Makes inline impl the default for 1.22+ with a polyfill for unreachable_unchecked.

Also editing the commit message to force a CI run. Woo!

Author: anp

build.rs

@@ -23,8 +23,8 @@ fn main() {
     let nightly_feature_enabled = is_var_set("CARGO_FEATURE_NIGHTLY");
     let spin_feature_enabled = is_var_set("CARGO_FEATURE_SPIN_NO_STD");
 
-    let version_geq_127 = version().unwrap() >= Version::new(1, 27, 0);
-    let unreachable_hint_supported = version_geq_127 || nightly_feature_enabled;
+    let version_geq_122 = version().unwrap() >= Version::new(1, 22, 0);
+    let drop_in_static_supported = version_geq_122 || nightly_feature_enabled;
 
     // precedence:
     // 1. explicit requests via cfg or spin_no_std feature
@@ -36,11 +36,17 @@ fn main() {
         "inline"
     } else if force_spin_cfg || spin_feature_enabled {
         "spin"
-    } else if unreachable_hint_supported {
+    } else if drop_in_static_supported {
         "inline"
     } else {
         "heap"
     };
 
     println!("cargo:rustc-cfg=lazy_static_{}_impl", impl_name);
+
+    let version_geq_127 = version().unwrap() >= Version::new(1, 27, 0);
+    let core_unreachable_unchecked_supported = version_geq_127 || nightly_feature_enabled;
+    if core_unreachable_unchecked_supported {
+        println!("cargo:rustc-cfg=lazy_static_core_unreachable_unchecked");
+    }
 }

src/inline_lazy.rs

@@ -31,7 +31,7 @@ impl<T: Sync> Lazy<T> {
         unsafe {
             match self.0 {
                 Some(ref x) => x,
-                None => core::hint::unreachable_unchecked(),
+                None => unreachable_unchecked(),
             }
         }
     }
@@ -46,3 +46,15 @@ macro_rules! __lazy_static_create {
         static mut $NAME: $crate::lazy::Lazy<$T> = $crate::lazy::Lazy::INIT;
     };
 }
+
+#[cfg(lazy_static_core_unreachable_unchecked)]
+use core::hint::unreachable_unchecked;
+
+#[cfg(not(lazy_static_core_unreachable_unchecked))]
+/// Polyfill for core::hint::unreachable_unchecked. Included to support Rust prior to 1.27. See
+/// [issue #102](https://github.com/rust-lang-nursery/lazy-static.rs/issues/102#issuecomment-400959779)
+/// for details.
+unsafe fn unreachable_unchecked() -> ! {
+    enum Void {}
+    match std::mem::uninitialized::<Void>() {}
+}