Auto merge of #51653 - mglagla:option-unreachable, r=dtolnay

Option::get_or_insert(_with): Replace unreachable! with unreachable_unchecked

Optimize codegen for both functions as the None branch is trivially not reachable.

src/libcore/option.rs

@@ -146,7 +146,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use iter::{FromIterator, FusedIterator, TrustedLen};
-use {mem, ops};
+use {hint, mem, ops};
 use mem::PinMut;
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
@@ -784,7 +784,7 @@ impl<T> Option<T> {
 
         match *self {
             Some(ref mut v) => v,
-            _ => unreachable!(),
+            None => unsafe { hint::unreachable_unchecked() },
         }
     }
 
@@ -817,7 +817,7 @@ impl<T> Option<T> {
 
         match *self {
             Some(ref mut v) => v,
-            _ => unreachable!(),
+            None => unsafe { hint::unreachable_unchecked() },
         }
     }