Auto merge of rust-lang#121300 - c410-f3r:perf-try, r=<try>

[Experiment] Eliminate possible `Vec::push` branches

Related to rust-lang#105156. Requesting a perf run.

```rust
pub fn push(v: &mut Vec<u8>) {
    let _ = v.reserve(4);
    v.push(1);
    v.push(2);
    v.push(3);
    v.push(4);
}
```

AFAICT, the codegen backend should infer the infallibility of these `push`s but unfortunately with LLVM 18 we still have unnecessary `reserve_for_push` branches.

For the sake of curiosity, `assert_unchecked` was included in `push` to see any potential impact of such change. Take a look at the generated assembly at https://godbolt.org/z/b5jjPhsf8.

AFAICT (again), the assumption of more available capacity for each `push` is not valid for all situations.

Author: bors

library/alloc/src/vec/mod.rs

@@ -905,10 +905,14 @@ impl<T, A: Allocator> Vec<T, A> {
     /// vec.reserve(10);
     /// assert!(vec.capacity() >= 11);
     /// ```
+    #[inline(always)]
     #[cfg(not(no_global_oom_handling))]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn reserve(&mut self, additional: usize) {
         self.buf.reserve(self.len, additional);
+        unsafe {
+            core::hint::assert_unchecked(self.len().unchecked_add(additional) <= self.capacity());
+        }
     }
 
     /// Reserves the minimum capacity for at least `additional` more elements to
@@ -1925,6 +1929,7 @@ impl<T, A: Allocator> Vec<T, A> {
             let end = self.as_mut_ptr().add(self.len);
             ptr::write(end, value);
             self.len += 1;
+            core::hint::assert_unchecked(self.len() <= self.capacity());
         }
     }
 

tests/codegen/reserve_local_push.rs

@@ -0,0 +1,16 @@
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn push(v: &mut Vec<u8>) {
+    let _ = v.reserve(4);
+    // CHECK-NOT: call {{.*}}reserve_for_push
+    v.push(1);
+    // CHECK-NOT: call {{.*}}reserve_for_push
+    v.push(2);
+    // CHECK-NOT: call {{.*}}reserve_for_push
+    v.push(3);
+    // CHECK-NOT: call {{.*}}reserve_for_push
+    v.push(4);
+}