Hint optimizer about reserved capacity

Author: kornelski

library/alloc/src/raw_vec.rs

@@ -291,6 +291,11 @@ impl<T, A: Allocator> RawVec<T, A> {
 
         if self.needs_to_grow(len, additional) {
             do_reserve_and_handle(self, len, additional);
+            if self.needs_to_grow(len, additional) {
+                unsafe {
+                    core::hint::unreachable_unchecked();
+                }
+            }
         }
     }
 
@@ -305,10 +310,14 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// The same as `reserve`, but returns on errors instead of panicking or aborting.
     pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
         if self.needs_to_grow(len, additional) {
-            self.grow_amortized(len, additional)
-        } else {
-            Ok(())
+            self.grow_amortized(len, additional)?;
+            if self.needs_to_grow(len, additional) {
+                unsafe {
+                    core::hint::unreachable_unchecked();
+                }
+            }
         }
+        Ok(())
     }
 
     /// Ensures that the buffer contains at least enough space to hold `len +
@@ -339,7 +348,15 @@ impl<T, A: Allocator> RawVec<T, A> {
         len: usize,
         additional: usize,
     ) -> Result<(), TryReserveError> {
-        if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
+        if self.needs_to_grow(len, additional) {
+            self.grow_exact(len, additional)?;
+            if self.needs_to_grow(len, additional) {
+                unsafe {
+                    core::hint::unreachable_unchecked();
+                }
+            }
+        }
+        Ok(())
     }
 
     /// Shrinks the buffer down to the specified capacity. If the given amount