2022-05-30

Auto merge of #12409 - lowr:fix/usize-overflow, r=Veykril

fix overflow during type inference for tuple struct patterns

The following code causes integer overflow during type inference for (malformed) tuple struct patterns.

```rust
struct S(usize);
let S(.., a, b) = S(1);
```

It has been panicking only in debug builds, and working in a way in release builds but it was inconsistent with type inference for tuple patterns:

```rust
struct S(usize);
let S(.., a, b) = S(1); // a -> unknown, b -> usize
let (.., a, b) = (1,);  // a -> usize, b -> unknown
```

With this PR, the overflow no longer happens by utilizing `saturating_sub()` like in other places and type inference for tuple struct patterns is in line with that for tuple patterns.