use unchecked mul to compute slice sizes

...since slice sizes can't signed wrap

see https://doc.rust-lang.org/std/slice/fn.from_raw_parts.html

> The total size len * mem::size_of::<T>() of the slice must be no larger than isize::MAX.

compiler/rustc_codegen_ssa/src/glue.rs

@@ -39,7 +39,12 @@ pub fn size_and_align_of_dst<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             // The info in this case is the length of the str, so the size is that
             // times the unit size.
             (
-                bx.mul(info.unwrap(), bx.const_usize(unit.size.bytes())),
+                // All slice sizes must fit into `isize`, so this multiplication cannot (signed) wrap.
+                // NOTE: ideally, we want the effects of both `unchecked_smul` and `unchecked_umul`
+                // (resulting in `mul nsw nuw` in LLVM IR), since we know that the multiplication
+                // cannot signed wrap, and that both operands are non-negative. But at the time of writing,
+                // `BuilderMethods` can't do this, and it doesn't seem to enable any further optimizations.
+                bx.unchecked_smul(info.unwrap(), bx.const_usize(unit.size.bytes())),
                 bx.const_usize(unit.align.abi.bytes()),
             )
         }