuration predicate, and finally `)`.
If the predicate is true, the thing is rewritten to not have the `cfg` attribute
on it. If the predicate is false, the thing is removed from the source code.

When a crate-level `cfg` has a false predicate, the behavior is slightly
different: any crate attributes preceding the `cfg` are kept, and any crate
attributes following the `cfg` are removed. This allows `#![no_std]` and
`#![no_core]` crates to avoid linking `std`/`core` even if a `#![cfg(...)]` has
removed the entire crate.

Some examples on functions:

```rust
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