Fix ICE in `loops` module
changelog: Fix ICE related to `needless_collect` when a call to `iter()` was not present.
I went for restoring the old suggestion of `next().is_some()` over `get(0).is_some()` given that `iter()` is not necessarily present (could be e.g. `into_iter()` or `iter_mut()`) and that the old suggestion could change semantics, e.g. a call to `filter()` could be present between `iter()` and the collect part.
Fixes#5872
Check whether locals are too large instead of whether accesses into them are too large
Essentially this stops const prop from attempting to optimize
```rust
let mut x = [0_u8; 5000];
x[42] = 3;
```
I don't expect this to be a perf improvement without #73656 (which is also where the lack of this PR will be a perf regression).
r? @wesleywiser
try_err: Consider Try impl for Poll when generating suggestions
There are two different implementation of `Try` trait for `Poll` type:
`Poll<Result<T, E>>` and `Poll<Option<Result<T, E>>>`. Take them into
account when generating suggestions.
For example, for `Err(e)?` suggest either `return Poll::Ready(Err(e))` or
`return Poll::Ready(Some(Err(e)))` as appropriate.
Fixes#5855
changelog: try_err: Consider Try impl for Poll when generating suggestions
Add derive_ord_xor_partial_ord lint
Fix#1621
Some remarks:
This PR follows the example of the analogous derive_hash_xor_partial_eq lint where possible.
I initially tried using the `match_path` function to identify `Ord` implementation like the derive_hash_xor_partial_eq lint currently does, for `Hash` implementations but that didn't work.
Specifically, the structs at the top level were getting paths that matched `&["$crate", "cmp", "Ord"]` instead of `&["std", "cmp", "Ord"]`. While trying to figure out what to do instead I saw the comment at the top of [clippy_lints/src/utils/paths.rs](f5d429cd76/clippy_lints/src/utils/paths.rs (L5)) that mentioned [this issue](https://github.com/rust-lang/rust-clippy/issues/5393) and suggested to use diagnostic items instead of hardcoded paths whenever possible. I looked for a way to identify `Ord` implementations with diagnostic items, but (possibly because this was the first time I had heard of diagnostic items,) I was unable to find one.
Eventually I tried using `get_trait_def_id` and comparing `DefId` values directly and that seems to work as expected. Maybe there's a better approach however?
changelog: new lint: derive_ord_xor_partial_ord
Handle mapping to Option in `map_flatten` lint
Fixes#4496
The existing [`map_flatten`](https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten) lint suggests changing `expr.map(...).flatten()` to `expr.flat_map(...)` when `expr` is `Iterator`. This PR changes suggestion to `filter_map` instead of `flat_map` when mapping to `Option`, because it is more natural
Also here are some questions:
* If expression has type which implements `Iterator` trait (`match_trait_method(cx, expr, &paths::ITERATOR) == true`), how can I get type of iterator elements? Currently I use return type of closure inside `map`, but probably it is not good way
* I would like to change suggestion range to cover only `.map(...).flatten()`, that is from:
```
let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `flat_map` instead: `vec![5_i8; 6].into_iter().flat_map
```
to
```
let _: Vec<_> = vec![5_i8; 6].into_iter().map(|x| 0..x).flatten().collect();
^^^^^^^^^^^^^^^^^^^^^^^^ help: try using `flat_map` instead: `.flat_map(|x| 0..x)`
```
Is it ok?
* Is `map_flatten` lint intentionally in `pedantic` category, or could it be moved to `complexity`?
changelog: Handle mapping to Option in [`map_flatten`](https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten) lint
needless_collect: catch x: Vec<_> = iter.collect(); x.into_iter() ...
changelog: Expand the needless_collect lint as suggested in #5627 (WIP).
This PR is WIP because I can't figure out how to make the multi-part suggestion include its changes in the source code (the fixed is identical to the source, despite the lint making suggestions). Aside from that one issue, I think this should be good.
There are two different implementation of Try trait for Poll type;
Poll<Result<T, E>> and Poll<Option<Result<T, E>>>. Take them into
account when generating suggestions.
For example, for Err(e)? suggest either return Poll::Ready(Err(e)) or
return Poll::Ready(Some(Err(e))) as appropriate.
The reason we do not trigger these lints anymore is that clippy sets the mir-opt-level to 0, and the recent changes subtly changed how the const propagator works.
