Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+% Conditional Compilation
+
+<small>There is a new edition of the book and this is an old link.</small>
+
+> Sometimes one wants to have different compiler outputs from the same code, depending on build target, such as targeted operating system, or to enable release builds.
+> Configuration options are either provided by the compiler or passed in on the command line using.
+> Rust code then checks for their presence using the `#[cfg(...)]` attribute
+
+```rust
+// The function is only included in the build when compiling for macOS
+#[cfg(target_os = "macos")]
+fn macos_only() {
+  // ...
+}
+```
+
+---
+
+This particular chapter does not exist in [the second edition][2].
+The best place to learn about it is [the Rust Reference][3].
+
+* **[In the Rust Reference: Ch 5 — Conditional Compilation][3]**
+* <small>[In the first edition: Ch 4.3 — Conditional Compilation][1]</small>
+
+
+[1]: https://doc.rust-lang.org/1.30.0/book/first-edition/conditional-compilation.html
+[2]: index.html
+[3]: ../reference/conditional-compilation.html