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. 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)`.

library/alloc/src/raw_vec.rs

@@ -291,6 +291,10 @@ 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
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
         }
     }
 
@@ -305,10 +309,13 @@ 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)?;
+            unsafe {
+                // Inform the optimizer that the reservation has succeeded
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
         }
+        Ok(())
     }
 
     /// Ensures that the buffer contains at least enough space to hold `len +
@@ -339,7 +346,14 @@ 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)?;
+            unsafe {
+                // Inform the optimizer that the reservation has succeeded
+                core::intrinsics::assume(!self.needs_to_grow(len, additional));
+            }
+        }
+        Ok(())
     }
 
     /// Shrinks the buffer down to the specified capacity. If the given amount

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

@@ -1,3 +1,3 @@
-thread 'main' panicked at library/alloc/src/raw_vec.rs:534:5:
+thread 'main' panicked at library/alloc/src/raw_vec.rs:548:5:
 capacity overflow
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace