Auto merge of rust-lang#118228 - Mark-Simulacrum:alloc-opt, r=scottmcm

Indicate that multiplication in Layout::array cannot overflow

Since rust-lang#113113, we have added a check that skips calling into the allocator at all if `capacity == 0`. The global, default allocator will not actually try to allocate though; it returns a dangling pointer explicitly. However, these two checks are not merged/deduplicated by LLVM and so we're comparing to zero twice whenever vectors are allocated/grown. Probably cheap, but also potentially expensive in code size and seems like an unfortunate miss.

This removes that extra check by telling LLVM that the multiplication as part of Layout::array can't overflow, turning the original non-zero value into a zero value afterwards. In my checks locally this successfully drops the duplicate comparisons.

See https://rust.godbolt.org/z/b6nPP9dcK for a code example.

```rust
pub fn foo(elements: usize) -> Vec<u32> {
    Vec::with_capacity(elements)
}
```

r? `@scottmcm` since you touched this in a32305a - curious if you have thoughts on doing this / can confirm my model of this being correct.

Author: bors

library/core/src/alloc/layout.rs

@@ -450,7 +450,11 @@ impl Layout {
                 return Err(LayoutError);
             }
 
-            let array_size = element_size * n;
+            // SAFETY: We just checked that we won't overflow `usize` when we multiply.
+            // This is a useless hint inside this function, but after inlining this helps
+            // deduplicate checks for whether the overall capacity is zero (e.g., in RawVec's
+            // allocation path) before/after this multiplication.
+            let array_size = unsafe { element_size.unchecked_mul(n) };
 
             // SAFETY: We just checked above that the `array_size` will not
             // exceed `isize::MAX` even when rounded up to the alignment.