Can't use `usize::unchecked_mul()` because "checked mul is not supported yet"

[tuguzT]

I've stumbled upon this while trying to use core::alloc::Layout type in my shader code.
You see, Layout::repeat() function uses checked math to return an error, but in my case this should not be possible, so I've reimplemented it using usize::unchecked_mul().
I expected my code to compile, but got this compiler error instead:

error: checked mul is not supported yet
      --> ***\.rustup\toolchains\nightly-2025-06-30-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\num\mod.rs:1224:5
       |
  1224 | /     uint_impl! {
  1225 | |         Self = usize,
  1226 | |         ActualT = u32,
  1227 | |         SignedT = isize,
  ...    |
  1241 | |         bound_condition = " on 32-bit targets",
  1242 | |     }
       | |_____^
       |
  note: used from within `<usize>::overflowing_mul`
      --> ***\.rustup\toolchains\nightly-2025-06-30-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\num\mod.rs:1224:5
       |
  1224 | /     uint_impl! {
  1225 | |         Self = usize,
  1226 | |         ActualT = u32,
  1227 | |         SignedT = isize,
  ...    |
  1241 | |         bound_condition = " on 32-bit targets",
  1242 | |     }
       | |_____^
  note: called by `<usize>::unchecked_mul::precondition_check`
      --> ***\.rustup\toolchains\nightly-2025-06-30-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\num\mod.rs:1224:5
       |
  1224 | /     uint_impl! {
  1225 | |         Self = usize,
  1226 | |         ActualT = u32,
  1227 | |         SignedT = isize,
  ...    |
  1241 | |         bound_condition = " on 32-bit targets",
  1242 | |     }
       | |_____^
  note: called by `<usize>::unchecked_mul`
      --> ***\.rustup\toolchains\nightly-2025-06-30-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\num\mod.rs:1224:5
       |
  1224 | /     uint_impl! {
  1225 | |         Self = usize,
  1226 | |         ActualT = u32,
  1227 | |         SignedT = isize,
  ...    |
  1241 | |         bound_condition = " on 32-bit targets",
  1242 | |     }
       | |_____^
  note: 

I guess it tries to compile assert_unsafe_precondition! macro call which contains a call to usize::overflowing_mul() (which is used to do checked multiplication), and fails to do so.
I thought it should be compiled out because I'm compiling my shader in release mode (as by default) which suggests that debug_assertions are set to false (which should disable unsafe preconditions), but it seems this is not the case here.

I think it will be best to resolve such issue by implementing checked mul (as it is already implemented in Zig SPIR-V compiler, which I guess was the inspiration for rust-gpu's checked math functions)...
But if it is possible to disable unsafe preconditions completely, I can do just that.

[LegNeato]

Will be fixed by #561