Auto merge of #12877 - y21:const_host_ice, r=dswij

Handle const effects inherited from parent correctly in `type_certainty`

This fixes a (debug) ICE in `type_certainty` that happened in the [k256 crate]. (I'm sure you can also specifically construct an edge test case that will run into type_certainty false positives visible outside of debug builds from this bug)

<details>
<summary>Minimal ICE repro</summary>

```rs
use std::ops::Add;
Add::add(1_i32, 1).add(i32::MIN);
```
</details>

The subtraction here overflowed:
https://github.com/rust-lang/rust-clippy/blob/436675b4772049a0ff535bd46d944689c90ed750/clippy_utils/src/ty/type_certainty/mod.rs#L209

... when we have something like `Add::add` where `add` fn has 0 generic params but the `host_effect_index` is `Some(2)` (inherited from the parent generics, the const trait `Add`), and we end up executing `0 - 1`.

(Even if the own generics weren't empty and we didn't overflow, this would still be wrong because it would assume that a trait method with 1 generic parameter didn't have any generics).

So, *only* exclude the "host" generic parameter if it's actually bound by the own generics

changelog: none

[k256 crate]: https://github.com/RustCrypto/elliptic-curves/tree/master/k256

0 files changed, 0 insertions, 0 deletions