define cfg_if! and cfg_if_expr! internal macros

examples/write_dir.rs

@@ -3,7 +3,7 @@
 use anyhow::Context;
 use clap::{Parser, ValueEnum};
 use std::io::prelude::*;
-use zip::{result::ZipError, write::SimpleFileOptions};
+use zip::{cfg_if_expr, result::ZipError, write::SimpleFileOptions};
 
 use std::fs::File;
 use std::path::{Path, PathBuf};
@@ -30,59 +30,51 @@ enum CompressionMethod {
     Zstd,
 }
 
-fn main() {
-    std::process::exit(real_main());
-}
-
-fn real_main() -> i32 {
+fn main() -> ! {
     let args = Args::parse();
     let src_dir = &args.source;
     let dst_file = &args.destination;
     let method = match args.compression_method {
         CompressionMethod::Stored => zip::CompressionMethod::Stored,
-        CompressionMethod::Deflated => {
-            #[cfg(not(feature = "deflate-flate2"))]
-            {
+        CompressionMethod::Deflated => cfg_if_expr! {
+            #[cfg(feature = "deflate-flate2")] => zip::CompressionMethod::Deflated,
+            _ => {
                 println!("The `deflate-flate2` feature is not enabled");
-                return 1;
+                std::process::exit(1)
             }
-            #[cfg(feature = "deflate-flate2")]
-            zip::CompressionMethod::Deflated
-        }
-        CompressionMethod::Bzip2 => {
-            #[cfg(not(feature = "bzip2"))]
-            {
+        },
+        CompressionMethod::Bzip2 => cfg_if_expr! {
+            #[cfg(feature = "bzip2")] => zip::CompressionMethod::Bzip2,
+            _ => {
                 println!("The `bzip2` feature is not enabled");
-                return 1;
+                std::process::exit(1)
             }
-            #[cfg(feature = "bzip2")]
-            zip::CompressionMethod::Bzip2
-        }
-        CompressionMethod::Xz => {
-            #[cfg(not(feature = "xz"))]
-            {
+        },
+        CompressionMethod::Xz => cfg_if_expr! {
+            #[cfg(feature = "xz")] => zip::CompressionMethod::Xz,
+            _ => {
                 println!("The `xz` feature is not enabled");
-                return 1;
+                std::process::exit(1)
             }
-            #[cfg(feature = "xz")]
-            zip::CompressionMethod::Xz
-        }
-        CompressionMethod::Zstd => {
-            #[cfg(not(feature = "zstd"))]
-            {
+        },
+        CompressionMethod::Zstd => cfg_if_expr! {
+            #[cfg(feature = "zstd")] => zip::CompressionMethod::Zstd,
+            _ => {
                 println!("The `zstd` feature is not enabled");
-                return 1;
+                std::process::exit(1)
             }
-            #[cfg(feature = "zstd")]
-            zip::CompressionMethod::Zstd
-        }
+        },
     };
     match doit(src_dir, dst_file, method) {
-        Ok(_) => println!("done: {src_dir:?} written to {dst_file:?}"),
-        Err(e) => eprintln!("Error: {e:?}"),
+        Ok(_) => {
+            println!("done: {src_dir:?} written to {dst_file:?}");
+            std::process::exit(0);
+        }
+        Err(e) => {
+            eprintln!("Error: {e:?}");
+            std::process::abort();
+        }
     }
-
-    0
 }
 
 fn zip_dir<T>(

src/compression.rs

@@ -2,6 +2,8 @@
 
 use std::{fmt, io};
 
+use crate::cfg_if_expr;
+
 #[allow(deprecated)]
 /// Identifies the storage format used to compress a file within a ZIP archive.
 ///
@@ -228,11 +230,10 @@ impl CompressionMethod {
 
 impl Default for CompressionMethod {
     fn default() -> Self {
-        #[cfg(feature = "_deflate-any")]
-        return CompressionMethod::Deflated;
-
-        #[cfg(not(feature = "_deflate-any"))]
-        return CompressionMethod::Stored;
+        cfg_if_expr! {
+            #[cfg(feature = "_deflate-any")] => CompressionMethod::Deflated,
+            _ => CompressionMethod::Stored
+        }
     }
 }
 

src/lib.rs

@@ -31,7 +31,7 @@
 //!
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![warn(missing_docs)]
-#![allow(unexpected_cfgs)] // Needed for cfg(fuzzing) on nightly as of 2024-05-06
+#![allow(unexpected_cfgs)] // Needed for cfg(fuzzing)
 pub use crate::compression::{CompressionMethod, SUPPORTED_COMPRESSION_METHODS};
 pub use crate::read::HasZipMetadata;
 pub use crate::read::ZipArchive;
@@ -71,3 +71,6 @@ zip = \"="]
 #[doc = "\"\n\
 ```"]
 pub mod unstable;
+
+#[doc(hidden)]
+pub mod macros;

src/macros.rs

@@ -0,0 +1,162 @@
+//! Macros used internally.
+//!
+//! These may technically be exported, but that's only to make them available to internal
+//! project dependencies. The `#[doc(hidden)]` mark indicates that these are not stable or supported
+//! APIs, and should not be relied upon by external dependees.
+
+/// The single macro export of the [`cfg-if`](https://docs.rs/cfg-if) crate.
+///
+/// It is packaged here to avoid pulling in another dependency. The stdlib does the same[^1].
+///
+/// [^1]: https://github.com/rust-lang/rust/blob/a2db9280539229a3b8a084a09886670a57bc7e9c/library/compiler-builtins/libm/src/math/support/macros.rs#L1
+#[doc(hidden)]
+#[macro_export]
+macro_rules! cfg_if {
+    // match if/else chains with a final `else`
+    (
+        $(
+            if #[cfg( $i_meta:meta )] { $( $i_tokens:tt )* }
+        ) else+
+        else { $( $e_tokens:tt )* }
+    ) => {
+        $crate::cfg_if! {
+            @__items () ;
+            $(
+                (( $i_meta ) ( $( $i_tokens )* )) ,
+            )+
+            (() ( $( $e_tokens )* )) ,
+        };
+    };
+
+    // match if/else chains lacking a final `else`
+    (
+        if #[cfg( $i_meta:meta )] { $( $i_tokens:tt )* }
+        $(
+            else if #[cfg( $e_meta:meta )] { $( $e_tokens:tt )* }
+        )*
+    ) => {
+        $crate::cfg_if! {
+            @__items () ;
+            (( $i_meta ) ( $( $i_tokens )* )) ,
+            $(
+                (( $e_meta ) ( $( $e_tokens )* )) ,
+            )*
+        };
+    };
+
+    // Internal and recursive macro to emit all the items
+    //
+    // Collects all the previous cfgs in a list at the beginning, so they can be
+    // negated. After the semicolon are all the remaining items.
+    (@__items ( $( $_:meta , )* ) ; ) => {};
+    (
+        @__items ( $( $no:meta , )* ) ;
+        (( $( $yes:meta )? ) ( $( $tokens:tt )* )) ,
+        $( $rest:tt , )*
+    ) => {
+        // Emit all items within one block, applying an appropriate #[cfg]. The
+        // #[cfg] will require all `$yes` matchers specified and must also negate
+        // all previous matchers.
+        #[cfg(all(
+            $( $yes , )?
+            not(any( $( $no ),* ))
+        ))]
+        $crate::cfg_if! { @__identity $( $tokens )* }
+
+        // Recurse to emit all other items in `$rest`, and when we do so add all
+        // our `$yes` matchers to the list of `$no` matchers as future emissions
+        // will have to negate everything we just matched as well.
+        $crate::cfg_if! {
+            @__items ( $( $no , )* $( $yes , )? ) ;
+            $( $rest , )*
+        };
+    };
+
+    // Internal macro to make __apply work out right for different match types,
+    // because of how macros match/expand stuff.
+    (@__identity $( $tokens:tt )* ) => {
+        $( $tokens )*
+    };
+}
+
+/// Similar to [`cfg_if`](cfg_if), but accepts a list of expressions, and generates an internal
+/// closure to return each value.
+///
+/// The main reason this is necessary is because attaching `#[cfg(...)]` annotations to certain
+/// types of statements requires a nightly feature, or `cfg_if` would be enough on its own. This
+/// macro's restricted interface allows it to generate a closure as a circumlocution that is legal
+/// on stable rust.
+///
+/// Note that any `return` operation within the expressions provided to this macro will apply to the
+/// generated closure, not the enclosing scope--it cannot be used to interfere with external
+/// control flow.
+///
+/// The generated closure is non-[`const`](const@keyword), so cannot be used inside `const` methods.
+#[doc(hidden)]
+#[macro_export]
+macro_rules! cfg_if_expr {
+    // Match =>, chains, maybe with a final _ => catchall clause.
+    (
+        $( $ret_ty:ty : )?
+        $(
+            #[cfg( $i_meta:meta )] => $i_val:expr
+        ),+ ,
+            _ => $rem_val:expr $(,)?
+    ) => {
+        (|| $( -> $ret_ty )? {
+            $crate::cfg_if_expr! {
+                @__items ();
+                $(
+                    (( $i_meta ) (
+                        #[allow(unreachable_code)]
+                        return $i_val ;
+                    )) ,
+                )+
+                    (() (
+                        #[allow(unreachable_code)]
+                        return $rem_val ;
+                    )) ,
+            }
+        })()
+    };
+    // Match =>, chains *without* any _ => clause.
+    (
+        $( $ret_ty:ty : )?
+        $(
+            #[cfg( $i_meta:meta )] => $i_val:expr
+        ),+ $(,)?
+    ) => {
+        (|| $( -> $ret_ty )? {
+            $crate::cfg_if_expr! {
+                @__items ();
+                $(
+                    (( $i_meta ) (
+                        #[allow(unreachable_code)]
+                        return $i_val ;
+                    )) ,
+                )+
+            }
+        })()
+    };
+
+    (@__items ( $( $_:meta , )* ) ; ) => {};
+    (
+        @__items ( $( $no:meta , )* ) ;
+        (( $( $yes:meta )? ) ( $( $tokens:tt )* )) ,
+        $( $rest:tt , )*
+    ) => {
+        #[cfg(all(
+            $( $yes , )?
+            not(any( $( $no ),* ))
+        ))]
+        $crate::cfg_if_expr! { @__identity $( $tokens )* }
+
+        $crate::cfg_if_expr! {
+            @__items ( $( $no , )* $( $yes , )? ) ;
+            $( $rest , )*
+        };
+    };
+    (@__identity $( $tokens:tt )* ) => {
+        $( $tokens )*
+    };
+}