Rollup merge of #61720 - alexcrichton:libstd-cfg-if-dep, r=sfackler

std: Remove internal definitions of `cfg_if!` macro

This is duplicated in a few locations throughout the sysroot to work
around issues with not exporting a macro in libstd but still wanting it
available to sysroot crates to define blocks. Nowadays though we can
simply depend on the `cfg-if` crate on crates.io, allowing us to use it
from there!

2 files changed, 25 insertions, 106 deletions

diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index 94eddbeda2b..519212bb6cb 100644
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -111,31 +111,31 @@ pub fn spin_loop() {
 /// This function is a no-op, and does not even read from `dummy`.
 #[inline]
 #[unstable(feature = "test", issue = "27812")]
+#[allow(unreachable_code)] // this makes #[cfg] a bit easier below.
 pub fn black_box<T>(dummy: T) -> T {
-    cfg_if! {
-        if #[cfg(any(
-            target_arch = "asmjs",
-            all(
-                target_arch = "wasm32",
-                target_os = "emscripten"
-            )
-        ))] {
-            #[inline]
-            unsafe fn black_box_impl<T>(d: T) -> T {
-                // these targets do not support inline assembly
-                let ret = crate::ptr::read_volatile(&d);
-                crate::mem::forget(d);
-                ret
-            }
-        } else {
-            #[inline]
-            unsafe fn black_box_impl<T>(d: T) -> T {
-                // we need to "use" the argument in some way LLVM can't
-                // introspect.
-                asm!("" : : "r"(&d));
-                d
-            }
-        }
+    // We need to "use" the argument in some way LLVM can't introspect, and on
+    // targets that support it we can typically leverage inline assembly to do
+    // this. LLVM's intepretation of inline assembly is that it's, well, a black
+    // box. This isn't the greatest implementation since it probably deoptimizes
+    // more than we want, but it's so far good enough.
+    #[cfg(not(any(
+        target_arch = "asmjs",
+        all(
+            target_arch = "wasm32",
+            target_os = "emscripten"
+        )
+    )))]
+    unsafe {
+        asm!("" : : "r"(&dummy));
+        return dummy;
+    }
+
+    // Not all platforms support inline assembly so try to do something without
+    // inline assembly which in theory still hinders at least some optimizations
+    // on those targets. This is the "best effort" scenario.
+    unsafe {
+        let ret = crate::ptr::read_volatile(&dummy);
+        crate::mem::forget(dummy);
+        ret
     }
-    unsafe { black_box_impl(dummy) }
 }
diff --git a/src/libcore/internal_macros.rs b/src/libcore/internal_macros.rs
index dd5b92857be..3acf2ec837d 100644
--- a/src/libcore/internal_macros.rs
+++ b/src/libcore/internal_macros.rs
@@ -117,84 +117,3 @@ macro_rules! impl_fn_for_zst {
         )+
     }
 }
-
-/// A macro for defining `#[cfg]` if-else statements.
-///
-/// The macro provided by this crate, `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
-///
-/// ```
-/// #[macro_use]
-/// extern crate cfg_if;
-///
-/// 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() {}
-/// ```
-macro_rules! cfg_if {
-    // match if/else chains with a final `else`
-    ($(
-        if #[cfg($($meta:meta),*)] { $($it:item)* }
-    ) else * else {
-        $($it2:item)*
-    }) => {
-        cfg_if! {
-            @__items
-            () ;
-            $( ( ($($meta),*) ($($it)*) ), )*
-            ( () ($($it2)*) ),
-        }
-    };
-
-    // match if/else chains lacking a final `else`
-    (
-        if #[cfg($($i_met:meta),*)] { $($i_it:item)* }
-        $(
-            else if #[cfg($($e_met:meta),*)] { $($e_it:item)* }
-        )*
-    ) => {
-        cfg_if! {
-            @__items
-            () ;
-            ( ($($i_met),*) ($($i_it)*) ),
-            $( ( ($($e_met),*) ($($e_it)*) ), )*
-            ( () () ),
-        }
-    };
-
-    // Internal and recursive macro to emit all the items
-    //
-    // Collects all the negated cfgs in a list at the beginning and after the
-    // semicolon is all the remaining items
-    (@__items ($($not:meta,)*) ; ) => {};
-    (@__items ($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
-        // Emit all items within one block, applying an approprate #[cfg]. The
-        // #[cfg] will require all `$m` matchers specified and must also negate
-        // all previous matchers.
-        cfg_if! { @__apply cfg(all($($m,)* not(any($($not),*)))), $($it)* }
-
-        // Recurse to emit all other items in `$rest`, and when we do so add all
-        // our `$m` matchers to the list of `$not` matchers as future emissions
-        // will have to negate everything we just matched as well.
-        cfg_if! { @__items ($($not,)* $($m,)*) ; $($rest)* }
-    };
-
-    // Internal macro to Apply a cfg attribute to a list of items
-    (@__apply $m:meta, $($it:item)*) => {
-        $(#[$m] $it)*
-    };
-}