diff --git a/Cargo.lock b/Cargo.lock
index 948074ce0fc48..40c8047e8a2ac 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1813,6 +1813,7 @@ name = "panic_unwind"
 version = "0.0.0"
 dependencies = [
  "alloc 0.0.0",
+ "cfg-if 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "compiler_builtins 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)",
  "core 0.0.0",
  "libc 0.2.54 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3338,6 +3339,7 @@ dependencies = [
  "alloc 0.0.0",
  "backtrace 0.3.29 (registry+https://github.com/rust-lang/crates.io-index)",
  "cc 1.0.35 (registry+https://github.com/rust-lang/crates.io-index)",
+ "cfg-if 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "compiler_builtins 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)",
  "core 0.0.0",
  "dlmalloc 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3936,6 +3938,7 @@ name = "unwind"
 version = "0.0.0"
 dependencies = [
  "cc 1.0.35 (registry+https://github.com/rust-lang/crates.io-index)",
+ "cfg-if 0.1.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "compiler_builtins 0.1.16 (registry+https://github.com/rust-lang/crates.io-index)",
  "core 0.0.0",
  "libc 0.2.54 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index 94eddbeda2b86..519212bb6cb4e 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 dd5b92857be63..3acf2ec837d88 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)*
-    };
-}
diff --git a/src/libpanic_unwind/Cargo.toml b/src/libpanic_unwind/Cargo.toml
index 1b3901ac11a96..47cd09f1b0510 100644
--- a/src/libpanic_unwind/Cargo.toml
+++ b/src/libpanic_unwind/Cargo.toml
@@ -16,3 +16,4 @@ core = { path = "../libcore" }
 libc = { version = "0.2", default-features = false }
 unwind = { path = "../libunwind" }
 compiler_builtins = "0.1.0"
+cfg-if = "0.1.8"
diff --git a/src/libpanic_unwind/lib.rs b/src/libpanic_unwind/lib.rs
index 72ddafb420ce7..2bb9ce6ab220b 100644
--- a/src/libpanic_unwind/lib.rs
+++ b/src/libpanic_unwind/lib.rs
@@ -38,10 +38,7 @@ use core::mem;
 use core::raw;
 use core::panic::BoxMeUp;
 
-#[macro_use]
-mod macros;
-
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
         mod imp;
diff --git a/src/libpanic_unwind/macros.rs b/src/libpanic_unwind/macros.rs
deleted file mode 100644
index 659e977285e35..0000000000000
--- a/src/libpanic_unwind/macros.rs
+++ /dev/null
@@ -1,35 +0,0 @@
-/// A macro for defining `#[cfg]` if-else statements.
-///
-/// This 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.
-macro_rules! cfg_if {
-    ($(
-        if #[cfg($($meta:meta),*)] { $($it:item)* }
-    ) else * else {
-        $($it2:item)*
-    }) => {
-        __cfg_if_items! {
-            () ;
-            $( ( ($($meta),*) ($($it)*) ), )*
-            ( () ($($it2)*) ),
-        }
-    }
-}
-
-macro_rules! __cfg_if_items {
-    (($($not:meta,)*) ; ) => {};
-    (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
-        __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
-        __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
-    }
-}
-
-macro_rules! __cfg_if_apply {
-    ($m:meta, $($it:item)*) => {
-        $(#[$m] $it)*
-    }
-}
diff --git a/src/libstd/Cargo.toml b/src/libstd/Cargo.toml
index 30e23f1007f20..a170dae2b08ca 100644
--- a/src/libstd/Cargo.toml
+++ b/src/libstd/Cargo.toml
@@ -15,6 +15,7 @@ crate-type = ["dylib", "rlib"]
 
 [dependencies]
 alloc = { path = "../liballoc" }
+cfg-if = "0.1.8"
 panic_unwind = { path = "../libpanic_unwind", optional = true }
 panic_abort = { path = "../libpanic_abort" }
 core = { path = "../libcore" }
diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index a3356e6be2c3f..e0ffc9ba92f11 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -336,6 +336,12 @@ extern crate libc;
 #[allow(unused_extern_crates)]
 extern crate unwind;
 
+// Only needed for now for the `std_detect` module until that crate changes to
+// use `cfg_if::cfg_if!`
+#[macro_use]
+#[cfg(not(test))]
+extern crate cfg_if;
+
 // During testing, this crate is not actually the "real" std library, but rather
 // it links to the real std library, which was compiled from this same source
 // code. So any lang items std defines are conditionally excluded (or else they
diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index ef7179f0b6f90..92b9a16f3e316 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -896,39 +896,3 @@ mod builtin {
         ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
     }
 }
-
-/// Defines `#[cfg]` if-else statements.
-///
-/// This 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.
-macro_rules! cfg_if {
-    ($(
-        if #[cfg($($meta:meta),*)] { $($it:item)* }
-    ) else * else {
-        $($it2:item)*
-    }) => {
-        __cfg_if_items! {
-            () ;
-            $( ( ($($meta),*) ($($it)*) ), )*
-            ( () ($($it2)*) ),
-        }
-    }
-}
-
-macro_rules! __cfg_if_items {
-    (($($not:meta,)*) ; ) => {};
-    (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
-        __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
-        __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
-    }
-}
-
-macro_rules! __cfg_if_apply {
-    ($m:meta, $($it:item)*) => {
-        $(#[$m] $it)*
-    }
-}
diff --git a/src/libstd/os/mod.rs b/src/libstd/os/mod.rs
index 44cbc180b8b01..94e8b7805cf93 100644
--- a/src/libstd/os/mod.rs
+++ b/src/libstd/os/mod.rs
@@ -3,7 +3,7 @@
 #![stable(feature = "os", since = "1.0.0")]
 #![allow(missing_docs, nonstandard_style, missing_debug_implementations)]
 
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(rustdoc)] {
 
         // When documenting libstd we want to show unix/windows/linux modules as
diff --git a/src/libstd/sys/mod.rs b/src/libstd/sys/mod.rs
index 3f3cedc53b729..21360e2e0f028 100644
--- a/src/libstd/sys/mod.rs
+++ b/src/libstd/sys/mod.rs
@@ -22,7 +22,7 @@
 
 #![allow(missing_debug_implementations)]
 
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(unix)] {
         mod unix;
         pub use self::unix::*;
@@ -54,7 +54,7 @@ cfg_if! {
 // Windows when we're compiling for Linux.
 
 #[cfg(rustdoc)]
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(any(unix, target_os = "redox"))] {
         // On unix we'll document what's already available
         #[stable(feature = "rust1", since = "1.0.0")]
@@ -77,7 +77,7 @@ cfg_if! {
 }
 
 #[cfg(rustdoc)]
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(windows)] {
         // On windows we'll just be documenting what's already available
         #[allow(missing_docs)]
diff --git a/src/libstd/sys/wasm/mod.rs b/src/libstd/sys/wasm/mod.rs
index 9ea8bd1653851..7d157709eb6bf 100644
--- a/src/libstd/sys/wasm/mod.rs
+++ b/src/libstd/sys/wasm/mod.rs
@@ -40,7 +40,7 @@ pub mod stdio;
 
 pub use crate::sys_common::os_str_bytes as os_str;
 
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(target_feature = "atomics")] {
         #[path = "condvar_atomics.rs"]
         pub mod condvar;
diff --git a/src/libstd/sys/wasm/thread.rs b/src/libstd/sys/wasm/thread.rs
index 1dc786cd5d7b6..61b4003cd3d14 100644
--- a/src/libstd/sys/wasm/thread.rs
+++ b/src/libstd/sys/wasm/thread.rs
@@ -59,7 +59,7 @@ pub mod guard {
     pub unsafe fn init() -> Option<Guard> { None }
 }
 
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(all(target_feature = "atomics", feature = "wasm-bindgen-threads"))] {
         #[link(wasm_import_module = "__wbindgen_thread_xform__")]
         extern {
diff --git a/src/libstd/sys_common/mod.rs b/src/libstd/sys_common/mod.rs
index c4daedefd8e6d..13a59f66c5c2f 100644
--- a/src/libstd/sys_common/mod.rs
+++ b/src/libstd/sys_common/mod.rs
@@ -65,7 +65,7 @@ pub mod bytestring;
 pub mod process;
 pub mod fs;
 
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(any(target_os = "cloudabi",
                  target_os = "l4re",
                  target_os = "redox",
diff --git a/src/libunwind/Cargo.toml b/src/libunwind/Cargo.toml
index 4ddc878997ee5..f0f1bab425d7a 100644
--- a/src/libunwind/Cargo.toml
+++ b/src/libunwind/Cargo.toml
@@ -19,6 +19,7 @@ doc = false
 core = { path = "../libcore" }
 libc = { version = "0.2.43", features = ['rustc-dep-of-std'], default-features = false }
 compiler_builtins = "0.1.0"
+cfg-if = "0.1.8"
 
 [build-dependencies]
 cc = { optional = true, version = "1.0.1" }
diff --git a/src/libunwind/lib.rs b/src/libunwind/lib.rs
index 0ccffea317053..9182e349b196e 100644
--- a/src/libunwind/lib.rs
+++ b/src/libunwind/lib.rs
@@ -11,10 +11,7 @@
 
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]
 
-#[macro_use]
-mod macros;
-
-cfg_if! {
+cfg_if::cfg_if! {
     if #[cfg(target_env = "msvc")] {
         // no extra unwinder support needed
     } else if #[cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))] {
diff --git a/src/libunwind/libunwind.rs b/src/libunwind/libunwind.rs
index 339b554ed6abd..5794e0b7683cb 100644
--- a/src/libunwind/libunwind.rs
+++ b/src/libunwind/libunwind.rs
@@ -1,10 +1,5 @@
 #![allow(nonstandard_style)]
 
-macro_rules! cfg_if {
-    ( $( if #[cfg( $meta:meta )] { $($it1:item)* } else { $($it2:item)* } )* ) =>
-        ( $( $( #[cfg($meta)] $it1)* $( #[cfg(not($meta))] $it2)* )* )
-}
-
 use libc::{c_int, c_void, uintptr_t};
 
 #[repr(C)]
@@ -82,7 +77,7 @@ extern "C" {
     pub fn _Unwind_GetDataRelBase(ctx: *mut _Unwind_Context) -> _Unwind_Ptr;
 }
 
-cfg_if! {
+cfg_if::cfg_if! {
 if #[cfg(all(any(target_os = "ios", target_os = "netbsd", not(target_arch = "arm"))))] {
     // Not ARM EHABI
     #[repr(C)]
@@ -206,7 +201,9 @@ if #[cfg(all(any(target_os = "ios", target_os = "netbsd", not(target_arch = "arm
         pc
     }
 }
+} // cfg_if!
 
+cfg_if::cfg_if! {
 if #[cfg(not(all(target_os = "ios", target_arch = "arm")))] {
     // Not 32-bit iOS
     extern "C" {
diff --git a/src/libunwind/macros.rs b/src/libunwind/macros.rs
deleted file mode 100644
index 659e977285e35..0000000000000
--- a/src/libunwind/macros.rs
+++ /dev/null
@@ -1,35 +0,0 @@
-/// A macro for defining `#[cfg]` if-else statements.
-///
-/// This 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.
-macro_rules! cfg_if {
-    ($(
-        if #[cfg($($meta:meta),*)] { $($it:item)* }
-    ) else * else {
-        $($it2:item)*
-    }) => {
-        __cfg_if_items! {
-            () ;
-            $( ( ($($meta),*) ($($it)*) ), )*
-            ( () ($($it2)*) ),
-        }
-    }
-}
-
-macro_rules! __cfg_if_items {
-    (($($not:meta,)*) ; ) => {};
-    (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
-        __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
-        __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
-    }
-}
-
-macro_rules! __cfg_if_apply {
-    ($m:meta, $($it:item)*) => {
-        $(#[$m] $it)*
-    }
-}