Auto merge of rust-lang#116568 - kornelski:hint-reserved, r=<try>

Hint optimizer about reserved capacity

The fact that `Vec` had capacity increased is not known to the optimizer, because functions like `do_reserve_and_handle` are not inlined. (rust-lang#82801) Because of that, code such as:

```rust
vec.try_reserve(123)?;
vec.extend_from_slice(&s[..123]);
```

Tries to reserve the capacity **twice**. This is unnecessary code bloat.

https://rust.godbolt.org/z/YWY16Ezej

Adding a hint after reserve optimizes out the next check of `self.needs_to_grow(len, additional)`.

Author: bors

library/alloc/src/raw_vec.rs

@@ -292,6 +292,11 @@ impl<T, A: Allocator> RawVec<T, A> {
         if self.needs_to_grow(len, additional) {
             do_reserve_and_handle(self, len, additional);
         }
+
+        unsafe {
+            // Inform the optimizer that the reservation has succeeded or wasn't needed
+            core::intrinsics::assume(!self.needs_to_grow(len, additional));
+        }
     }
 
     /// A specialized version of `reserve()` used only by the hot and

tests/ui/hygiene/panic-location.run.stderr

@@ -1,3 +1,3 @@
-thread 'main' panicked at library/alloc/src/raw_vec.rs:545:5:
+thread 'main' panicked at library/alloc/src/raw_vec.rs:550:5:
 capacity overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace