Add a copy of cfg_if to core's internal_macros.rs

core can't depend on external crates the way std can. Rather than revert
usage of cfg_if, add a copy of it to core. This does not export our
copy, even unstably; such a change could occur in a later commit.

Author: joshtriplett

library/core/src/internal_macros.rs

@@ -187,3 +187,96 @@ macro_rules! impl_fn_for_zst {
         )+
     }
 }
+
+/// A macro for defining `#[cfg]` if-else statements.
+///
+/// `cfg_if` is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade
+/// of `#[cfg]` cases, emitting the implementation which matches first.
+///
+/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code without having to
+/// rewrite each clause multiple times.
+///
+/// # Example
+///
+/// ```
+/// cfg_if! {
+///     if #[cfg(unix)] {
+///         fn foo() { /* unix specific functionality */ }
+///     } else if #[cfg(target_pointer_width = "32")] {
+///         fn foo() { /* non-unix, 32-bit functionality */ }
+///     } else {
+///         fn foo() { /* fallback implementation */ }
+///     }
+/// }
+///
+/// # fn main() {}
+/// ```
+// This is a copy of `cfg_if!` from the `cfg_if` crate.
+// The recursive invocations should use $crate if this is ever exported.
+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 )* }
+    ) => {
+        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 )* }
+        )*
+    ) => {
+        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 is 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 ),* ))
+        ))]
+        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.
+        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 )*
+    };
+}